std::map 和带有复制构造函数的类

为什么这段代码，它试图推送一对 -int和一个带有复制构造函数的类，不起作用？

int main() {
    std::map<int, A> m;

    A a;

    m.insert(std::make_pair(0, a));

    return 0;
}

A类：

A::A() = default;

A::A(A &a) {
    //smth
}

错误文字：

/home/wcobalt/Files/InstalledPrograms/clion-2018.2.4/bin/cmake/linux/bin/cmake --build /home/wcobalt/Files/test/cmake-build-debug --target test -- -j 2
Scanning dependencies of target test
[ 66%] Building CXX object CMakeFiles/test.dir/A.cpp.o
[ 66%] Building CXX object CMakeFiles/test.dir/main.cpp.o
In file included from /usr/include/c++/7/bits/stl_algobase.h:64:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from /home/wcobalt/Files/test/main.cpp:1:
/usr/include/c++/7/bits/stl_pair.h: In instantiation of ‘struct std::pair<int, A>’:
/home/wcobalt/Files/test/main.cpp:11:33:   required from here
/usr/include/c++/7/bits/stl_pair.h:292:17: error: ‘constexpr std::pair<_T1, _T2>::pair(const std::pair<_T1, _T2>&) [with _T1 = int; _T2 = A]’ declared to take const reference, but implicit declaration would take non-const
       constexpr pair(const pair&) = default;
                 ^~~~
/usr/include/c++/7/bits/stl_pair.h: In instantiation of ‘struct std::pair<const int, A>’:
/usr/include/c++/7/type_traits:787:65:   required by substitution of ‘template<class _Tp, class> static std::true_type std::__do_is_destructible_impl::__test(int) [with _Tp = std::pair<const int, A>; <template-parameter-1-2> = <missing>]’
/usr/include/c++/7/type_traits:798:35:   required from ‘struct std::__is_destructible_impl<std::pair<const int, A> >’
/usr/include/c++/7/type_traits:809:12:   required from ‘struct std::__is_destructible_safe<std::pair<const int, A>, false, false>’
/usr/include/c++/7/type_traits:824:12:   required from ‘struct std::is_destructible<std::pair<const int, A> >’
/usr/include/c++/7/type_traits:143:12:   [ skipping 2 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ]
/usr/include/c++/7/type_traits:1070:12:   required from ‘struct std::__is_direct_constructible_new<std::pair<const int, A>, std::pair<int, A>&&>’
/usr/include/c++/7/type_traits:1078:12:   required from ‘struct std::__is_direct_constructible<std::pair<const int, A>, std::pair<int, A>&&>’
/usr/include/c++/7/type_traits:1118:12:   required from ‘struct std::__is_constructible_impl<std::pair<const int, A>, std::pair<int, A>&&>’
/usr/include/c++/7/type_traits:1129:12:   required from ‘struct std::is_constructible<std::pair<const int, A>, std::pair<int, A>&&>’
/usr/include/c++/7/bits/stl_map.h:805:32:   required by substitution of ‘template<class _Pair, class> std::pair<std::_Rb_tree_iterator<std::pair<const int, A> >, bool> std::map<int, A>::insert<_Pair, <template-parameter-1-2> >(_Pair&&) [with _Pair = std::pair<int, A>; <template-parameter-1-2> = <missing>]’
/home/wcobalt/Files/test/main.cpp:11:34:   required from here
/usr/include/c++/7/bits/stl_pair.h:292:17: error: ‘constexpr std::pair<_T1, _T2>::pair(const std::pair<_T1, _T2>&) [with _T1 = const int; _T2 = A]’ declared to take const reference, but implicit declaration would take non-const
/home/wcobalt/Files/test/main.cpp: In function ‘int main()’:
/home/wcobalt/Files/test/main.cpp:11:34: error: no matching function for call to ‘std::map<int, A>::insert(std::pair<int, A>)’
     m.insert(std::make_pair(0, a));
                                  ^
In file included from /usr/include/c++/7/map:61:0,
                 from /home/wcobalt/Files/test/main.cpp:2:
