访问时，对象是一个<map>值实例方法

Question

什么是正确的方式访问prime作为的std ::地图的值时，将调用该方法print()：

#include <iostream> 
#include <map> 

using namespace std; 

class Prime { 
private: 
    int n; 

public: 
    Prime(int n): n(n) {} 

    Prime(const Prime &P) { 
     n = P.n; 
    } 

    void print() { 
     cout << "value: " << n << endl; 
    } 
}; 

int main() { 
    Prime prime(5); 

    map<string, Prime> myMap; 
    myMap.emplace("five", prime); 

    // this line appears to be failing 
    myMap["five"].print(); 

    return 1; 
} 

当我运行这段代码$ g++ -std=c++11 main.cpp && ./a.out：

In file included from /usr/include/c++/4.8/bits/stl_map.h:63:0, 
        from /usr/include/c++/4.8/map:61, 
        from main.cpp:4: 
/usr/include/c++/4.8/tuple: In instantiation of ‘std::pair<_T1, _T2>::pair(std::tuple<_Args1 ...>&, std::tuple<_Args2 ...>&, std::_Index_tuple<_Indexes1 ...>, std::_Index_tuple<_Indexes2 ...>) [with _Args1 = {std::basic_string<char, std::char_traits<char>, std::allocator<char> >&&}; long unsigned int ..._Indexes1 = {0ul}; _Args2 = {}; long unsigned int ..._Indexes2 = {}; _T1 = const std::basic_string<char>; _T2 = Prime]’: 
/usr/include/c++/4.8/tuple:1079:63: required from ‘std::pair<_T1, _T2>::pair(std::piecewise_construct_t, std::tuple<_Args1 ...>, std::tuple<_Args2 ...>) [with _Args1 = {std::basic_string<char, std::char_traits<char>, std::allocator<char> >&&}; _Args2 = {}; _T1 = const std::basic_string<char>; _T2 = Prime]’ 
/usr/include/c++/4.8/bits/stl_tree.h:140:49: required from ‘std::_Rb_tree_node<_Val>::_Rb_tree_node(_Args&& ...) [with _Args = {const std::piecewise_construct_t&, std::tuple<std::basic_string<char, std::char_traits<char>, std::allocator<char> >&&>, std::tuple<>}; _Val = std::pair<const std::basic_string<char>, Prime>]’ 
/usr/include/c++/4.8/ext/new_allocator.h:120:4: required from ‘void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::_Rb_tree_node<std::pair<const std::basic_string<char>, Prime> >; _Args = {const std::piecewise_construct_t&, std::tuple<std::basic_string<char, std::char_traits<char>, std::allocator<char> >&&>, std::tuple<>}; _Tp = std::_Rb_tree_node<std::pair<const std::basic_string<char>, Prime> >]’ 
/usr/include/c++/4.8/bits/alloc_traits.h:254:4: required from ‘static typename std::enable_if<std::allocator_traits<_Alloc>::__construct_helper<_Tp, _Args>::value, void>::type std::allocator_traits<_Alloc>::_S_construct(_Alloc&, _Tp*, _Args&& ...) [with _Tp = std::_Rb_tree_node<std::pair<const std::basic_string<char>, Prime> >; _Args = {const std::piecewise_construct_t&, std::tuple<std::basic_string<char, std::char_traits<char>, std::allocator<char> >&&>, std::tuple<>}; _Alloc = std::allocator<std::_Rb_tree_node<std::pair<const std::basic_string<char>, Prime> > >; typename std::enable_if<std::allocator_traits<_Alloc>::__construct_helper<_Tp, _Args>::value, void>::type = void]’ 
/usr/include/c++/4.8/bits/alloc_traits.h:393:57: required from ‘static decltype (_S_construct(__a, __p, (forward<_Args>)(std::allocator_traits::construct::__args)...)) std::allocator_traits<_Alloc>::construct(_Alloc&, _Tp*, _Args&& ...) [with _Tp = std::_Rb_tree_node<std::pair<const std::basic_string<char>, Prime> >; _Args = {const std::piecewise_construct_t&, std::tuple<std::basic_string<char, std::char_traits<char>, std::allocator<char> >&&>, std::tuple<>}; _Alloc = std::allocator<std::_Rb_tree_node<std::pair<const std::basic_string<char>, Prime> > >; decltype (_S_construct(__a, __p, (forward<_Args>)(std::allocator_traits::construct::__args)...)) = <type error>]’ 
/usr/include/c++/4.8/bits/stl_tree.h:408:36: required from ‘std::_Rb_tree_node<_Val>* std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_create_node(_Args&& ...) [with _Args = {const std::piecewise_construct_t&, std::tuple<std::basic_string<char, std::char_traits<char>, std::allocator<char> >&&>, std::tuple<>}; _Key = std::basic_string<char>; _Val = std::pair<const std::basic_string<char>, Prime>; _KeyOfValue = std::_Select1st<std::pair<const std::basic_string<char>, Prime> >; _Compare = std::less<std::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::basic_string<char>, Prime> >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Link_type = std::_Rb_tree_node<std::pair<const std::basic_string<char>, Prime> >*]’ 
/usr/include/c++/4.8/bits/stl_tree.h:1669:64: required from ‘std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_emplace_hint_unique(std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::const_iterator, _Args&& ...) [with _Args = {const std::piecewise_construct_t&, std::tuple<std::basic_string<char, std::char_traits<char>, std::allocator<char> >&&>, std::tuple<>}; _Key = std::basic_string<char>; _Val = std::pair<const std::basic_string<char>, Prime>; _KeyOfValue = std::_Select1st<std::pair<const std::basic_string<char>, Prime> >; _Compare = std::less<std::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::basic_string<char>, Prime> >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator = std::_Rb_tree_iterator<std::pair<const std::basic_string<char>, Prime> >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::const_iterator = std::_Rb_tree_const_iterator<std::pair<const std::basic_string<char>, Prime> >]’ 
/usr/include/c++/4.8/bits/stl_map.h:484:8: required from ‘std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](std::map<_Key, _Tp, _Compare, _Alloc>::key_type&&) [with _Key = std::basic_string<char>; _Tp = Prime; _Compare = std::less<std::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::basic_string<char>, Prime> >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = Prime; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = std::basic_string<char>]’ 
main.cpp:31:15: required from here 
/usr/include/c++/4.8/tuple:1090:70: error: no matching function for call to ‘Prime::Prime()’ 
      second(std::forward<_Args2>(std::get<_Indexes2>(__tuple2))...) 
                     ^
/usr/include/c++/4.8/tuple:1090:70: note: candidates are: 
main.cpp:15:3: note: Prime::Prime(const Prime&) 
    Prime(const Prime &P) { 
    ^
main.cpp:15:3: note: candidate expects 1 argument, 0 provided 
main.cpp:13:3: note: Prime::Prime(int) 
    Prime(int n): n(n) {} 
    ^
main.cpp:13:3: note: candidate expects 1 argument, 0 provided 

Answer 1

写Prime(int n = 0): n(n) {}是你的修复。

即使您使用的是emplace，C++标准也要求Prime类具有默认的构造函数，而您目前没有这样做。