Why a non-member == operator overloading needed when using a std::pair as a std::unordered_map key

Question

In the below example I have overloaded the conversion functions for uint8_t and bool. And used a pair as the map key with boost::hash. There was a previous error ambiguity in conversion of 'const Mem' to 'float' (not sure why it's requesting a Mem -> float conversion) so I decided to add an overloading for == operator. This will give an compile error in clang++ if I only implement the member operator overloading for the == operator.

In file included from /opt/compiler-explorer/gcc-11.2.0/lib/gcc/x86_64-linux-gnu/11.2.0/../../../../include/c++/11.2.0/iostream:39:
In file included from /opt/compiler-explorer/gcc-11.2.0/lib/gcc/x86_64-linux-gnu/11.2.0/../../../../include/c++/11.2.0/ostream:38:
In file included from /opt/compiler-explorer/gcc-11.2.0/lib/gcc/x86_64-linux-gnu/11.2.0/../../../../include/c++/11.2.0/ios:40:
In file included from /opt/compiler-explorer/gcc-11.2.0/lib/gcc/x86_64-linux-gnu/11.2.0/../../../../include/c++/11.2.0/bits/char_traits.h:39:
In file included from /opt/compiler-explorer/gcc-11.2.0/lib/gcc/x86_64-linux-gnu/11.2.0/../../../../include/c++/11.2.0/bits/stl_algobase.h:64:
/opt/compiler-explorer/gcc-11.2.0/lib/gcc/x86_64-linux-gnu/11.2.0/../../../../include/c++/11.2.0/bits/stl_pair.h:467:51: error: use of overloaded operator '==' is ambiguous (with operand types 'const Mem' and 'const Mem')
    { return __x.first == __y.first && __x.second == __y.second; }

Why do I need a non-member overload of == since __x and __y are same type and can be supported from member operator. Example run

#include <iostream>
#include <cstdint>
#include <unordered_map>
#include <string>

#include <boost/functional/hash.hpp>

struct Mem {
    bool inv : 1;
    uint8_t bit : 3;

    Mem(bool v = false, uint8_t bs = 0) : inv(v), bit(bs) {}

    operator uint8_t() const
    {
        return (bit << 1 | inv);
    }
    
    operator bool() const
    {
        return inv;
    }
    
    // not working for member overloading
    bool operator==(const Mem& other) 
    { 
        return inv == other.inv;
    }

};

// only work for non-member overload
// inline bool operator==(const Mem& lhs, const Mem& rhs) 
// { 
//     return lhs.inv == rhs.inv;
// }

inline std::size_t hash_value(const Mem& mem)
{
    return mem.bit;
}


int main()
{
    std::unordered_map<std::pair<std::string, Mem>, std::string, 
                       boost::hash<std::pair<std::string, Mem>>> map {};

    auto i = Mem();
    std::string k = "T";

    auto p = map.find({k, i});
     
    return 0;
}

Answer 1

Your member operator== needs to be declared as const (like your other member operators are), eg:

bool operator==(const Mem& other) const
                                   ^^

Without that, your member operator== is ignored when the left-hand operand of a == comparison is a const Mem object. Your non-member operator== can by used instead because its lhs parameter takes a const Mem& object reference, which accepts both non-const Mem and const Mem objects.