Monthly Archives: September 2014

C++11

[ C++11 – Part 4 ] STL-Iterators

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Standard Template Library

A major part of C++ is the Standard Template Library ( STL ). It is quite large and it contains both containers ( like vector, stack and maps ) with a lot of functionality for using these. In this post, we’ll look at one part of the STL, iterator, and in the following post we’ll look at other parts of STL like the algorithms library

Iterators

Iterators can be looked at like pointers. They reference an item in a data structure( including string ). They can also be used with streams. The iterators enables us to iterate through them using various operations like ++.

But we need different types of iterators because data structures can often be iterated in different ways. Some ways are efficient on some containers while they might be inefficient or even impossible on other containers.

Let’s take a look at the different type of iterators.



Here’s a simple overview :

Iterators overview


The properties of the various iterators here are the bare minimum of the “pure” version of the iterators. Some implementations might support more operations.

Note that forward iterator, bidirectional iterator and random access iterator does not implement output iterators. This means that they can’t, by definition, write to the object they refer to. We’ll get back to this later.

Let’s start by looking at the first type of iterator, output iterator.

Output iterator

As mentioned earlier, output iterator is a very limited type of iterator it is also the only iterator that can write to the object.

You can’t go back with an output iterator, and you can’t iterate over the same range twice. And there is no guarantee that you can write to the same position twice without incrementing the operator. And you can’t compare two output iteratrs either. What you’re basically doing is writing into a “black hole”; you’re just writing and you can’t see what you have written.

This might seem very constrictive, there’s very little we can do. But these rules only apply in some cases, like when writing to streams, which we’ll look at later. You can look at the iterators that follows everything described above as pure output iterators

Most iterators, though implement output iterators but also has other traits that enables more functionality. We’ll look at these later.

A simple example

This is a very simple example of the intended usage for output iterators :

Or, in other words :

  • Write
  • Move forward
  • Write
  • Move forward
  • …

A bit more complex example

In C++ you have streams, which allows you to to either write or read from them using the >> and << operators. Examples of this is cout ( write ) cin ( read ). This makes cout output operators ( because they output to the terminal. )

cout is of the type ostream. You also have an iterator to this type, called ostream_iterator that can be set to an ostream Writing to this iterator writes to the ostream it’s an iterator to.

Using this knowledge, we can create an iterator to cout, this enables us to write to the terminal using an iterator

Output :

11
44
33

The ostream_iterators are output iterators. Using cout like this shows how output iterators works ; you can only write and the writing has to be sequential. You can newer go back. Once you’ve written something, you can’t “go back” and write something in the position before what you just wrote.

Input iterator

The counterpart of output iterators is input iterators. But where output iterators can only write, input iterators can read but there is also a small changes in the restrictions :

  • You can compare two iterators
    • Comparing two  iterators is not guaranteed to work unless at least one of them is past the end ( like std::end )
  • Since you can read, you can also use the -> operator to access member variables

    A simple example

    The following example shows the essential use of an input iterator :

    Or, in other words :

    • Read
    • Move forward
    • Read
    • Move forward
    • …

    A more complex example

    In the previous example, we saw that ostream_iterators are output iteratorss. Which means we can use them to write to cout. Similar to this, we have istream_iterators that are input operators. And where we can use ostream_iterators with cout, we can use istream_iterators with cin which reads input.

    Here too we have to work consecutively, we can’t go back or skip ahead, we can read input in a similar to when we wrote to cout

    Example input :

    3
    2
    6
    f

    Output :

    Write 3
    You wrote 3
    2
    Write 2
    You wrote 2
    6
    Write 6
    You wrote 6
    f

    Since we’ve specified int as the template argument of the input_iterator, the input_iterator ( intReader ) will become the same as invalid, which in C++ means the iterator will point to the element past the end element. This is the same as readerEnd and the loop will terminate.

    Forward iterator

    If you take a look at the iterator overview above, you’ll see that the forward iterator implements input iterator. This means a forward iterator can do anything an output iterator can do.

    Forward vs input operator

    A forward iterator has a few extra features that input iterators don’t have. These are :

    Equality check

    Two forward iterators are guaranteed to compare as equal if they refer to the same element

    • An input iterator can, as we saw above, be compared to another output iterator, but the operation is only guaranteed to be true  if one of them is past the end of the container

    Example :

    The compare operation ( it1 == it2 ) is not guaranteed to be true for  output iterators or input iterators, but it is for forward iterators

    Forward iteration

    A forward iterator can also iterate forward as many times as you like before reading

    • With  output iterators or input iterators we would have to read or write in between each time we iterate forward.

    Example :

    If we had used an output iterator or an input iterator to do the multiple iterations would have caused the iterators to become invalid.

    Iteration over same range

    Finally, a forward iterator can iterate over the same range twice.

    • output iterators or input iterators are intended to only iterate over the same range once ( single-pass ) but are intended to be able to do this ( multi-pass )

    Example :

    Here we’ve effectively moved back and continued moving forward, which again would not be possible with code>output iterators or input iterators.
    As you can see, using  forward iterators is in many ways more convenient than using output iterators or input iterators, but some object only has very few operations, like the streams we looked at earlier.

    Luckily, most ( if not all ) structures in the C++11 supports at least the functionality of forward iterators. Most of them provide iterators that has more properties, but this time we’ll look at a structure that only provides forward iterators

    Forward list

    A linked list is a container that works a bit different from the array-like containers. Basically, each element has a pointer to the next element. So that all the elements are chained together using pointers. If you want to go from one element to another, you use the next pointer. This can mean a lot of operations if the list is very large.

    The objects will not be one contiguous piece of memory like arrays are. This means there are no quick ways to go from the first to the last element. So if we wanted to go from Node 1 to Node 2 in the above illustration, we would have to go via Node 3. And if we wanted to go back from Node 3 to Node 1 we would have a problem. There is no pointer that goes back in a forward list. If you want to get to a previous node, you would need a pointer to an earlier element. The forard_list itself keeps a pointer to the first element in order to be able to access all elements.

    A container in like this didn’t exist in C++ until C++11 which has the container forward list.

    Iterating forward lists

    The forward list is a great way of showing how the forward iterators works. The forward list can only be iterated forward and the forward iterator can only iterate forwards.

    Let’s look at an example :

    We are now at the fifth element ( 5 ), but say we want to print the previous element ( 0 )? We need to go back to the begin element and increment like above.

    Output :

    43
    63
    11
    0
    5
    0

    Forward iterators and output iterator

    • Per definition, forward iterators  does not include the properties of output iterators. 
      • Most implementations of forward iterators does however support assignment like output iterators. These iterator are called mutable forward iterators

    Bidirectional iterator

    The bidirectional iterator is very similar to forward iterator. The only different is that it also supports backwards iteration. And that’s all the difference between code>forward iterator and bidirectional iterator.

    Linked list

    In the forward iterator section we looked at singly linked lists which has a pointer to the next element. But most linked lists also has a pointer to the previous element.These are called doubly linked lists or simply just linked list.

    The container type doubly linked lists are identical to singly linked lists, with the one exception that each Node now has a pointer to the previous Node too.

    The prev pointer enables us to iterate backwards, so now we need an iterator that can iterate backwards and forwards. This is where the bidirectional iterator comes in.

    An example

    The following example is the similar to the example for forward list but this time we solve the last problem in a different way.

    We are now at the fifth element, but say we wanted to go back to the previous element? Since doubly linked lists support bidirectional iterators, we can use -- to iterate back.

    -it;

    Output :

    43
    63
    11
    0
    5
    0

    As you can see, bidirectional iterators can iterate back and forth without problems. Now let’s look at the final iterator, random access iterator.

    Random access iterator

    The final iterator is also the one with the most features. It can be used like a pointer. In fact, some implementations of STL uses pointers as random access iterators.

    Here’s a list of the features of a random access iterator

    • All the features of bidirectional itearator
    • Random access
      • Access items by [] operator
        • it[n] returns the value of the position it + n
        • it[n] is the same as *( it + n )
    • Iterator arithmetics
      • This means you can add or subtract integer values and iterators
        • it += n moves it forward n steps
        • it -= n moves it backward n steps
        • it + n returns an iterator to the element n steps after it
        • n - it returns an iterator to the element n steps before it
        • it - it2 returns the distance between it and it2
    • Iterator comparison
      • This means you can check which if one iterator is before another iterator
        • it1 < it2 returns true if it1 is before it2
        • it1 > it2 returns true if it1 is after it2
        • it1 >= it2 returns true if it1 is not before it2
        • it1 >= it2 returns true if it1 is not after it2

          • not it1 < it2 

    Array-like containers

    Array like containers are all containers that are organized as a continuous piece of memory. This means you can predict where the next item is, since it the next item will always be the previous item + the size of each item. Using this knowledge, it’s easy to jump forward or backwards. An example of these containers is vector, which I covered in a previous part.

    The above example shows how any array-like container ( vector, array, queue, deque, … ) is laid out in memory. Everything is in one continuous block which means we always know where the next element is.

    An example

    The following example shows usage of random access iterators:

    Conclusion

    There are several types of iterators in C++. Especially random-acces iterators can be particularly useful. We will see more of this in the next post. The next part will also introduce a few new iterators, but they are all variations of the iterators in this post.

    Feel free to comment if you have anything to say or ask questions if anything is unclear. I always appreciate getting comments.

