在下面的例子中，我重载了uint8_t和bool的转换函数。并使用一个pair作为Map键boost::hash。之前有一个错误ambiguity in conversion of 'const Mem' to 'float'（不知道为什么它请求Mem -> float转换），所以我决定为==操作符添加一个重载。如果我只为==运算符实现成员运算符重载，这将在clang++中给予编译错误。

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; }

为什么我需要==的非成员重载，因为__x和__y是相同的类型，可以从成员操作符支持。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;
}