多态性与指向数组的指针

问题描述

I have a class A:

class A
{
    public:
        virtual double getValue() = 0;
}

And a class B:

class B : public A
{
    public:
        virtual double getValue() { return 0.0; }
}

And then in main() I do:

A * var;
var = new B[100];
std::cout << var[0].getValue(); //This works fine
std::cout << var[1].getValue(); //This, or any other index besides 0, causes the program to quit

If instead I do:

B * var;
var = new B[100];
std::cout << var[0].getValue(); //This works fine
std::cout << var[1].getValue(); //Everything else works fine too

Everything compiles fine, but it seems as though there is something wrong with my polymorphism perhaps? I'm puzzled.

You can't treat arrays polymorphically, so while new B[100] creates an array of B objects and returns a pointer to the array - or equivalently the first element of the array - and while it is valid to assign this pointer to a pointer to a base class, it is not valid to treat this as a pointer into an array of A objects.

The principal reason that you can't is that (typically) derived objects are a different size to their base classes, so attempting to access the array as an array of base class objects will not use the correct offset to get a pointer to the next base class subobject of the next member of the derived class array.

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