    You can also email me : olevegard@headerphile.com
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    C++11

    [ C++11 – Part 3 ] Smart pointers

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    Memory in C++

    One of the major parts of C++11 is the new smart pointers. But why do we need them, and why should you steer away from the old style pointers?

    This post is intended both as an introduction, and a reference guide to the smart pointers found in C++11. It will cover what they are, how to use them and how they work.

    Memory allocation in C++

    C++ supports dynamic memory allocation ( new / delete ). It’s a way of creating a object whenever you need it. The object will live until you call delete on it. Otherwise it won’t be deleted, not even when it goes out of scope. And if you don’t delete it, none will, which in turns causes memory leaks. It might also cause undefined behavior ( which means anything can happen ) if, for instance, you delete something and try to use it afterwards.

    Smart pointers will help you with these issues and make your code cleaner and better.

    Smart pointers

    Smart pointers are like regular pointers, but they provide extra functionality such as automatic checks or other safety mechanisms. C++11 introduces three smart pointer types :

    • unique_ptr
      • Only one unique_ptr can point to the object at the time
    • shared_ptr
      • Several shared_ptrs can point to the object at a time.
      • Object is released when no shared_ptrs point to the object
    • weak_ptr
      • Non-owning, needs to be converted into an shared_ptr to use the object it points to

    Using smart pointers

    Once you have created the smart pointer, you can use it as a regular pointer :

    • ->
    • *
    • !
    • ==
    • ...

    All of the above operations are supported for the smart pointers in C++11. This is not true for weak_ptr, but we’ll get to that later.

    You can also use them in ifs like this :

    It will enter the if as long as the smart pointer is initialized and is managing a valid object ( meaning it’s not a NULL pointer )

    We’ll now take an in-depth look at the smart pointers, one at the time.

    unique_ptr

    An unique_ptr is the sole owner of an object that it points to. Two unique_ptrs can also swap objects, but they can never manage the same object. And as with all smart pointers, it takes care of deleting the object. 

    Initialization and allocation

    There are two ways of doing this, regular constructor or make_unique

    constructor

    The constructor is pretty straight forwards. You just set the template arguments ( in this case Foo ) and pass an allocated ( new‘ed ) object :

    This is pretty straight forwards ; template parameter is the type and you supply an allocated ( new‘ed ) object.

    make_unique

    The second alternative is using a function named make_unique, this is in most cases better, because it constructs the object and it helps prevent errors. It also follows the naming convention ofshared_ptr which can be constructed in the same way with the make_shared method.

    This is how to use make_unique :

    What make_unique does is that it creates a new unique_ptr that manages a new object of the template type ( Foo in our case. ) It takes the arguments of the make_unique function itself and passes it to the constructor of the new Foo. Then the function returns the unique_ptr it created and the =operator assigns it to the unique_ptr we declared ( p2 )

    This function requires C++14, it should have been included in C++11, a simple oversight lead to it not being included in the standard. But don’t worry, it’s easy to get this to compile:

    • On Linux / Mac
      • Change -std=c++11 to -std=c++1y in your compile command
    • On Windows
      • Newer versions VisualStudio should support this without you having to do anything.

    If neither of the above works, you can copy-paste this :

    Destruction and deallocation

    Since the unique_ptr is a smart pointer, it handles deallocation automatically. In general, this means that the unique_ptr calls delete on the object it holds. This happens when :

    • The unique_ptr is being destroyed.
      • Happens when the unique_ptr goes out of scope.
    • You set the unique_ptr to point to something else
      • More on this below.

    Releasing the object

    Releasing the object means that the unique_ptr stops managing the object. This means you can let a regular pointer “take over” the object the unique_ptr was managing. So when the unique_ptr goes out of scope or is destroyed in any way, the pointer object it was managing wont be affected.

    Here’s an example :

    • fooUPtr now manages pFoo1
    • pFoo1 is unchanged

    • fooUPtr no longer manages the object pFoo1 points to
    • pFoo2 now points to the same object as pFoo1

    But now pFoo1/pFoo2 needs to be deleted. Since they both point to the same object, we just need to delete one of them :

    Changing the object being pointed to / reassignment

    Changing what the unique_ptr points to is quite easy. It can be done in two ways :

    These are used in a similar work

    And

    Both of these properly deallocates the original object managed by foo1, calling its destructor in the process. Then the new object returned by new is assigned to foo1 and foo1 now manages it just like it did with the previous object.

    Transferring ownership

    You can’t set two unique_ptrs to manage the same object, but you can transfer ownership from one unique_ptr to another. And you can swap the managed object of two unique_ptrs.

    Swapping owenership

    Swapping ownership can be done in two ways ; a member function, or the std::swap() function.

    Member function
    The member function is very straight forwards :

    std::swap
    std::swap() is a very simple, but also very useful algorithm defined in the algorithm header ( until C++11 and utility since C++11 ). It basically swaps two objects of whatever type you pass as arguments. It can be two ints, two std::strings or two objects of a class/struct or anything else you want.

    The version used for unique_ptrs is just an overload of this that uses the above swap( std::unique_ptr ) member function function internally. Here’s how to use it :

    So in reality, the member function and swap() are identical. The result of both of these are as you would expect :

    • foo1 now manages foo2
    • foo2 now managesfoo1

    Reseting

    You can use the reset() to reset the unique_ptr This means that we change what pointer the unique_ptr manages, deleting the old one. This method can only be used with raw pointers, not smart pointers.

    Example :

    What this does

    • Crates a new uniqe_ptr that manages a new Foo object
    • Deletes the old managed Foo object and replaces it with a new one

    Transfering owenership

    You can use the = operator to transfer ownership :

    This takes the pointer from foo2 and gives it to foo1. This means that :

    • foo2 no longer handles any object ( like calling )
      • This is because only one unique_ptr can manage an object at a time
    • The object that foo1 was holding is deleted
      • Because foo1 is going to be managing foo2

    This is the same as doing :

    Note that this is not the same as swapping two unique_ptr

    If we swap, the result would be :

    • foo1 manages foo2 
    • foo2 manages foo1

    In this case the result is.

    • foo1 manages foo2 
    • foo2 doesn’t manage any object

    Destroying the object

    When you want to destroy the object the unique_ptr manages, there are two ways you can do it:

    • Destructor
      • This simply means letting the unique_ptr go out of scope
    • The reset() method
      • You can use this with either a NULL pointer or another pointer, the managed object will get deleted in both cases

    The bool operator

    The bool operator dictates what happens when used in a boolean expression like if () . It will return true if the unique_ptr manages an object. If not, it’ll return false. In other words ; if, and only if, you can use it, it’ll return true, so this is exactly the same as you do when you’re checking if a pointer is NULL or nullptr

    The other operators

    The other operators works just like regular pointers, so I won’t discuss them here.

    shared_ptr

    shared_ptr is, as the name implies, a smart pointer that allows sharing. This means you can have several shared_ptrs that point to the same object. This is not permitted by unique_ptr as we saw in the previous section.

    Implementation

    The shared_ptr is a bit more complex than unique_ptr. This is because a shared_ptr needs to keep track of how many other shared_ptrs are managing this object, But for unique_ptrs there will always be only one pointer managing the same resource, so there is no need to keep track of the number of other unique_ptrs that are managing this object ( because it’s always just one! )

    In order to keep track of this information, shared_ptr keeps a pointer to a structure called a control block. This structure has three member variables :

    •  shared_ptr reference count
      • How many shared_ptrs are managing to the object
    • weak_ptr reference count
      • How many weak_ptrs are referring to this object
        • More on  weak_ptrs later
    • A pointer to the object the shared_ptr manages

    The shared_ptr also keeps a pointer to the object it manages. And this is where it gets a little complicated, because this pointer is closely related to the one in the control block. But why? The reason for this is a bit complicated and it has to do with the two ways of creating a shared_ptr, make_shared and constructor. I will discuss this in the next section :

    Initialization

    When it comes to unique_ptr, the difference between using make_unique and the regular constructor is slight ( that doesn’t mean you shouldn’t use make_unique as often as you can! ) But in a shared_ptr things are different.

    As stated above, a shared_ptr has two elements : a control block and a pointer to the object it manages. Both of these needs to be allocated, but before we get into that, let’s look at how we can create a shared_ptr :

    Or

    Using constructor

    In the first example, we first allocate Foo using new before pass it to the shared_ptr constructor. This means that the shared_ptr has no control over the allocation of Foo so all it can do is to create the control block and have it point to Foo. The figure below will show the procedure for when you create a shared_ptr using constructor :

     
    Foo and ControlBlock allocated in two steps

    As you can see, the control block and the Foo needs to be allocated in two steps. First the object, then the control bloc.

    Using make_shared

    But if we let make_shared handle the allocation of Foo, it can allocate both the control block and Foo in one go. It’ll look something like this :

    Foo is now part of the FooControlBlock

    So make_shared creates the object and the control block together in one operation. This makes the operation faster than creating them in two step, but it requires them to be one object, so here Foo is part of the control block itself.
    make_shared is available in C++11 so you can use it without enabling C++14

    When not to use make_shared?

    There are two cases when you can’t use make_shared :

    • If you are using a pointer that’s already created somewhere else
      • Using make_shared means the object would be re-allocated
      • Pass the pointer in the constructor instead ( where we passed new Foo() in the example above )
    • If you don’t want to use the default delete
      • You can’t specify a custom deleter using make_shared
      • This is a bit complicated, so I won’t go into details

    Destruction and deallocation

    The destructor for shared_ptr is also a bit different from unique_ptr because an unique_ptr will always be the sole manager of an object ( not other unique_ptr or shared_ptrswill be managing it. ) This means it’s always safe to delete, so that’s what the unique_ptr will do.

