Question

I was going through some code in CodeProject and came across the following code for C++ casting.

template <class OutputClass, class InputClass>
union horrible_union{
    OutputClass out;
    InputClass in;
};
template <class OutputClass, class InputClass>
inline OutputClass horrible_cast(const InputClass input){
    horrible_union<OutputClass, InputClass> u;
    u.in = input;
    return u.out;
}

Why is the cast implemented the above way. Why can’t we just do a manual cast. Can someone give an example of when a normal cast wouldn’t work ?

Answer 1

This approach basically lets you get away with any cast, although it relies on undefined behavior.

A normal cast would complain when casting between unrelated types, whereas this wouldn’t.

struct A{};
struct B{};

template <class OutputClass, class InputClass>
union horrible_union{
    OutputClass out;
    InputClass in;
};
template <class OutputClass, class InputClass>
inline OutputClass horrible_cast(const InputClass input){
    horrible_union<OutputClass, InputClass> u;
    u.in = input;
    return u.out;
}

int main()
{
    A a;
    B b;
    a = horrible_cast<A,B>(b);   //this compiles
    a = reinterpret_cast<A>(b);  //this doesn't
} 

Bottom line: it’s horrible, don’t do it.