How to reuse operator overloads for different types?

Question

I have several enums defined as follows:

enum class Suit {
    spades = 1, hearts, diamonds, clubs,
    first = spades, last = clubs
};

enum class Rank {
    six = 6, seven, eight, nine, ten, jack, queen, king, ace,
    first = six, last = ace
};

For each of these enums I have overloaded some operators:

Suit operator++(Suit& r) { return r = (Suit)(static_cast<std::underlying_type_t<Suit>>(r) + 1); }
Rank operator++(Rank& r) { return r = (Rank)(static_cast<std::underlying_type_t<Rank>>(r) + 1); }
// more overloads ...

Note that the implementation of the operator overloads is the same for both types. How can I avoid this code duplication?

I could use templates...

template<class T>
T operator++(T& r) { return r = (T)(static_cast<std::underlying_type_t<T>>(r) + 1); }

but these overloads should apply to my custom types only.

Answer 1

Here's how you can restrict your templates in a scalable manner.

#include <type_traits>

template <class> 
constexpr bool is_my_enum = false;       // most types are not my enums

enum class Suit {
    spades = 1, hearts, diamonds, clubs,
    first = spades, last = clubs
};
template<> 
constexpr bool is_my_enum<Suit> = true;  // this is my enum

enum class Rank {
    six = 6, seven, eight, nine, ten, jack, queen, king, ace,
    first = six, last = ace
};
template<> 
constexpr bool is_my_enum<Rank> = true;  // this one is mine too

enum class Moo { moo = 0 };              // but this one isn't



// define functions for my enums only
template<class T>
using for_my_enums = std::enable_if_t<is_my_enum<T>, T>;

template<class T> 
for_my_enums<T>                        
operator++(T& r) {return r = (T)(static_cast<std::underlying_type_t<T>>(r) + 1);}

template<class T> 
for_my_enums<T>                        
operator++(T& r, int) { 
    auto k = r; 
    r = (T)(static_cast<std::underlying_type_t<T>>(r) + 1); 
    return k;
}

// ... more functions ...


int main()
{
   Suit a = Suit::spades;
   ++a;                   // ok
   Moo b = Moo::moo;
   ++b;                   // compilation error, it's not my enum
}