    But when it comes to shared_ptrs, we can’t do that before we make sure that no other shared_ptrs are managing it. So what we do is that we look on the control block and how many shared_ptrs are managing it. If this is 0, we are the last owner and we can safetely delete it.

    The weak_ptr reference count it not checked at this point. I’ll get into why in the next section that discusses weak_ptr and how the relate to shared_ptrs.

    Changing the object being pointed to / reassignment

    Similar to unique_ptrs but here we need to do some extra work. There’s two different cases for this ; setting the shared_ptr to be the same as another shared_ptr and setting the shared_ptr to manage a new pointer.

    In both of these cases, it will decrement the shared_ptr reference count in the control block. And if this count reaches 0 it will delete the object being pointed to ( but not necessarily the control block, more on this later. )

    Assigning to a different shared_ptr

    Assigning a shared_ptr to a different shared_ptr is done using the =operator.

    Here’s a simple example

    Result

    • The original ptr1‘s shared_ptr count is now 0, and the object it manages will be deleted
    • ptr1 and ptr2 will now both manage the same object as the original ptr2 with a shared_ptr count of 2

    Assigning shared_ptr to a new object

    Assigning a shared_ptr to a new object/pointer is done using the reset() function :

    Here’s a simple example

    Result

    • The shared_ptr reference count for sharedPtr is decremented as if we were calling the destructor.
      • The Foo object sharedPtr was originally manging may get deleted
    • sharedPtr now manages the object rawPtr points to.

    As you can see from the examples, you use opeator= for reassigning to another shared_ptr but reset() for reassigning to a different raw pointer. You can’t use them the other way around. This can help prevent bugs by giving an error if the programmer uses the wrong versions.

    There is a way you can use operator= to assign to a new pointer;  using make_shared to create a new object :

    This works because make_shared creates and returns a fully constructed shared_ptr ( just like make_unique described above ) and the =operator assigns it just like in the example above.

    Swapping

    The syntax for swapping shared_ptrs is the exact same as for swapping two unique_ptrs :

    Member function :

    std::swap :

    This will, as you would expect, swap both the control block and pointer for both the shared_ptr. It needs to swap the control block since this is what keeps tracks of the number of references to the pointer so these needs to be consistent.

    The bool operator

    The bool operator dictates what happens when used in a boolean expression like if () . It will return true if the shared_ptr manages an object. If not, it’ll return false. In other words ; if, and only if, you can use it, it’ll return true, so this is exactly the same as you do when you’re checking if a pointer is NULL or nullptr

    This is exactly the same as for unique_ptr.

    The other operators

    The other operators, just like with unique_ptr, works just like regular pointers, so I won’t discuss them here.

    weak_ptr

    As mentioned in the introduction, weak_ptr doesn’t actually manage a pointer. It just holds a non-owning ( "weak" ) reference to an object that is managed by a shared_ptr. It also keeps a pointer to the control block ( the exact same as the one in the shared_ptr who manages the object. This means it has to be created from a shared_ptr so that it can get a pointer to the control block.

    wrak_ptr and the control block

    The control block, as we saw in the previous section, keeps a count of both shared_ptr and weak_ptr who’s using the object. We also saw that the object will get deleted if the count of shared_ptrs using the object is 0 regardless of how what the weak_ptrs count is. This is part of the point of weak_ptr; it is not supposed to keep objects alive except for in situations we explicitly tell it to.

    But even though the managed object will get deleted if the count of shared_ptrs is 0, the control block will remain intact. The control block will only be deleted if both the conut of shared_ptr and weak_ptr uses. This is because the weak_ptr uses the control block to check if the object is alive.

    Creating a weak_ptr

    There are two ways of creating a weak_ptr from a shared_ptr: constructor and =operator. This is very straight forwards :

    All of these will set up the pointer and the control block and increment weak count by 1.

    Creating a weak_ptr from a raw pointer won’t work simply because it isn’t designed to be managing a pointer by itself. And you can’t use them with unique_ptrs because unique_ptrs are supposed to be the sole owner.

    Reseting a weak_ptr

    The object a weak_ptr can be reset ( so that it no longer references any object ) using the destructor or the reset() function.

    Using a weak_ptr

    weak_ptr has the function lock(). What this function does is that it makes a shared_ptr of itself. This shared_ptr will work exactly as any other shared_ptrs maintaining the object. It will increase the shared_ptr count, so this shared_ptr will keep the object alive.

    If the object has been deleted, it will still return a shared_ptr. But this shared_ptr doesn’t point to anything, it’s essentially a nullptr. It will return false if you use it in a if or loop condition, just like a regular shared_ptr or unique_ptr that doesn’t manage an object.

    This function is the crucial part of the weak_ptr. It enables us to keep weak_ptrs that refers to object maintained by a shared_ptr without preventing it from being deleted and we can still use it safely when we want.

    So let’s look at an example :

    Usage 1

    Here everything works as intended. The weak_ptr refers to sharedPtr which has a valid object inside this scope.

    • lock() returns a shared_ptr that manages the same object as sharedPtr
    • The returned shared_ptr is valid so it enters the if
    • While in this if, the shared_ptr, fromWeakPtr1 keeps the object alive, even if all other shared_ptr should be destroyed in other threads in the meantime

    Usage 2

    Our shared_ptrs has gone out of scope ( both the original and the one we created from the weak_ptr and the object has been deleted. But the weak_ptr, weakPtr still exists as it was declared out of the scope. So we try to use it again :

    •  We create our shared_ptr from the weak_ptra
    • The shared is created, but since the original object was deleted, this is essentially a nullptr
    • The shared_ptr returns false and it doesn’t enter the if.

    Leaving the scope

    Finally we leave the scope and we delete weakPtr. Now the weak_ptr count will be 0 and the control block will be deleted ( shared_ptr count was already 0 )

    Releasing the object

    A weak_ptr can’t release the object because it doesn’t have control over it, and the object might be NULL. For this reason, weak_ptr does not have a release() function. The closest thing is the reset() function described above.

    Swapping

    The syntax for swapping weak_ptrs is the exact same as for swapping two shared_ptrs and code>unique_ptrs:

    std::weak_ptr< Foo >( new Foo ) foo2;
    std::weak_ptr< Foo >( new Foo ) foo2;

    Member function :

    foo1.swap( foo2 );

    std::swap :

    std::swap( foo1, foo2 );

    This will, as you would expect, swap both the control block and pointer for both the weak_ptrs. It needs to swap the control block since this is what keeps tracks of the number of references to the pointer so these needs to be consistent.

    Changing the object being pointed to / reassignment

    Similar to shared_ptrs but with a few differences :

    • Since we are not managing the object, we don’t need to worry about deleting it.
      • It might already have been deleted at this point, but this will not cause the opeartion to fail
    • Decrement count for weak_ptr, not shared_ptr
      • If weak_ptr count reaches 0 and the count for shared_ptr is also 0, we delete the control block
    • Now we can set the pointer to the object and control block
      • Both of these will already have been created by a shared_ptr

    You can create reassign a weak_ptr to both a shared_ptr and another weak_ptr

    Feel free to comment if you have anything to say or ask questions if anything is unclear. I always appreciate getting comments.

    You can also email me : olevegard@headerphile.com
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    SDL2

    [ SDL2 – Part 11 ] Text styling

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    Text styles using SDL2_ttf

    In the last post we looked at how to render text. Now let’s take this a step further and change the appearance of the font. There are two ways you can change how the font looks. Font style and font outline.

    Fon styles

    SDL TTF allows you to set the style of the font. It supports the following font styles

    • bold
    • italic
    • underlined
    • strikthrough

    You can combine these in any way you like. We’ll start off with just setting a single font style at a time, and then move on to see how we can apply several of them at once.

    Font styles

    Settnig font styles in TTF is easy, it just requires a single function. The function let us you set one or more font styles. Let’s start off by looking at how to set just one font style

    Setting the font style

    We can set the font style using the following function

    Arguments :

    •  TTF_Font *font – the font to set the style on
    • int style       – the style to set on the font

    As you can see, the style parameter is an int and not an enum. I’ll get back to why that is later, but for now let’s look at the possible values for style, these are all self-explanatory so I won’t be adding a description.

    • TTF_STYLE_NORMAL
    • TTF_STYLE_BOLD
    • TTF_STYLE_ITALIC
    • TTF_STYLE_UNDERLINE
    • TTF_STYLE_STRIKETRHOUGH

    Any text you render after setting this font style will have the new effect, but it won’t change any text you have written with a different style. So when you set the style to TTF_STYLE_BOLD, all text you render from that point and until you set a different style will be bold. And as long as you pass any of the above values to the function, the font will only have the one last style you set.

