オブジェクトへの参照をC++のマップに追加する

Question

オブジェクトへの参照をC++のマップに挿入する際に問題があります。ここで

は私が何をしたいのか示すためにいくつかのサンプルコードです：私はコンパイルするとき

#include <iostream> 
#include <map> 

class Foo { 
public: 
    // ... 

    void addToMap(); 
}; 

std::map<std::string, Foo&> myMap; 

void Foo::addToMap() { 
    myMap.insert(std::make_pair(std::string("hello"), *this)); 
} 

int main() { 
    Foo foo; 
    foo.addToMap(); 
} 

を、私はどのように作るには考えている

main.cpp: In member function ‘void Foo::addToMap()’: 
main.cpp:12:48: error: no matching function for call to ‘std::map<std::__cxx11::basic_string<char>, Foo&>::insert(std::pair<const char*, Foo>)’ 
    myMap.insert(std::make_pair("hello", *this)); 
               ^
In file included from /usr/include/c++/6.3.1/map:61:0, 
       from main.cpp:2: 
/usr/include/c++/6.3.1/bits/stl_map.h:731:7: note: candidate: std::pair<typename std::_Rb_tree<_Key, std::pair<const _Key, _Tp>, std::_Select1st<std::pair<const _Key, _Tp> >, _Compare, typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<std::pair<const _Key, _Tp> >::other>::iterator, bool> std::map<_Key, _Tp, _Compare, _Alloc>::insert(const value_type&) [with _Key = std::__cxx11::basic_string<char>; _Tp = Foo&; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, Foo&> >; typename std::_Rb_tree<_Key, std::pair<const _Key, _Tp>, std::_Select1st<std::pair<const _Key, _Tp> >, _Compare, typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<std::pair<const _Key, _Tp> >::other>::iterator = std::_Rb_tree_iterator<std::pair<const std::__cxx11::basic_string<char>, Foo&> >; std::map<_Key, _Tp, _Compare, _Alloc>::value_type = std::pair<const std::__cxx11::basic_string<char>, Foo&>] 
     insert(const value_type& __x) 
     ^~~~~~ 
/usr/include/c++/6.3.1/bits/stl_map.h:731:7: note: no known conversion for argument 1 from ‘std::pair<const char*, Foo>’ to ‘const value_type& {aka const std::pair<const std::__cxx11::basic_string<char>, Foo&>&}’ 
/usr/include/c++/6.3.1/bits/stl_map.h:739:9: note: candidate: template<class _Pair, class> std::pair<typename std::_Rb_tree<_Key, std::pair<const _Key, _Tp>, std::_Select1st<std::pair<const _Key, _Tp> >, _Compare, typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<std::pair<const _Key, _Tp> >::other>::iterator, bool> std::map<_Key, _Tp, _Compare, _Alloc>::insert(_Pair&&) [with _Pair = _Pair; <template-parameter-2-2> = <template-parameter-1-2>; _Key = std::__cxx11::basic_string<char>; _Tp = Foo&; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, Foo&> >] 
     insert(_Pair&& __x) 
     ^~~~~~ 
/usr/include/c++/6.3.1/bits/stl_map.h:739:9: note: template argument deduction/substitution failed: 
/usr/include/c++/6.3.1/bits/stl_map.h:735:32: error: no type named ‘type’ in ‘struct std::enable_if<false, void>’ 
     template<typename _Pair, typename = typename 
           ^~~~~~~~ 
/usr/include/c++/6.3.1/bits/stl_map.h:752:7: note: candidate: void std::map<_Key, _Tp, _Compare, _Alloc>::insert(std::initializer_list<std::pair<const _Key, _Tp> >) [with _Key = std::__cxx11::basic_string<char>; _Tp = Foo&; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, Foo&> >] 
     insert(std::initializer_list<value_type> __list) 
     ^~~~~~ 
/usr/include/c++/6.3.1/bits/stl_map.h:752:7: note: no known conversion for argument 1 from ‘std::pair<const char*, Foo>’ to ‘std::initializer_list<std::pair<const std::__cxx11::basic_string<char>, Foo&> >’ 
/usr/include/c++/6.3.1/bits/stl_map.h:781:7: note: candidate: std::map<_Key, _Tp, _Compare, _Alloc>::iterator std::map<_Key, _Tp, _Compare, _Alloc>::insert(std::map<_Key, _Tp, _Compare, _Alloc>::const_iterator, const value_type&) [with _Key = std::__cxx11::basic_string<char>; _Tp = Foo&; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, Foo&> >; std::map<_Key, _Tp, _Compare, _Alloc>::iterator = std::_Rb_tree_iterator<std::pair<const std::__cxx11::basic_string<char>, Foo&> >; std::map<_Key, _Tp, _Compare, _Alloc>::const_iterator = std::_Rb_tree_const_iterator<std::pair<const std::__cxx11::basic_string<char>, Foo&> >; std::map<_Key, _Tp, _Compare, _Alloc>::value_type = std::pair<const std::__cxx11::basic_string<char>, Foo&>] 
     insert(const_iterator __position, const value_type& __x) 
     ^~~~~~ 
/usr/include/c++/6.3.1/bits/stl_map.h:781:7: note: candidate expects 2 arguments, 1 provided 
/usr/include/c++/6.3.1/bits/stl_map.h:792:9: note: candidate: template<class _Pair, class> std::map<_Key, _Tp, _Compare, _Alloc>::iterator std::map<_Key, _Tp, _Compare, _Alloc>::insert(std::map<_Key, _Tp, _Compare, _Alloc>::const_iterator, _Pair&&) [with _Pair = _Pair; <template-parameter-2-2> = <template-parameter-1-2>; _Key = std::__cxx11::basic_string<char>; _Tp = Foo&; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, Foo&> >] 
     insert(const_iterator __position, _Pair&& __x) 
     ^~~~~~ 
/usr/include/c++/6.3.1/bits/stl_map.h:792:9: note: template argument deduction/substitution failed: 
main.cpp:12:48: note: candidate expects 2 arguments, 1 provided 
    myMap.insert(std::make_pair("hello", *this)); 
               ^
In file included from /usr/include/c++/6.3.1/map:61:0, 
       from main.cpp:2: 
/usr/include/c++/6.3.1/bits/stl_map.h:807:9: note: candidate: template<class _InputIterator> void std::map<_Key, _Tp, _Compare, _Alloc>::insert(_InputIterator, _InputIterator) [with _InputIterator = _InputIterator; _Key = std::__cxx11::basic_string<char>; _Tp = Foo&; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, Foo&> >] 
     insert(_InputIterator __first, _InputIterator __last) 
     ^~~~~~ 
/usr/include/c++/6.3.1/bits/stl_map.h:807:9: note: template argument deduction/substitution failed: 
main.cpp:12:48: note: candidate expects 2 arguments, 1 provided 
    myMap.insert(std::make_pair("hello", *this)); 

の非常に不可解なエラーメッセージが表示されますポインタthisを参照に挿入してマップに挿入することができます。コードを修正するにはどうすればいいですか？

Answer 1

標準ライブラリのコンテナには参照を含めることはできません。代わりにポインタを使用するか、代わりにTへの参照をstd::reference_wrapper<T>にラップしてください。

#include <functional> 
#include <map> 
#include <string> 

std::map<std::string, std::reference_wrapper<Foo> > myMap;