以隐式方式在不同类型之间进行类型转换

Question

I need an in implicit conversion from A* to C*; i cannot change A's definition or implementation.

class A
{
};

struct B: public A
{
};

struct C: public B
{

};

when i write the following:

A* p;
C* q = p;

i am getting an error C2440; cannot convert from A* to C*. what can i do giving the fact i cannot change A. both classes are plain structs of primitive data.

Answer 1

The only way you can do this is to use a cast:

// a C++ style static_cast:
C* q = static_cast<C*>(p);

// or the less verbose C-style cast
C* q = (C*)p;

Because C is a derivative of A and not every A is a C , it cannot be implicitly casted (like you could implicitly cast a C* to an A* because every C is an A (ie A has "less or equal features" than C , but not more)).

I doubt that you really must have an implicit cast between pointer types. What is it making you think you do?