    Let’s do a simple example

    Any text rendered at this point will be normal with no font styles

    Any text rendered at this point will be bold

    Any text rendered at this point will be in italics, but not bold

    Any text rendered at this point will be normal with no font styles

    Any text rendered at this point will be underlined

    As you can see, this is pretty straight forwards. So let’s make things a little bit trickier by setting multiple font styles at once. To do this, we must first look a bit at the binary number system

    Binary numbers

    In order to learn about how to combine these flags, we need to look at binary numbers first of all. If you don’t already know about binary numbers, you should take a look at the above link. It’s not crucial, but it is highly recommended to know a little about them. I might create a blog post about them at some point. For now, I’ll just talk a tiny bit about the binary number system. But as I said, I highly recommend understanding it fully to the point where you can convert back and forth between binary and decimal numbers

    The binary number system

    On a daily basis, we use the decimal number system. The binary numbers system is just a different way of representing numbers. Any numbers can be converted from any other number system. So you can convert binary numbers to decimal numbers ( and the other way around ).

    A computer stores numbers as individual bits ( 0‘s and 1‘s ). They correspond to on / off or true / false.

    Let’s take a look at an 8 bit binary number ( 1 byte )

    1010 0101

    As you can see, it has 8 digits. So that’s eight different flags. Each of these flags have two different possible values : 0 / 1 or false / true. So that’s 8 bools for the price of a single byte!

    Bitwise operations

    So how do we use these 8 booleans? As you know, we have the following boolean operations in C++:

    • and ( && )
    • or ( || ) 

    These work on an entire variable. An int, for instance will be false if its value is 0, otherwise its true.

    But there are similar operations that does this on all bits of a variable. These are called bitwise operations, simply because they operate on a simple byte. To do a bitwise operation, we need two variables of equal size ( same number of digits ), for instance two bytes. The result of a bitwise operation is a third variable of the same size. So if we do a bitwise operation between two bytes, we get a third byte back as a result.

    Let’s create two bytes, we’ll use these for a few examples

    Byte 1 : 0101 0011 ( 64 + 16 + 2 + 1 = 85 )
    Byte 2 : 0110 0010 ( 32 + 64 + 2 = 98 )

    We’ll be referring to each digit as a position. So the digits in the first position is 0 in both or bytes. In the second position it’s 1 in both bytes and in the third it’s 0 in the first byte and 1 in the second byte.

    Bitwise OR

    A bitwise OR operation means we look at all positions as check if either of them is 1 if so, we set the digit in that position to 0. If no digit in that position is 1, we set it to 0

    The operator for bitwise OR in C++ is | ( just one |, not two )

    Here is a simple example of bitwise OR between two bytes

    0101 0011
    OR
    0110 0010
    =
    0111 0011

    Bitwise AND

    In a bitwise AND operation, we look at each position and see if both of them are 1. If so, we set the digit in that position to 1, otherwise we set it to 0. So in OR we set it to 1 if any of the two is 1, here we only set it to 1 if both are 1.

    The operator for bitwise AND in C++ is & ( just one &, not two )

    Here’s the a simple example :

    0101 0011
    AND
    0110 0010
    =
    0100 0010

    Bitwise XOR

    XOR or exclusive OR is slightly less known than OR and AND. In an XOR operation, we check if the two values are different. So this is equivalent to != in C++.

    • ( true  != false ) = true
    • ( true  != true  ) = false
    • ( false != true  ) = true
    • ( false != false ) = false

    Simply put, an XOR operation is true if the two parts are different. So in a bitwise XOR operation, we look at each position and see if the two digits are different. If so we set the digit at that position to 1, otherwise we set it to 0.

    The operator for bitwise XOR in C++ is !=

    Here is an example :

    0101 0011
    XOR
    0110 0010
    =
    0011 0001

    Bitwise NOT

    We also have the a bitwise version of the NOT opeartion this is done by using the ~ operator in C++. If we used ! we would get the result of NOT on the entire variable, not the individual bits which is what we want. This operation only takes a single element and flips all bits ( turns 1‘s into 0‘s and 0‘s into 1‘s. ). Let’s test it on our two bytes

    The operator for bitwise NOT in C++ is !

    Byte 1 :

    NOT 0101 0011
    =   1010 1100

    Byte 2 :

    NOT 0110 0010
    =   1001 1101

    Setting and checking individual bits

    So now that we know how to do bitwise operations, we need a way of checking and setting the individual bits. This is done simply by using OR, AND and XOR. Before we take a look at how to do this, let’s define a few values to check.

    Remember that the different font styles are ints? This is because they are used to perform bitwise operations to set and unset different bits. Here they are again, this time with their values. For simplicity, I’ll only list the last four bits ( the others are always 0 ). The values are in decimal with the binary representation in parenthesis

    • TTF_STYLE_NORMAL = 0 ( 0000 )
    • TTF_STYLE_BOLD = 1 ( 0001 )
    • TTF_STYLE_ITALIC = 2 ( 0010 )
    • TTF_STYLE_UNDERLINE = 4 ( 0100 )
    • TTF_STYLE_STRIKETRHOUGH = 8 ( 1000 )

    As you can see, they all have only one ( or zero ) bit set. This means we can use AND, OR or XOR on just one bit.

    Setting a bit

    To set a bit ( without affect any other bit ) we use the ORoperation. So say that we have four bits set to 0, 0000 and we want to set the bit for bold on it ( 0001 ). In other words, we want the result 0001. What we do is : that we take our original 4 bits ( 0001 ) and set it to the original 4 bits ( 0001 ) OR‘ed with the bitmask for bold ( 0001 ) :

    0000
    OR
    0001     ( value of TTF_STYLE_BOLD )
    =
    0001

    Simple as that! This woks for any of the other flags in the same way. They all will end up setting one bit.

    Note that this will not change any other bits. If we try set the italics font style on the above variable we get :

    0001
    OR
    0010     ( value of TTF_STYLE_ITALIC )
    =
    0011 ( TTF_STYLE_BOLD and TTF_STYLE_ITALIC set )

    Let’s make a simple function that adds a style to a mask.

    Unsetting a bit

    Sometimes we want to unset a bit. Say for instance we want to remove the italics from the font above. How do we do that without affection the other values? This is a bit more complex, because it requires two operations. What we are trying to do is the following :

    Say we have a bitmask ( 0000 1011 ) and we want to unset the bit for bold text, but leave the rest unchanged. So we need to be able to go :

    From 1011 to 1010

    To do this, we need to use an AND operation. This is because we can’t turn of a bit using OR, and XOR would only flip it back and forth between 0 and 1

    But we can’t use AND with the flag we want to unset alone, because that would keep the flag at 1 and change every other to 0!

    0000 0101
    AND
    0000 0001
    =
    0000 0001

    This is the opposite of what we want! Wait? Opposite you say? Well how about we use the NOT operator here to get the opposite result? This works perfectly, because NOT 0000 0001 is 1111 1110. And, as we saw earlier, doing AND 1 won’t change the element we’re AND‘ing with. So we get :

    0000 0101
    AND
    1111 1110
    =
    0000 0100

    Success! Only the bit we were trying to unset has changed. So let’s make a function that does this for us :

    Checking a bit

    To check a bit, we also need to use the bitwise AND operation. And since we are only checking and not setting, we don’t have to store the value anywhere which means we don’t have to worry about changing anything.

    To check a bitmask, simply do an AND operation with the value you want to check for ( in this case, any of the TTF_STYLE_.... values ). So, to check if a text is bold, we do an AND between our mask and TTF_STYLE_BOLD :

    0011 ( our bit mask, TTF_STYLE_BOLD and TTF_STYLE_ITALIC set )
    AND
    0001
    =
    0001

    As you can see, we only check the bit that’s set in our variable ( TTF_STYLE_ITALIC set ) the others will be 0 no matter what our mask is. The value 0001 is not 0, and thus this evaluates to true and we now know that the font is bold.

    If our mask didn’t have the bold bit set ( only the italic one ), our mask would be 0010. An AND between 0010 AND 0001 is false ( they have no bit set to 1 in common ) and the result is 0 aka false.

    So let’s create a function for that too!

    Conclusion

    With a little knowledge about binary numbers and bitwise operations, we can easily set, add, remove and check various font styles in SDL_TTF.

    Since it does involve a little low level code, I made a simple class that does the apparitions for us in a more intuitive way. I strongly suggest using this opposed to “raw” TTF_Fount*

    Feel free to comment if you have anything to say or ask questions if anything is unclear. I always appreciate getting comments.

    You can also email me : olevegard@headerphile.com
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    SDL2

    [ SDL2 – Part 10 ] Text rendering

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    Rendering text

    In the previous parts, we’ve look at how to render rectangles and images, both with and without transparency. Now it’s time to look at how we can render text.

    Rendering text is tricky. You’ll want to be able to render any font, in any size and preferably every possible character. Luckily, with the SDL_ttf library, this is easy.

    SDL2_ttf

    SDL2_ttf, just like SDL2_image, is an additional library for SDL2. It can use just about every font, and you can set the size and text styling too!

    What’s TTF?

    TTF, or TrueType Fonts is a type of fonts developed by Apple and Microsoft in the late 90’s. True Type Fonts offers a high degree of control on how the font looks. The internals of TTF fonts and how they work isn’t important here. The important part is that they’re easy to use, will look really nice ( even scaled up. ) And they’re also widely used, so finding fonts shouldn’t be a problem.

    SDL2 TTF?

    As with SDL2_image, SDL2_ttf is an additional library for SDL2 that deals with rendering text and makes it very easy. It is based on libfreetype, a library for writing text using TTF fonts. However, it’s not very practical to use. SDL2_TTF makes using it a lot easier. But if you do want to use it yourself, you can take a look at their tutorial.

    Setting up SDL2_TTF

    Setting up SDL2 requires a tiny bit more work than SDL2_image, but don’t be scared, it’s still very easy. First we need to install the ttf library.

