How to cast enums? - c++

Question

I have a third-party unscoped enum (that I can't modify) that I'd really like to cast to my own scoped enum. How can I provide something like a conversion operator?
What I'd like to do is something like this:
#include <iostream>
enum ThirdPartyLetter {
A=4,
B=5
};
enum class MyNumber {
ONE=1,
TWO=2
// This doesn't compile, of course
/*Number(const ThirdPartyLetter& rhs) {
if(rhs == ThirdPartyLetter::A) {
return ONE;
}
else {
return TWO;
}
}*/
};
int main() {
ThirdPartyLetter letter = ThirdPartyLetter::A;
MyNumber number = static_cast<MyNumber>(letter);
// Without the cast, this prints 4 (an invalid enum value!)
std::cout << int(number) << std::endl;
}
Is there a way to provide some kind of casting from ThirdPartyNumber to MyNumber?

Answer 1

An idiomatic way to do that at compile-time in C++ is using traits.
As an example:
enum Foo { ONE, TWO };
enum Bar { THREE, FOUR };
template<Foo> struct conv;
template<> struct conv<Foo::ONE> { static constexpr Bar value = Bar::THREE; };
template<> struct conv<Foo::TWO> { static constexpr Bar value = Bar::FOUR; };
If you want to do that at runtime, maybe a switch is well suited.
Anyway, you can still use traits to centralize your conversion logic and do something like this:
Bar get(Foo choice) {
switch(choice) {
case Foo::ONE:
return conv<ONE>::value;
case Foo::TWO:
return conv<TWO>::value;
}
}