Making a vector of instances of different subclasses

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Yes, but you’ll need to use either pointers or smart pointers (I’d go with this).

struct X
{
    virtual ~X() {}  //<--- as pointed out in the comments
                     // a virtual destructor is required
                     // for correct deletion
    virtual void foo() = 0;
};
struct Y : X
{
    virtual void foo() { }
};

int main()
{
    std::vector<X*> a;
    a.push_back(new Y);
    a[0]->foo();
    for ( int i = 0 ; i < a.size() ; i++ )
        delete a[i];
    return 0;
}

Don’t forget to delete the allocated memory.

Why you can’t use actual objects:

Assume std::vector<X>. This is illegal because:

  1. If you want to initialize your vector with some number of elements, the allocation would fail. A vector stores objects internally in continuous memory. A preallocation would fail because it would mean it needed to create objects, which can’t be done for abstract classes.

  2. Even if you could, or the base class wasn’t abstract, it wouldn’t help too much, as you’d suffer from object slicing.

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