    Installation

    Installing SDL2_ttf is done exactly like SDL2_image. Just replace SDL2_image with SDL2_ttf

    Here’s the short version :

    Linux

    For Linux you can use need to install -lSDL2_ttf or -libSDL2_ttf or -SDL2_ttf ( the actual name might be different in different distributions. )

    The linker flag is -lSDL2_ttf

    The process is more or less identical to that of setting up SDL2_image.

    If you can’t find SDL2_ttf in any repositories and it’s not installed by default, you might have to compile it yourself. For more information, see my blog post about setting up SDL2.

    Windows

    Similar to setting up SDL2 base.

    The difference is that you have to download the development files for SDL2_ttf

    And similarly add SDL2_ttf.lib to library includes and add SDL2_ttf.lib to the library flags ( where you previously added SDL2_image.lib )

    And with that, it should work.

    Mac

    See the first part of my tutorial. Just install SDL2_ttf instead of SDL2

    Initialization

    Unlike SDL2_image does need to be initialized. Why? Because libfreetype, the library that SDL2_ttf builds upon needs to be initialized, so naturally SDL_ttf needs to be initialized too.

    Initializing SDL2_ttf requires a single function, TTF_Init() :

    Just like SDL_Init(Uint32 flags) this function returns -1 on error, but unlike SDL_Init(Uint32 flags), this method does not have any flags.

    Sine this function can fail and return -1, we should print an error if this happens. This means our routine for initializing SDL2_ttf will be the similar to SDL2, just with the two functions above :

    TTF_Font

    The basic object for SDL_TTF is TTF_Font. A TTF_Font basically holds information about a font like the font itself and data about styling and size. The exact internals of TTF_Fonts is only known to the library using it so I won’t go into depths about it.

    The only thing you need to remember about TTF_Fonts is that they hold all information about the font that SDL_TTF needs to render it, and that they need to be loaded and unloaded ( we’ll look at this later. )

    Loading fonts

    This is the central structure of SDL2_ttf. It holds the font itself, the size and some other style information ( I’ll go into this in the next part ). So, in order for us to use an TTF_Font we need to load it. This is done using a load function :

    Arguments :

    • const char *file – a pointer to the .ttf file
    • int ptsize – the size of the font

    Return value :

    A pointer to the created TTF_Font<

    The function returns a NULL pointer of it can’t find the file, or there is another error ( like SDL2_ttf isn’t initialized. So this too should be handled by priting the error using SDL_GetError(), just like when initializing ttf

    Cleaning up fonts

    Just like we with SDL_Texture* and SDL_Surface*, we need to clean our fonts when done. This is just as easy for TTF_Fonts as with SDL_Texture* and SDL_Surface*. We simply call a function that does it for us :

    Rendering text

    There are three functions you can use to render text, depending on what you want. Let’s start with the first one :

    TTF_RenderText_Solid

    This function is used for quick and simple rendering of a text, using a specific font and a font color. The background of this is transparent. Here’s the signature:

    Arguments :

    •  TTF_Font *font – the font to use
    • const char *text – the text to render
    • SDL_Color fg –  the color to use for the text

    Return value :

    A SDL_Surface with the rendered text

    The function returns a NULL pointer of it can’t find the file, or there is another error ( like SDL2_ttf isn’t initialized. So this too should be handled by priting the error using SDL_GetError(), just like when initializing ttf

    The result will look something like this :

    TTF_RenderText_Blended

    The next function is very similar to the previous one

    Arguments :

    •  TTF_Font *font – the font to use
    • const char *text – the text to render
    • SDL_Color fg –  the color to use for the text
    • SDL_Color fg –  the color to use for the text

    Return value :

    A SDL_Surface with the rendered text

    The function returns a NULL pointer of it can’t find the file, or there is another error ( like SDL2_ttf isn’t initialized. So this too should be handled by priting the error using SDL_GetError(), just like when initializing ttf

    As you can see, both the arguments and return value is the same for TTF_RenderText_Solid and TTF_RenderText_Blended. So what’s the difference between TTF_RenderText_Solid and TTF_RenderText_Blended? The difference is that TTF_RenderText_Solid is very quick, but TTF_RenderText_Blended produces a better result. In our game, we won’t be updating our text surfaces all that often, and there’s not a lot of them either, so TTF_RenderText_Blended is a good choice.

    Here’s what TTF_RenderText_Blended looks like :

    And here’s a comparison between TTF_RenderText_Solid and TTF_RenderText_Blended :

    The difference is not huge, but in the actual game it will be more clear. And the difference might also vary from font to font.

    TTF_RenderText_Shaded

    This function is a bit different from the two other ones. It will render the texture with a specified background color.

    Arguments :

    •  TTF_Font *font – the font to use
    • const char *text – the text to render
    • SDL_Color fg –  the color to use for the text
    • SDL_Color bg –  the color to use for the background

    Return value :

    A SDL_Surface with the rendered text

    The function returns a NULL pointer of it can’t find the file, or there is another error ( like SDL2_ttf isn’t initialized. So this too should be handled by priting the error using SDL_GetError(), just like when initializing ttf

    So it’s almost the same as the other two, just with a fourth argument for the background color. The return value is also the same as the other two. The difference in the resulting function is that you will get a texture with a background color. The background color ( bg ) will fill the entire rect around the text. The text will be rendered on top of it with the specified foreground color ( fg ).

    The result will look something like this :

    An example

    Below is a simple example that should run and compile out of the box. For compilation details, look below.

    Compilation notes

    Running it is just as simple as with SDL2_image. So that means compilation on Windows is already set up when you installed TTF

    Linux / Mac

    If you are compiling using the compiler, you have to add -lSDL2_ttf to the compile string like so :

    clang++ main.cpp -std=c++11 -o Game -lSDL2 -lSDL2_image -lSDL2_ttf

    If you want to run it, you simply do

    ./Game

    Updated game code

    I have done a bit of cleaning up in the game code. I’ve added a new Texture class for text, cleaned up include, removed ( and added ) comments, improve delta calculation++ Everything should be explained in comments, but, of course, if you have any questions of any kinds, just comment or contact me, I’ll be happy to help.

    You can find the code here.

    Conclusion

    Text rendering can be hard, but SDL2 makes it quite easy. Just load your TTF_Fonts and you can easily get them as a SDL_Surface.

    Feel free to comment if you have anything to say or ask questions if anything is unclear. I always appreciate getting comments.

    You can also email me : olevegard@headerphile.com
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    SDL2

    [ SDL2 – Part 9 ] No more delays

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    Getting rid of SDL_Delay

    Up until now, we’ve been regulating movement by just setting a delay of 16 milliseconds at the end of each frame like so :

    This would work if the frames always lasted less than 1 millisecond, but they don’t. There is absolutely no grantee that the each frame will last as long as the previous one. And when the duration of each frame differs, the animation will become choppy and not smooth like we want it to be. So how can we get smooth animation?

    Method 1 – our current solution

    We’ve set an aim for 60 frames per second. Since there is 1000 millisconds ( ms ) in a second, each frame should take 1000 ms / 60 frames = 16 ms.Since our code is very light, we’ve just assumed that each frame takes less than 1 ms, so we’ve added a 1ms delay.

    Method 2 – using delta

    Another, better way we could limit framerate is to calculate delta time and use that to set the delay. Delta time is the time between each frame ( or update. )

    If a frame takes 3 ms, delta will be 3 ms. In method 1 we had a 16ms delay and ignore the delta time completely. This means there would always be a 16ms delay. If the frame took 3ms, the delay would still be 16ms. This means the entire frame would take 3ms + 16ms = 19ms. If this happened often, the duration of a frame would jump up and down, and the animation would be choppy.

    Using delta time, though, we could simply subtract that from the 16ms to ensure that every frame lasted exactly 16ms. So if a frame lasts 3ms the delay would be 13ms for a total of 16.

    Let’s create a simple example function that takes the delta time and does the delay :

    This is a valid solution, and your game will run relatively smoothly in 60 fps. You can, of course change the framerate as you please simply by 1000ms by the desired framerate to get the delay amount :

    This method will produce smooth animation, but the third method shows an even better method!

    Method 3 – using delta, no delay

    Method 1 and 2 has worked by limiting frame rate. But why limit it? We can just skip the delay altogether and just use the delta time to adjust animation. Most modern games works this way. Since we adjust movement with delta time, it doesn’t matter if we run in 60 fps or 6000 fps. For smooth animation, this method is superior to the ones above. Waiting 16 ms between frame can make objects jump a tiny bit. And having your game run at 200fps will make a difference, even though the human eye sees 24fps as smooth.

    It’s also very simple, just multiply the movement with the delta time.

    Implementation of method 3

    Implementing frame independent movement is quite simple, but there are a few things you need to be aware of in order to get it right.

    Delta time

    Probably the most important point. In order to get animations right, the delta tie needs to be accurate. Very accurate, preferably with microsecond ( µs ), or nanoseconds ( ns )

    There are various ways to do this.

