C++ Array of pointers to class names

Question

I'd like to be able to hold an array of class names and simply pass an index to a function which would create an instance of a certain class. I have memory constraints so I'd like to not simply create an array of objects.

Here's my most descriptive pseudo-code:

Classes:

class Shape
{
public:
    Shape();
};

class Square : public Shape
{
public:
    Square();
};

class Triangle : public Shape
{
public:
    Triangle();
};

Main:

int main()
{
    pointerToShapeClass arrayOfShapes[2];       // incorrect syntax - but
    arrayOfShapes[0] = pointerToSquareClass;    // how would i do this?
    arrayOfShapes[1] = pointerToTriangleClass;

    cin << myNumber;

    // Depending on input, create instance of class corresponding to array index
    if (myNumber == 0) { // create Square   instance }
    if (myNumber == 1) { // create Triangle instance }

    return 0;
}

If this is confusing, I can try to elucidate. Thanks in advance!

Edit:

Really, I think I just need a pointer to a class without actually instantiating the class.

Answer 1

Sounds like you want some kind of factory:

class Shape
{
public:
    Shape();
    static Shape* create(int id);
};

Shape* Shape::create(int id) {
  switch (id) {
    case 0: return new Square();
    case 1: return new Triangle();
  }
  return NULL;
}

Then when you want to create a particular Shape given user input, you can do:

int myNumber;
cin >> myNumber;
Shape* shape = Shape::create(myNumber);

This is it in it's simplest form. I would, however, recommend having the create function return a std::unique_ptr<Shape>, rather than a raw pointer. I would also set up static constants to represent the different IDs.

class Shape
{
public:
    Shape();
    static std::unique_ptr<Shape> create(int id);

    enum class Id { Square = 0, Triangle = 1 };
};

std::unique_ptr<Shape> Shape::create(Shape::Id id) {
  switch (id) {
    case Shape::Id::Square: return new Square();
    case Shape::Id::Triangle: return new Triangle();
  }
  return nullptr;
}