In a diamond inheritance structure, is there a way to cast between the branches?

Question

I have a diamond inheritance structure in my code in which I have a pointer to the bottom object. I tried to case this to a pointer to the left of the two diamond sides, cast it again to the top of the diamond, and again to the right side. But apparently, C++ kind of remembers the order of casting and things don't work as expected. Example code:

#include <iostream>

class A
{
};

class B1 : public A
{
public:
    virtual int Return1() = 0;
};

class B2 : public A
{
public:
    virtual int Return2() = 0;
};

class C : public B1, public B2
{
public:
    virtual int Return1() { return 1; }
    virtual int Return2() { return 2; }
};

int main()
{
    C c;
    B1* b1 = &c;
    A* a = b1;
    B2* b2 = (B2*)a;
    std::cout << "Return2() = " << b2->Return2();
}

This results in Return2() = 1, so apparently this approach is wrong. I know that something like this works in C#, so my question would be: Is there a way in C++ to do what I'm attempting here or - if not - why is this not an option?

Answer 1

• As inheritance is not virtual (for A), you have "Y" inheritance (2 A),

A     A
|     |
B1    B2
 \   /
   C

not a diamond (1 A).

• Avoid C-cast which might result in reinterpret_cast, and most reinterpret_cast usage leads to Undefined Behavior (UB).

• You might use dynamic_cast in your case to have expected behavior (A need to be polymorphic for that, default virtual destructor does the job):

class A
{
public:
    virtual ~A() = default; // Added to allow dynamic_cast
};

class B1 : public A
{
public:
    virtual int Return1() = 0;
};

class B2 : public A
{
public:
    virtual int Return2() = 0;
};

class C : public B1, public B2
{
public:
    // override used for extra check from compiler.
    int Return1() override { return 1; }
    int Return2() override { return 2; }
};

int main()
{
    C c;
    B1* b1 = &c;
    A* a = b1;
    B2* b2 = dynamic_cast<B2*>(a); // C-cast replaced by dynamic_cast
    assert(b2 != nullptr);
    std::cout << "Return2() = " << b2->Return2();
}

Demo