    • The C way
      • – Confusing
        •   Functions takes a struct and fills it with time / date
      • – Not always accurate
      • – Hard to use correctly
      • – Platform dependent
        • No one way to do it in Linux and Winows
      • + No library needed,
      • + Doesn’t need C++11 support
    • Boost
      • – You’d need to download, and link with boost
      • – Adds and a dependency, just for time
      • – Syntax can be hard to understand
      • + Cross-platform
      • + Does everything you need easily
      • + Doesn’t need C++11 support
    • C++11

    In this part, we’ll be using the chrono library. I suggest using it over the old C way. If you can’t compile C++11, you can use the boost version.

    chorno

    chrono is the new timing library in C++11. I covered this in my previous post, and I recommend that you read it before you read on. If you choose not to read it, I will give you the short version of the various part of the chrono library :

    • duration
      • Holds the amount of time between two points in time
      • Can be stored as any time unit ( seconds, millisconds, years, fortnights, .. )
      • Can use both double and int
    • time_point
      • Holds a point in time
      • Subtracting a time_point from another returns a duration
    • Clock
      • Three kinds: steady_clock, system_clock, high_resolution_clock
      • Used to get current time as a time_point

    Getting the delta

    So here’s the code for our simple delta timer :

    Using delta to calculate movements

    Now that the worst part is over, it’s time for some minor details on how to implement frame independent movement.

    First of all, the animated objects needs to have speed. And for better precision, we use doubles. Let’s make a simple struct for holding the speed. Our speed will have values between -1.0 and 1.0.

    We also need an Update( double delta ) functions that’s used to update the position of the object.

    There’s a few things wrong with this, though. First of all, delta values will be really small, and our speed is between-1.0, and 1.0 so the resulting movement will be really small. We can solve this by multiplying with a constant.

    But there is a second issue here : both speed and delta are likely to do decimal numbers. So the result of the multiplication will almost always be a decimal number as well. But we just put it into an int and this means we’ll loose the decimal precision. So if the result is 0.8, we’ll end up adding 0. And if the same things happen in the next frame, we’ve “lost” 1.6 worth of movement. A whole pixel! It might seem like a small thing, but it would make the movement really choppy. So instead we introduce two new member variables to our Texture struct to hold the result of the multiplication.

    Final function

    Here’s our final Update function:

    In Texture :

    Calling it :

    Conclusion

    That concludes the tutorial on delta timers. I’ve made a few updates, put things into different classes and other minor improvements. The full code is too big to include in this blog post, but you can find a full zip of it here.

    I’ve added a new .cpp file, so you need to add that to the compilation string if you’re using clang or gcc

    clang++ main.cpp Texture.cpp -std=c++11 -lSDL2 -lSDL2_image -o Game

    Feel free to comment if you have anything to say or ask questions if anything is unclear. I always appreciate getting comments.

    You can also email me : olevegard@headerphile.com
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    C++11

    [ C++11 – Part 2 ] Timing

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    std::chrono

    Today, we’ll learn how to use the chrono library. It’s used for timing functionality. It supports a variety of different operations and can be quite daunting in the beginning. The library contains several new types, but I’ll try to explain all of them shortly. But first a few practical tips that will make using std::chrono easier.

    using std::chrono

    For working with chrono I suggest writing using namespace std::chrono somewhere your code ( preferably at the top ). This means you don’t have to write std::chron:: in front of everything.

    Is easier to read than :

    Usually, using namespace statements are discouraged to avoid name collisions, but since we are writing std::chrono we could only get name collisions with items in the std::chrono library, which is less likely to happens.

    Includes

    To make sure everything compiles as it should, please add the headers and using to the top of your file :

    About this part

    I also want to point out that this guide is rather comprehensive. This is because it’s a good idea to have deep understanding how the various components of the chrono library works. Just writing this guide helped me to understand a lot of it. My hope is that it will help you too.

    If you want more information, you can look at the en.cppreference documentation for chrono. This resource covers most, if not all of the features in C++11 with detailed information and good examples. It’s very useful for learning and as a reference.

    Overview

    Here is a quick overview of the chrono library.

    I will now go over each of these components, starting with the basic one, ratio

    ratio

    The ratio is actually not a direct part of chrono but it’s very useful for working with chrono and just useful in general. A ratio is just that, a ratio or a fraction if you will. For example the ratio between a minute and a second is 60 / 1 since you need 60 seconds to make a minute. ration is defined in it’s own header file, simply called ratio

    Definition

    The class is templated. The definition looks like the following :

    Even though this is a templated class, which can be complicated and hard to understand, the template arguments are quite straight forwards.

    Template arguments :

    • Num  – the numerator, or simply the top part of a fraction
    • Denom  -is the denomerator or bottom part of fraction

    These two numbers forms a fractions :

    Num / Denom

    Usually these numbers are dividable by 10, so you end up with numbers like 1000 or 0.001

    Both variables are of the type std::intmax_t, which means they are the biggest possible type of signed int available, usually at least 64 bits. For example int64_t.

    Predefined ratios

    In the chrono library ticks are stored in fractions of second. A few examples :

    • A second : ratio < 1 /1 >
      • A second is 1 / 1 of a second.
      • Can also be written as std::ratio< 1 > since the denomerator defaults to 1
    • A millisecond : ratio < 1 /1000 >
      • A millisecond is 1 / 1000 of a second
    • A minute : ratio< 60 /1 >
      • A minute is 60 / 1 seconds.
      • Can also be written as ratio< 60 > since the denomerator defaults to 1

    Resource on ratios

    As a part of the ratio header, there’s a lot of predefined ratios ( milli, nano, kilo, mega, … ). Because ratio is not directly a part of the chrono library ( it has other uses than just chrono ), there is no predefined values for minutes, hours, etc. There’s just the ones based on the standard units that’s used for distance or weight. You can refer to either of these using the std:: prefix. Since the ratios are not part of the chrono library, it’s just std:: and not std::chrono::.

    For a full list of the predefined values, take a look at the documentation.

    duration

    duration( or std::chrono::duration ) is one of the basic classes of the chrono library. As the name suggests, it stores a time duration. Or a distance between to points in time if you like.

    Member variables

    It’s quite simple in its structure, it only has two member variables :

    • rep
      • an arithmetic type ( basically any unit that can hold a number ) that stores the number of ticks ( number of seconds, minutes, years, etc.. ).
    • period
      • a ration containing the type of unit rep stores. In effects this means “how many seconds/minutes/years,… is one tick.”

    Constructing a duration

    The creation of a duration is a bit complex as it involves two template arguments that sets representation type and a ratio plus the constructor.

    teamplate arguments

    The actual templated part looks like this :

    Template parameters :

    • rep – the type to hold the ticks
      • Typically int or double
    • ratio – the type units used by duration
      • Covered above

    The ratio defaults to 1 / 1, meaning seconds. The number of ticks the duration should use are sent to the constructor in the ordinary way using the parenthesis after the variable name.

    To create a duration for containing seconds, you can do :

    This creates a duration for holding seconds.

    Other ratios

    To create different durations we can use the ratio like above.  For example 10 millisecond becomes :

    We can simplify this by using the predefined ratios already in C++11 :

    Okay, but what about 10 minutes? This is quite similar to the above :

    We do std::ratio< 60, 1 > because a minute is 60 / 1 seconds which or simply 60 seconds. But we can’t simply this like above because there is not predefined ratio for minutes. The predefined ratios are all “clean” ratios like 1 / 100, 1000 / 1, 1 / 10, 10 / 1, etc…

    Predefined durations

    But there is another, even simpler way! Just like in ratio there are predefined values in duration And they’re very simple to use.

    So say you wanted a duration of 10 milliseconds :

    Or 10 minues :

    Constructor

    The duration has three major constructors

    • Default, empty constructor
      • Looks something like this : duration( )
      • Initializes the duration
    • Constructor with rep variable
      • Looks something like this : duration( rep ) where red can be int, doube or similar
      • Sets the internal tick variable to the variable passed in constructor
      • This is the one we’ve used up until now
    • Construct for other duration
      • Looks something like this : duration( duration )
      • Sets the internal tick variable based on the passed duration
      • The = operator works in the same way
    From rep type
    Takes a value ( rep ) as argument and sets the ticks of the duration to it.

    From other duration type
    Takes a duration as argument and sets the ticks of the duration to it. Will convert between the two types of duration based on the ration. This means that if you pass a minutes duration to a seconds duration, you will get the number of minutes * 60 ( because there are 60 seconds in a minute.

    Example

    This creates a 10 second duration

    Duration functions

    Duration has a few functions, but I’ll just cover two of them now

    • count
      • Returns the number of ticks
    • duration_cast 
      • Converts a duration of duration into another.

    count

    This function simply returns the number of ticks. So for our 10 millisecond example, this will be 10.

    Code example :

    Output :

    duration_cast

    When we’re working with two different types of durations things can get a bit weird. A simple example : what’s 3 minutes minus 36 seconds. The answer is quite simple for a human to figure out. But the results in your program needs to be a duration but which unit? Since 3:24 can’t be expressed exactly in minutes, we need to change either the 3 minutes to seconds, or the 36 seconds to minutes. Here’s where duration_cast<> comes in.

    This simply casts our 3 minutes into 180 seconds. After doing that, we can simply do :

    And we’ll get the result in seconds ( 144. ) If you had converted this to minutes, it would take the whole minutes and strip the seconds. Meaning you would end up with 2

    Floating points

    Up until now, we’ve only used integer values for representing ticks. This means we can only represent whole tick. So a duration of seconds using an int to represent ticks means it can only represent whole seconds. Consider the following code :

    This will fail ( won’t compile ) because you loose precision. You could use a duration_cast to fix this, but you’ll loose the precision and end up with 0. duration_cast is way of telling the compiler “yeah, I know this will loose precision, just do it!”

