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Chapter 53: C++ 1

C++ is in some ways a combination of Java and C. It has syntax and a lot of other features borrowed from C, but also supports object-oriented programming similar to Java. One interesting aspect of C++ programming is that it is (typically, for most use cases and with a few small syntax changes) backwards compatibile with C. You can, for all intents and purposes, write a program in C and use a C++ compiler to compile it. OpenGL is also a C API, so we could write OpenGL programs using only C. However, C++ adds a lot of features that are useful for computer graphics applications. And two of the libraries we will be using, glfw and glm, are C++ only.

Also, C++ is very commonly used in the graphics industry. So we’re going to spend some time learning and using it this quarter!

Structs

C++ objects are similar to C structs, but they allow you to define both data and behavior, unlike C structs which only support data definitions. The behavior of a C++ object takes the form of member functions, which should be familiar to you as a Java programmer.

Take a look at Vector.h (sample files listed at the bottom of notes) for an example of a C++ class.

First let’s look at our constructors and destructors:

Vector();                 // default constructor
Vector(Vector *);         // constructor
Vector(const Vector &);   // copy constructor
~Vector();                // destructor

Constructors are special member functions used to create instances of our class. We can have multiple constructors if there are multiple ways to create instances. In the case of our Vector class, we declare a default constructor and two other constructors to give three ways to make a Vector. You might be confused about the use of the & symbol in our third constructor, but we’ll talk about this in a second - it’s a C++ concept called a reference.

Finally, we have a destructor. This function is responsible for destroying the class when it’s time for it to be deallocated.

Initializer Lists

Constructors can have initializer lists to initialize member variables.

Vector(float x, float y)
 : X(x), Y(y)
{}

Note that there is a “new” concept in C++, std::initializer_list that has the same name but is a different thing.

References

void swap(int *a, int *b)
{
    int temp = *a;
    *a = *b;
    *b = temp;
}

// elsewhere

int x, y;
swap(&x, &y);

void swap(int &a, int &b)
{
    int temp = a;
    a = b;
    b = temp;
}

// elsewhere

int x, y;
swap(x, y);

References are a lot like pointers except:

  1. You don’t need a * or a -> to use them.
  2. You can’t change what they point to - once a reference is created, it always points to the same thing.
  3. You’re not allowed (supposed) to point a reference to null.

Inheritance

Check out GeomObject.h.

Syntax for declaring inheritance:

class GeomObject
{
    // ...
};

class Sphere : public GeomObject
{
    // ...
};

Memory Allocation

If we want to create instances of our classes, there are two ways to do it. We can either create them on the stack (limited to current scope) or allocate them on the heap.

// Stack allocation
{
    Vector v1;
    Vector v2 = v1;
    v1.p_x = 3.0;
} // v1 and v2 go "out of scope" here and are deallocated

Stack allocation is refered to as automatic storage.

// Heap allocation
{
    Sphere *sphere = new Sphere();
    Sphere *other = new Sphere();

    scene->AddGeomObject(sphere);

    delete other; // 'other' is deallocated now
} // The pointers 'sphere' and 'other' go out of scope, but what they point to is not deallocated.
// The Sphere instance we created on the first line still exists, and 'scene' can still use it

Heap/dynamic allocation is referred to as dynamic storage.

Here, new and delete replace the C usage of malloc and free. There should be no mallocs or frees anywhere in your code!

Note that to allocate arrays of something (instead of just a single copy), there are special commands:

int * my_int_array = new int[10];
my_int_array[4] = 2;
delete[] my_int_array;

Example Files

References

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