如何将 std::unordered_map 与 std::pair 一起使用？

Question

I have this error:

binary '==': 'std::pair<const EntityId,EntityData>' does not define this operator or a conversion to a type acceptable to the predefined operator*

In this code:

using EntityId = unsigned int;
using ComponentContainer = std::vector<C_Base*>;
using EntityData = std::pair<Bitmask,ComponentContainer>;

// ERROR below at line:
using EntityContainer = std::unordered_map<EntityId, EntityData>;

Here is the C_Base class:

#include <iostream>
#include <sstream>
#include "ECS_Types.h"

class C_Base{
public:
    C_Base(const Component& l_type): m_type(l_type){}
    virtual ~C_Base(){}

    Component GetType(){ return m_type; }

    friend std::stringstream& operator >>(
        std::stringstream& l_stream, C_Base& b)
    {
        b.ReadIn(l_stream);
        return l_stream;
    }

    virtual void ReadIn(std::stringstream& l_stream) = 0;
protected:
    Component m_type;
};

And the Bitmask class:

#include <stdint.h>

using Bitset = uint32_t;

class Bitmask{
public:
    Bitmask() : bits(0){}
    Bitmask(const Bitset& l_bits) : bits(l_bits){}

    Bitset GetMask() const{ return bits; }
    void SetMask(const Bitset& l_value){ bits = l_value; }

    bool Matches(const Bitmask& l_bits, 
        const Bitset& l_relevant = 0)const
    {
        return(l_relevant ? 
            ((l_bits.GetMask() & l_relevant) == (bits & l_relevant))
            : (l_bits.GetMask() == bits));
    }

    bool GetBit(const unsigned int& l_pos)const{
        return ((bits&(1 << l_pos)) != 0);
    }
    void TurnOnBit(const unsigned int& l_pos){
        bits |= 1 << l_pos;
    }
    void TurnOnBits(const Bitset& l_bits){
        bits |= l_bits;
    }
    void ClearBit(const unsigned int& l_pos){
        bits &= ~(1 << l_pos);
    }
    void ToggleBit(const unsigned int& l_pos){
        bits ^= 1 << l_pos;
    }

    void Clear(){ bits = 0; }
private:
    Bitset bits;
};

When I try the below code, I get this error using EntityContainer = std::unordered_map;:

'operator __surrogate_func': no matching overloaded function found

struct pair_hash {
    template <class T1, class T2>
    std::size_t operator () ( const std::pair<T1, T2>& p ) const {
        auto h1 = std::hash<T1>{}( p.first );
        auto h2 = std::hash<T2>{}( p.second );

        // Mainly for demonstration purposes, i.e. works but is overly simple
        // In the real world, use sth. like boost.hash_combine
        return h1 ^ h2;
    }
};

using EntityId = unsigned int;

using ComponentContainer = std::vector<C_Base*>;
using EntityData = std::pair<Bitmask,ComponentContainer>;

// ERROR below at line:
using EntityContainer = std::unordered_map<EntityId, EntityData, pair_hash>;

Answer 1

Although std::pair has operator== implemented it relies on each of the two component parts of the pair having a viable operator== definition of their own (until c++20 which provides synthesized_three-way_comparison ).

So you need to define an equality operator for Bitmask ...

class Bitmask {
public:
  ...
  bool operator== (const Bitmask &other) const
    {
      return bits == other.bits;
    }
};