/usr/include/c++/7/bits/stl_map.h:621:7: note: candidate: std::map<_Key, _Tp, _Compare, _Alloc>::insert_return_type std::map<_Key, _Tp, _Compare, _Alloc>::insert(std::map<_Key, _Tp, _Compare, _Alloc>::node_type&&) [with _Key = int; _Tp = A; _Compare = std::less<int>; _Alloc = std::allocator<std::pair<const int, A> >; std::map<_Key, _Tp, _Compare, _Alloc>::insert_return_type = std::_Node_insert_return<std::_Rb_tree_iterator<std::pair<const int, A> >, std::_Node_handle<int, std::pair<const int, A>, std::allocator<std::_Rb_tree_node<std::pair<const int, A> > > > >; std::map<_Key, _Tp, _Compare, _Alloc>::node_type = std::_Node_handle<int, std::pair<const int, A>, std::allocator<std::_Rb_tree_node<std::pair<const int, A> > > >]
       insert(node_type&& __nh)
       ^~~~~~
/usr/include/c++/7/bits/stl_map.h:621:7: note:   no known conversion for argument 1 from ‘std::pair<int, A>’ to ‘std::map<int, A>::node_type&& {aka std::_Node_handle<int, std::pair<const int, A>, std::allocator<std::_Rb_tree_node<std::pair<const int, A> > > >&&}’
/usr/include/c++/7/bits/stl_map.h:626:7: note: candidate: std::map<_Key, _Tp, _Compare, _Alloc>::iterator std::map<_Key, _Tp, _Compare, _Alloc>::insert(std::map<_Key, _Tp, _Compare, _Alloc>::const_iterator, std::map<_Key, _Tp, _Compare, _Alloc>::node_type&&) [with _Key = int; _Tp = A; _Compare = std::less<int>; _Alloc = std::allocator<std::pair<const int, A> >; std::map<_Key, _Tp, _Compare, _Alloc>::iterator = std::_Rb_tree_iterator<std::pair<const int, A> >; std::map<_Key, _Tp, _Compare, _Alloc>::const_iterator = std::_Rb_tree_const_iterator<std::pair<const int, A> >; std::map<_Key, _Tp, _Compare, _Alloc>::node_type = std::_Node_handle<int, std::pair<const int, A>, std::allocator<std::_Rb_tree_node<std::pair<const int, A> > > >]
       insert(const_iterator __hint, node_type&& __nh)
       ^~~~~~
/usr/include/c++/7/bits/stl_map.h:626:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/7/bits/stl_map.h:795:7: 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 = int; _Tp = A; _Compare = std::less<int>; _Alloc = std::allocator<std::pair<const int, A> >; 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 int, A> >; std::map<_Key, _Tp, _Compare, _Alloc>::value_type = std::pair<const int, A>]
       insert(const value_type& __x)
       ^~~~~~
/usr/include/c++/7/bits/stl_map.h:795:7: note:   no known conversion for argument 1 from ‘std::pair<int, A>’ to ‘const value_type& {aka const std::pair<const int, A>&}’
/usr/include/c++/7/bits/stl_map.h:802:7: 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(std::map<_Key, _Tp, _Compare, _Alloc>::value_type&&) [with _Key = int; _Tp = A; _Compare = std::less<int>; _Alloc = std::allocator<std::pair<const int, A> >; 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 int, A> >; std::map<_Key, _Tp, _Compare, _Alloc>::value_type = std::pair<const int, A>]
       insert(value_type&& __x)
       ^~~~~~
/usr/include/c++/7/bits/stl_map.h:802:7: note:   no known conversion for argument 1 from ‘std::pair<int, A>’ to ‘std::map<int, A>::value_type&& {aka std::pair<const int, A>&&}’
/usr/include/c++/7/bits/stl_map.h:809:2: 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 = int; _Tp = A; _Compare = std::less<int>; _Alloc = std::allocator<std::pair<const int, A> >]
  insert(_Pair&& __x)
  ^~~~~~
/usr/include/c++/7/bits/stl_map.h:809:2: note:   substitution of deduced template arguments resulted in errors seen above
/usr/include/c++/7/bits/stl_map.h:823:7: note: candidate: void std::map<_Key, _Tp, _Compare, _Alloc>::insert(std::initializer_list<std::pair<const _Key, _Tp> >) [with _Key = int; _Tp = A; _Compare = std::less<int>; _Alloc = std::allocator<std::pair<const int, A> >]
