Looking for "is_comparable" typetrait

Question

I'm looking for an "is_comparable" typetrait but can't find any.

It's very easy to build one that checks if an operator== for a class was implemented, but this excludes global defined operators.

Is it impossible to implement a is_comparable typetait?

Answer 1

I take it you mean a trait that, for two types L and R and objects lhs and rhs of those types respectively, will yield true if the lhs == rhs will compile and false otherwise. You appreciate that in theory lhs == rhs might compile even though rhs == lhs, or lhs != rhs, does not.

In that case you might implement the trait like:

#include <type_traits>

template<class ...> using void_t = void;

template<typename L, typename R, class = void>
struct is_comparable : std::false_type {};

template<typename L, typename R>
using comparability = decltype(std::declval<L>() == std::declval<R>());

template<typename L, typename R>
struct is_comparable<L,R,void_t<comparability<L,R>>> : std::true_type{};

This applies a popular SFINAE pattern for defining traits that is explained in the answer to this question

Some illustrations:

struct noncomparable{};

struct comparable_right
{
    bool operator==(comparable_right const & other) const {
        return true;
    }
};

struct any_comparable_right
{
    template<typename T>
    bool operator==(T && other) const {
        return false;
    }
};

bool operator==(noncomparable const & lhs, int i) {
    return true;
}

#include <string>

static_assert(is_comparable<comparable_right,comparable_right>::value,"");
static_assert(!is_comparable<noncomparable,noncomparable>::value,"");
static_assert(!is_comparable<noncomparable,any_comparable_right>::value,"");
static_assert(is_comparable<any_comparable_right,noncomparable>::value,"");
static_assert(is_comparable<noncomparable,int>::value,"");
static_assert(!is_comparable<int,noncomparable>::value,"");
static_assert(is_comparable<char *,std::string>::value,"");
static_assert(!is_comparable<char const *,char>::value,"");
static_assert(is_comparable<double,char>::value,"");

If you want the trait to require that equality is symmetric and that inequality also exists and is symmetric you can see how to elaborate it yourself.