std :: make_pair，std :: unordered_map以及键类型中的move构造函数的用法

Question

Given following struct:

struct FieldNo
{
    FieldNo() : a('0'), b('0') {}
    FieldNo(char a_, char b_) : a(a_), b(_b) {}
    // copy construction and assigment not allowed
    FieldNo(const FieldNo& other) = delete;
    FieldNo& operator=(const FieldNo& other) = delete;
    // move construction and assignment ok
    FieldNo(FieldNo&& other) = default;
    FieldNo& operator=(FieldNo&& other) = default; 

    char a;
    char b;
};

enum class Members : int8_t
{
    FOO,
    BAR
};

I am using FieldNo as key and Member as value for an std::unordered_map. Ommitting the code for the hash creation function, my map is defined as following:

typedef std::unordered_map<FieldNo, Members, FieldNoHasher> MyMapT;

Later I use following method to initialize and return to the caller

const MyMapT& map()
{
   static const MyMapT fields = 
   {
       std::make_pair(FieldNo('0', '5'), Members::FOO),
       std::make_pair(FieldNo('1', 'X'), Members::BAR)
   }
   return fields;
}

At first the copy constructor was not deleted and everything worked fine.

As I deleted to copy constructor I am getting loads of errors in insertion and put here the most relevant one:

[build] 
[build] /usr/include/c++/8/ext/new_allocator.h:136:4: error: use of deleted function ‘constexpr std::pair<_T1, _T2>::pair(const std::pair<_T1, _T2>&) [with _T1 = const FieldNo; _T2 = Members]’
[build]   { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }
[build]     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[build] In file included from /usr/include/c++/8/bits/stl_algobase.h:64,
[build]                  from /usr/include/c++/8/bits/char_traits.h:39,
[build]                  from /usr/include/c++/8/ios:40,
[build]                  from /usr/include/c++/8/istream:38,
[build]                  from /usr/include/c++/8/fstream:38,
[build]                  from 
[build] /usr/include/c++/8/bits/stl_pair.h:303:17: note: ‘constexpr std::pair<_T1, _T2>::pair(const std::pair<_T1, _T2>&) [with _T1 = const FieldNo; _T2 = Members]’ is implicitly deleted because the default definition would be ill-formed:
[build]        constexpr pair(const pair&) = default;
[build]                  ^~~~
[build] /usr/include/c++/8/bits/stl_pair.h:303:17: error: use of deleted function ‘FieldNo::FieldNo(const libtraco::FieldNo&)’

I know the errors are related due to the missing copy constructor. But why does it even try to copy things? FieldNo('0', '5') in std::make_pair is clearly a rvalue . Am I missing something?

Thanks!

EDIT: Minimal reproducible example added

#include <unordered_map>

struct FieldNo
{
    FieldNo() : a('0'), b('0') {}
    FieldNo(char a_, char b_) : a(a_), b(b_) {}
    // copy construction and assigment not allowed
    FieldNo(const FieldNo& other) = delete;
    FieldNo& operator=(const FieldNo& other) = delete;
    // move construction and assignment ok
    FieldNo(FieldNo&& other) = default;
    FieldNo& operator=(FieldNo&& other) = default;

    bool operator==(const FieldNo& lhs) const
    {
        return a == lhs.a && b == lhs.b;
    }

    char a;
    char b;
};

template <class T>
inline void hash_combine(std::size_t& seed, const T& v)
{
    std::hash<T> hasher;
    seed ^= hasher(v) + 0x9e3779b9 + (seed<<6) + (seed>>2);
}

struct FieldNoHasher 
{
    std::size_t operator()(const FieldNo& input) const
    {
        std::size_t seed = 0;
        hash_combine(seed, input.a);
        hash_combine(seed, input.b);
        return seed;
    }
};

enum class Members : int8_t
{
    FOO,
    BAR
};

typedef std::unordered_map<FieldNo, Members, FieldNoHasher> MyMapT;

const MyMapT& map()
{
   static const MyMapT fields = 
   {
       std::make_pair(FieldNo('0', '5'), Members::FOO),
       std::make_pair(FieldNo('1', 'X'), Members::BAR)
   };

   return fields;
}

int main()
{
    map();
}

Answer 1

Here's a minimal reproducible example:

#include <unordered_map>
#include <utility>

class C {
public:
    C() = default;
    C(const C&) = delete;
    C& operator=(const C&) = delete;
    C(C&&) = default;
    C& operator=(C&&) = default;
};

bool operator==(const C&, const C&)
{
    return true;
}

struct Hash {
    constexpr std::size_t operator()(const C&) const
    {
        return 42;
    }
};

int main()
{
    std::unordered_map<C, int, Hash> map {
        std::make_pair(C{}, 0)
    };
}

( live demo )

The problem is that you are calling the initializer_list constructor. Because of the way initializer_list works, the underlying elements of an initializer_list are const , meaning the elements can only be copied, not moved. You cannot use the initializer_list constructor if your type is move only. You have to use the other facilities:

std::unordered_map<C, int, Hash> map;
map.emplace(C{}, 0);

or you can replace initializer_list with another container that supports moving (a vector, for example):

std::vector<std::pair<const C, int>> values {
    std::make_pair(C{}, 0)
};
std::unordered_map<C, int, Hash> map(values.cbegin(), values.cend());

( live demo )