    So instead we can create a duration of seconds represented as a double :

    Output :

    Count : 0.016

    This is not the same as the predefined duration seconds because all the predefined durations uses int.

    Unlike the previous example, this won’t fail because you don’t loose precision. In fact, you could change the ratio to store years :

    This will convert the 16 ms to years. If you did yearsAsDouble.count()
    you would get roughly 5.07356672 × 10^-9 ( 0.00000000507356672 ) years.

    Clocks

    Clocks are just objects that is used for finding the current time. They are static, which means that there will only be one of each clock type. There is also just one member function ( also static member function, now() . The function now() returns the current time as time_point. I will cover time_points later.

    Clock also have a tick rate which state how accurate they are. For instance : a clock has a tick rate of seconds, it can’t be used to measure milliseconds. The tick rates are represented as ratios.

    Clock types

    There are three available clocks in the chrono library.

    • system_clock
      • Full name : std::chrono::system_clock
      • The wall clock, use this if you just want to know the current time and date
      • This clock may be adjusted by either daylight saving time ( DST ) or leap seconds.
      • Can be mapped to C-style points and can therefore be easily printed.
    •  steady_clock
      • Full name : std::chrono::steady_clock
      • This clock is monotonic, this means will never be adjusted, so it’ll never be affected by things like DST and leap seconds
      • Best suited for measuring intervals
    •  high_resolution_clock
      • Full name : std::chrono::high_resolution_clock
      • Shortest tick available ( will be updated most often )
      • Might be alias of std::chrono::system_clock or std::chrono::steady_clock, or a third, independent clock.
        • This means it’s not guaranteed to be monotonic like steady_clock
      • Use this for benchmarking

    That’s really all you need to know about clocks, but in the next part we’ll be exploring them more.

    time_point

    time_point<( or std::chrono::time_point) is the central class of the chrono library. I’ll spare the implementation details for now and just say that it contains various information about the current point it in time.

    Construction

    The constructor for a time point looks something like this :

    As you can see, its a templated class that takes both a clock, and a duration. The duration is the duration the time_point uses. This defaults to the duration of the clock ( as you can see from the typename Clock::duration part. )

    Simplified constructor

    Luckily though don’t have to specify either of the arguments. So a really simple way to construct a time_point is :

    You can use stady_clock or system_clock instead of high_resolution_clock.

    This is equal to writing :

    A bit more complex, but it means the exact same time : a time_point that uses a high_resolution_clock and has the same duration as the high_resolution_clock. I see no reason to use this method as opposed to the simpler one, but I want to explain the template arguments as well as I can.

    Using custom ratio

    But what if we wanted to specify our own duration? Well then we have to set the second template argument as well. Say we wanted to specify milliseconds as our duration time unit. Remember that duration has predefined constants for this.

    This means we can simply do :

    Full constructor

    Now let’s do something even worse. Say we wanted a time_point that uses half minutes ( 30 seconds ) as the time units. I have no idea why anyone would want this, but it’s just a ( somewhat contrived ) example. As with some other examples, I do not encourage writing code like this. It’s horribly hard to read and very hard to understand unless you know the different part of the chrono libraries. Okay, here we go :

    Yup, that’s a template argument, inside a template argument, inside a template argument! Don’t write code like this! If you need a specific time ratio, at least put the ratio and duration in separate objects ( I’ve shown you how to use the constructors above. ) Also make sure they’re well named like ratio30sec and duration30sec. That should give the reader at least a small hope at knowing what’s going on.

    I added this example because if you understand this constructor and how it works, you understand the major parts of std::chrono

    Initialization

    As stated above, the static member function of clocks, now() returns a time_point of the current point in time.

    That means we can just use auto to create a time_point too!

    Simple as that, we’ve created a time point. The type of this object will be exactly the same as

    Only it’s even simpler, and we’ve initialized it to the current point in time at the same time!

    Difference between two time_points

    And finally we can do some actual timing! Time points supports arithmetic operations. This means you can say time_point_x - time_point_y. This will result in a duration But what is the kind of duration. Seconds? Milliseconds? Years? Actually there is no one answer, it’s implementation defined. But the result is almost certain to be less than one second.

    Since we can’t know what the resulting value is, we have two options. Say we have the following code :

    And wanted to know the result of t1 - t2 Our two options are :

    1. Use auto
        • Nice and simple
        • But we can’t control the type of result.
    2. Use duration_cast
        • Takes the result of t2 - t1 and casts it into a duration of milliseconds and uses it to set result
        • This is a bit harder, but now we have full control over the type of duration.
        • If you want higher precision, you can use microseconds or nanoseconds
        • If you want higher precision, you can use microseconds or nanoseconds

    Delta timer

    So now let’s use all this knowledge and make a delta timer. A delta timer is used in games to tell the difference between two frames. The code is relatively simple and will show one ( of many ) ways to use the chrono library.

    Notes

    1. Get the current time
      • We need this to calculate the difference in the next step
    2. Take the dif and store it in a duration
      • Stored in seconds ( since it’s the default )
      • Represented as double
    3. Reset time_prev
      • We need this point because we always need to know the time_point of the previous frame
    4. Return the results
      • Finally we return the time since last frame as fractions of a second
      • Can be a very small value like 0.000000001 ( 1ns )

    A note about time units

    This section just describes my reasoning for using seconds stored as double. It’s very focused around game programming and not crucial to understanding chrono.

    I choose to use seconds as double here because double allows higher accuracy than int. And even though the game will almost certainly use less than 1 second on one frame, using seconds over milliseconds as time units means that the number will always be on the form 0.x. It also ensure that the delta won’t be exactly 0.

    If the delta is 0 on one frame, no movement would happen ( I will get into this in the next SDL2 part ). If it only happens in one frame, the effect might not be that bad. But if it happens 100 frames in a row, that would be 100 frames without movements. And that would be bad!

    So, in my opinion, it’s better to always have a delta of 0.x as it ensure that there will always be at least a tiny fraction of movement. The fact that the delta is a small number can be countered by multiplying with a great number in the function that calculates movement ( we’ll see this in the next SDL2 part too! )

    Conclusion

    The chrono library is very useful and makes it easy to deal with timing. It enables high precision timing, and makes it easy to find difference between two points in time. There are also other libraries that uses chrono like the threading library. I might write about the threading library later. There is also a post about timing in games coming up soon ( as mentioned in the above post. )

    Feel free to comment if you have anything to say or ask questions if anything is unclear. I always appreciate getting comments.

    You can also email me : olevegard@headerphile.com
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    SDL2

    [ SDL2 – Part 8 ] Image rotation

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    Introduction

    In the last tutorial we learned how to load and render png files with alpha layers. This time we’re gonna dive into rotating textures.  Or more precisely ; rendering them rotated.

     

    Today we’ll be making a simple, but working, analog clock. This will show how to rotate textures in SDL2. We’ll also improve how we represent textures

    SDL_RenderCopyEx

    Until now, we have been rendering using SDL_RencerCopy. But in order to rotate, we need a different function ;  int SDL_RenderCopyEx it takes a couple more arguments. I’ll try to explain all of them.

    Parameters :

    • renderer – the SDL_Renderer we always use for rendering
    • texture  – the SDL_Texture we want to render
    • srcrect  – which part of the SDL_Texture to render. null for everything.
    • dstrect  – where to render it. Used exactly like in SDL_RenderFillRect)
    • angle     – angle of rotation
    • center    – the center of rotation
    • flip      – flipping of texture ( vertical, horizontal, no flip )

    Return value

    • 0 on success

    As you can see, the first four parameters are identical to SDL_RenderCopy you can read more about them in part 6.

    Texture flip

    Texture flip is represented as an SDL_TextureFlip

    It is simply an enum with the following three values :

    • SDL_FLIP_NONE         – don’t flip at all
    • SDL_FLIP_HORIZONTAL   – flip horizontaly
    • SDL_FLIP_VERTICAL     – flip vertically

    For now, we will just be using SDL_FLIP_NONE.

    Center of rotation

    The center of rotation is given as a position seen from the top-left of the texture ( remember; in SDL 0,0 is the top-left of a texture. ) If the center of rotation is 0,0 you rotate it from the top-left corner.

    Finding the center of rotation is usually quite simple, it’ll usually just be the center of the texture you are using. For a circle, like a spinning wheel, the center of rotation will simply be the center of the wheel. Or for a Tetris piece it will be the center of the Tetros piece. The center of a texture is easy to find. Since a center of any rectangle will be halfway along its x axis and halfway along its y axis, the position can be calculate as follows ;

    The type of center is one we haven’t used before, SDL_Point. But an SDL_Point is simply just a struct with an x and y value.

    In our case we are going to make a working analog wall clock. So we can’t rotate it around the middle ( that would be a weird clock! ) What we need to do, is find the base of the hands ( where they connect to the center of the clock )


    Here is a picture of on our clock hands. The white hole in the middle is where we want the center of rotation to be. All three hands look very similar to this, all hands have a hole in the base, and all of them rotate around that point.

    If you look closely at the green dots, you’ll see that the distance to the hole from either of the sides and the bottom is all the same ( three green dots. ) The green dots span the entire width of the base. So to find the center x value, we simply take half of the width. Looking at the picture, you’ll see that the distance from the bottom to the hole is the same as the distance from the sides subtract half of the width to find the y position of the hole.

