为什么不允许这个static_cast？

我有一个A类的对象,我想在自定义堆栈对象上分配.为此,我只需将堆栈指针移动到对象大小的字节数并返回其先前的值:

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

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

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

        _top += sizeof(T);

        return ptr;
    }

    //...

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

stack _stack;

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

    return 0;
}