Why is this static_cast not allowed?

Question

I have an object of class A that I want to allocate on a custom stack object. To do this, I simply move the stack pointer as many bytes as the object is in size and return its previous value:

class A : public B {}; //B is from a precompiled library

class stack {
public:
    stack(void): _top(&_storage[0]) {}

    template <typename T>
    inline T* push(void) {
        T* ptr = static_cast<T*>(_top);

        _top += sizeof(T);

        return ptr;
    }

    //...

private:
    char _storage[1024];
    char* _top;
};

stack _stack;

int main(int argc, char** argv) {
    A* a = _stack.push<A>(); //ignore the lack of a constructor call

    return 0;
}

Visual C++ simply tells me that static_cast cannot convert from char* to A*. A regular C style cast does not give me this error, but I'd rather be more explicit and avoid a dynamic cast (A inherits from another class, but does not contribute to the vtable it doesn't have). Is there any difference between the two in this case?

Answer 1

As others have said, the solution is to use reinterpret_cast, which is meant to be used when casting between unrelated pointer types:

T* ptr = reinterpret_cast<T*>(_top);

If you use placement new instead, you not only avoid the casting problem but you also fix the problem of not calling the constructor for types that have a constructor:

T* ptr = new(_top) T();