       insert(std::initializer_list<value_type> __list)
       ^~~~~~
/usr/include/c++/7/bits/stl_map.h:823:7: note:   no known conversion for argument 1 from ‘std::pair<int, A>’ to ‘std::initializer_list<std::pair<const int, A> >’
/usr/include/c++/7/bits/stl_map.h:853:7: 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 = int; _Tp = A; _Compare = std::less<int>; _Alloc = std::allocator<std::pair<const int, A> >; std::map<_Key, _Tp, _Compare, _Alloc>::iterator = std::_Rb_tree_iterator<std::pair<const int, A> >; std::map<_Key, _Tp, _Compare, _Alloc>::const_iterator = std::_Rb_tree_const_iterator<std::pair<const int, A> >; std::map<_Key, _Tp, _Compare, _Alloc>::value_type = std::pair<const int, A>]
       insert(const_iterator __position, const value_type& __x)
       ^~~~~~
/usr/include/c++/7/bits/stl_map.h:853:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/7/bits/stl_map.h:863:7: note: candidate: std::map<_Key, _Tp, _Compare, _Alloc>::iterator std::map<_Key, _Tp, _Compare, _Alloc>::insert(std::map<_Key, _Tp, _Compare, _Alloc>::const_iterator, std::map<_Key, _Tp, _Compare, _Alloc>::value_type&&) [with _Key = int; _Tp = A; _Compare = std::less<int>; _Alloc = std::allocator<std::pair<const int, A> >; std::map<_Key, _Tp, _Compare, _Alloc>::iterator = std::_Rb_tree_iterator<std::pair<const int, A> >; std::map<_Key, _Tp, _Compare, _Alloc>::const_iterator = std::_Rb_tree_const_iterator<std::pair<const int, A> >; std::map<_Key, _Tp, _Compare, _Alloc>::value_type = std::pair<const int, A>]
       insert(const_iterator __position, value_type&& __x)
       ^~~~~~
/usr/include/c++/7/bits/stl_map.h:863:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/7/bits/stl_map.h:870:2: 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 = int; _Tp = A; _Compare = std::less<int>; _Alloc = std::allocator<std::pair<const int, A> >]
  insert(const_iterator __position, _Pair&& __x)
  ^~~~~~
/usr/include/c++/7/bits/stl_map.h:870:2: note:   template argument deduction/substitution failed:
/home/wcobalt/Files/test/main.cpp:11:34: note:   candidate expects 2 arguments, 1 provided
     m.insert(std::make_pair(0, a));
                                  ^
In file included from /usr/include/c++/7/map:61:0,
                 from /home/wcobalt/Files/test/main.cpp:2:
/usr/include/c++/7/bits/stl_map.h:886:2: note: candidate: template<class _InputIterator> void std::map<_Key, _Tp, _Compare, _Alloc>::insert(_InputIterator, _InputIterator) [with _InputIterator = _InputIterator; _Key = int; _Tp = A; _Compare = std::less<int>; _Alloc = std::allocator<std::pair<const int, A> >]
  insert(_InputIterator __first, _InputIterator __last)
  ^~~~~~
/usr/include/c++/7/bits/stl_map.h:886:2: note:   template argument deduction/substitution failed:
/home/wcobalt/Files/test/main.cpp:11:34: note:   candidate expects 2 arguments, 1 provided
     m.insert(std::make_pair(0, a));
                                  ^
CMakeFiles/test.dir/build.make:62: recipe for target 'CMakeFiles/test.dir/main.cpp.o' failed
make[3]: *** [CMakeFiles/test.dir/main.cpp.o] Error 1
CMakeFiles/Makefile2:72: recipe for target 'CMakeFiles/test.dir/all' failed
make[2]: *** [CMakeFiles/test.dir/all] Error 2
CMakeFiles/Makefile2:84: recipe for target 'CMakeFiles/test.dir/rule' failed
make[1]: *** [CMakeFiles/test.dir/rule] Error 2
Makefile:118: recipe for target 'test' failed
make: *** [test] Error 2