    So the code for finding the center of rotation will be as follows:

    Putting things together

    Previously we have worked with simply storing the SDL_Texture and SDL_Rect as separate objects in our main.cpp. But now we also need to store the center of rotation and the angle. So that’s four variables for each texture. With so many variables, it’s a good idea to split it into a separate struct :

    We also need to change main.cpp to make the clock work :

    As you can see, there is very little code needed in main since a lot of it is moved into Texture.h

    Conclusion

    The SDL_RenderCopyEx function can be very useful and is a very simple way of rotating textures. An by wrapping the SDL_Texture in a separate struct we can make the remaining code smaller and simpler while still keeping all the functionality we need.

    The full code can be downloaded here.

    Feel free to comment if you have anything to say or ask questions if anything is unclear. I always appreciate getting comments.

    You can also email me : olevegard@headerphile.com
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    SDL2

    [ SDL 2 – Part 7 ] Loading PNG files

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    Using PNG

    In the previous post we learned how to load .bmp files. Which is not optimal. The .bmp format means the files grow large and there is no alpha layer ( no transparency. ) Which means you end up with ugly single-colored edges instead of transparency

    Original with .bmp images

    SDL2_image

    Up until now, we’ve only been using the standard SDL2.0. But the standard SDL2.0 doesn’t support loading of .png files. But fortunately, there is a second SDL library that does this. And since it’s still SDL, it will work perfectly with or other SDL code!

    Installing is almost exactly the same as installing SDL2 itself; you just follow the instructions in part 1 of my tutorial. But instead of installing SDL2, you install SDL2_image.

    New version with png files.

    Note: I apologize for vague and not very thorough descriptions in the below points. I will update them when I have access to Windows and Mac. I also apologize for the horrible background and sprites.

    Linux

    For Linux you can use need to install -lSDL2_image– or -libSDL2_image– ( the actual name might be different in different distributions. )

    The linker flag is -lSDL2_image

    The process is more or less identical to that of setting up SDL2 base. For more information, see my blog post about setting up SDL2.

    Windows

    Similar to setting up SDL2 base.

    The difference is that you have to download the development files for SDL2_image

    And similarly add SDL2_image.lib to library includes and add SDL2_image.lib to the library flags ( where you previusly added SDL2.lb )

    And with that, it should work.

    Mac

    See the first part of my tutorial. Just install SDL2_image instead of SDL2

    Again; I apologize for my poor description on this. I don’t run Mac and have no experience with it. I hope my short descriptions is of any hope, though.

    Loading PNGs using SDL2_image

    In order to use SDL2_image, you need to add the header file, like so :

    #include <SDL2/SDL_image.h>

    The actual loading of .png files is just as easy as loading .bmp files. But since it uses an extension library, we have to use a different function :

    This function will load a file and return it as a SDL_Texture*. This means we don’t have to convert it, so we can just do:

    Note that this function, unlike the one for loading bmps, needs a pointer to the SDL_Renderer this is because it returns a SDL_Texture. As we saw in the previous post, SDL_Texture are hardware optimized, and SDL2 needs the SDL_Renderer to do the actual hardware optimization.

    Running it

    Just like last time, I’m not gonna show the entire code as one long gist ( too much code, and there are images too. )

    Here is a link to the code in .zip form

    Linux / Max

    If you are compiling using the compiler, you have to add -lSDL2_image to the compile string like so :

    clang++ main.cpp -std=c++11 -o Game -lSDL2 -lSDL2_image

    If you want to run it, you simply do

    ./Game

    Feel free to comment if you have anything to say or ask questions if anything is unclear. I always appreciate getting comments.

    You can also email me : olevegard@headerphile.com
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    SDL2

    [ SDL2 – Part 6 ] Using SDL_Textures

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    Loading textures

    In this tutorial you will learn how to load images ( just .bmp for now, the next part will cover .png ). First of all, a little info about the two new structs we’ll be using

    SDL_Surface

    This is part of the old SDL ( before 2.0. ) It was used for just about everything related to rendering.

    SDL_Surface is used for holding the raw texture data. Basically it says what the color for every pixels is. Sort of like this :

    • pixel[ 0, 0 ] = 128, 64, 255, 255 ( 128 red, 64 green, 255 blue, 255 alpha )
    • pixel[ 1, 0 ] = 255, 32, 128, 128 ( 255 red, 32 green, 128 blue, 128 alpha )
    • ..etc

    ( This might not be 100% true for all cases, but it’s generally how SDL_Surface stores the pixels. )

    The SDL_Surface structure also holds other data about the texture :

    • width/height ( these are read-only )
    • the pixel format, stores as an SDL_PixelFormat
    • clip_rec, a SDL_Rect used for bliting ( similar to rendering )

    These values are useful to know about if you want to create your own SDL_Surfaces using code or want to use the old form of rendering ( bliting ). I do, however, not recommend learning about it. Bliting is less intuitive and it uses only software rendering. This basically means that the CPU will be doing the rendering, something CPUs are really bad at. We want optimized hardware rendering on the GPU. And that’s where the second struct comes in.

    SDL_Texture

    Like SDL_Surface, SDL_Texture holds texture data. But unlike SDL_Surface, SDL_Texture holds an optimized, driver specific representation of the pixel data. This means rendering will be executed on the GPU, where it belongs. And THAT means it is really, really fast!

    So what’s the point of SDL_Surfaces?

    Well, as I said, the pixel data in SDL_Texture is optimized. And what’s more, it can be very different depending on your driver ( nVidia vs ATI. ) Basically this means that you can’t use the pixel data for anything other than rendering. A second reason will come up when we dive into another new part of SDL, namely image loading.

    Loading images

    Loading a BMP image in SDL is very easy. It can be done using the following function

    Parameters

    • file – the file we want to load

    Return value

    The loaded image as a SDL_Surface. null if failed

    Converting SDL_Surface to SDL_Texture

    We’re not gonna use the SDL_Surface*, we want a SDL_Texture* instead. To get it, we need to use a second function :

    Parameters

    • renderer – our SDL_Renderer
    • surface – the surface we want to make a SDL_Texture from

    Return value

    The resulting SDL_Texture. null if failed. 

    Since we won’t be using the SDL_Surface, we can delete it and free the memory :

    Parameters

    • surface – the SDL_Surface to delete

    Now that all of this is in place, we can wrap it all up in a nice little function :

    Getting the images on screen

    Now that we have loaded the textures, we need to render then on screen. That means no more SDL_RenderFillRect() and THAT means no more rectangles and squares. Finally! To render them, we just need a simple function :

    Parameters

    • renderer – the SDL_Renderer we always use for rendering
    • texture – the SDL_Texture we want to render
    • srcrect – which part of the SDL_Texture to render. null for everything.
    • dstrect – where to render it. Used exactly like in SDL_RenderFillRect)

    Return value

    0 on success

    Even though this function has a few parameters, it’s really quite simple. We can use nullptr for srcrect and the SDL_Rects we used for SDL_RenderFillRect() as dstrect. And then we can remove the SDL_SetRenderDrawColor and end up with a shorter Render() function.

    Putting it all together

    I’m not gonna put the entire code on the blog anymore since this creates a whole wall of text to scroll through. And now there are images you’ll need too. But you can download the images and the source code here.

    Feel free to comment if you have anything to say or ask questions if anything is unclear. I always appreciate getting comments.

    You can also email me : olevegard@headerphile.com
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    SDL2

    [ SDL2 – Part 5 ] Collision detection

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    Collision detection

    Collision detection is one of the key aspects of game programming. For all non-rectangular objects it gets harder and harder the more accurate you want it to be. But today we’re gonna continue just using rectangles. This part describes a simple algorithm to check for intersections.

    After we’ve created a functions that does rectangle-rectangle collision detections, we’ll use all of what we’ve learned so far and make a simple game!

    Rectangle – rectangle collisions

    Instead of checking if two rectangles are inside or touching each other, we’re gonna check for the opposite; if they are outside of each other. The algorithm to do so consists of two steps.

    1. Find the x coordinate of the left and right and the y coordinate for top and bottom.

      Say we have two rectangles, rect1, and rect2.

      Finding the x/y of left/right/top/bottom is very easy.

      The process is exactly the same for the second SDL_Rect.

    2. Use the 8 edges to check if they are not colliding

      Take a look at the drawing below

      As you can see, the red rectangle is farther to the right than the blue one. But how do can we easily check that? Simply by checking if redLeft is further to the right than blueRight. If it is ( like it is in our case ) there is no collision. Then it’s just a simple matter of repeating it for the other edges. So we end up with something like this :

      This is the final function for you to copy and test out.

      If you want to shorten the code, you can remove the comments and replace the variables with the rect. x / y / w / h values like so :

      I chose to not do this as the longer version is a bit easier to read and understand.

    Our first game!

    Our first game is of the “lead the chicken safely across the road” kind of things. Or, as in our case, “lead the square from the rectangle, past all the other squares and to the other rectangle.” You control the square with the arrow keys. If you hit a red square, you’ll end up where you started. If you make it to the other side, you’ll also end up where you started, but at least you have that tiny sense of victory for helping our poor little square!

    The code just uses the different things we’ve learned so far, so I won’t explain it other than in code comments and ( hopefully ) descriptive names.

    So without further ado, here’s the code :

    Feel free to comment if you have anything to say or ask questions if anything is unclear. I always appreciate getting comments.

    You can also email me : olevegard@headerphile.com
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