I wanted to create an item into a map using C++17 (g++-6 or higher).
The value_type of the map is a POD Bar which contains a non-copyable, non-moveable member of type Foo.
By looking at stack-overflow, I found the following posts that told me to piecewise construct the map-item:
std::map emplace without copying value
Adding struct containing not copyable/moveable object to std::map
But I am currently not able to apply the instructions of both posts in order to construct Bar piecewise using the following example: (I get the compiler error below):
#include<string>
#include<map>
struct Foo
{
Foo(int a, std::string b) {}
Foo(const Foo&) = delete;
Foo(Foo&&) = delete;
};
struct Bar
{
Foo x;
int y;
};
int main() {
const int idx{0};
const int a{1};
const std::string b{"my"};
const int y{2};
std::map<int, Bar> m;
m.emplace(std::piecewise_construct,
std::forward_as_tuple(idx),
std::forward_as_tuple({a, b}, y));
}
The compile error on g++-6 is as following:
tes.cpp: In function ‘int main()’:
tes.cpp:24:46: error: too many arguments to function ‘constexpr std::tuple<_Elements&& ...> std::forward_as_tuple(_Elements&& ...) [with _Elements = {}]’
std::forward_as_tuple({a, b}, y));
^
In file included from /usr/include/c++/6/bits/stl_map.h:63:0,
from /usr/include/c++/6/map:61,
from tes.cpp:2:
/usr/include/c++/6/tuple:1403:5: note: declared here
forward_as_tuple(_Elements&&... __args) noexcept
^~~~~~~~~~~~~~~~
/usr/include/c++/6/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 = {const int&}; long unsigned int ..._Indexes1 = {0ul}; _Args2 = {}; long unsigned int ..._Indexes2 = {}; _T1 = const int; _T2 = Bar]’:
/usr/include/c++/6/tuple:1579:63: required from ‘std::pair<_T1, _T2>::pair(std::piecewise_construct_t, std::tuple<_Args1 ...>, std::tuple<_Args2 ...>) [with _Args1 = {const int&}; _Args2 = {}; _T1 = const int; _T2 = Bar]’
/usr/include/c++/6/ext/new_allocator.h:120:4: required from ‘void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::pair<const int, Bar>; _Args = {const std::piecewise_construct_t&, std::tuple<const int&>, std::tuple<>}; _Tp = std::_Rb_tree_node<std::pair<const int, Bar> >]’
/usr/include/c++/6/bits/alloc_traits.h:475:4: required from ‘static void std::allocator_traits<std::allocator<_CharT> >::construct(std::allocator_traits<std::allocator<_CharT> >::allocator_type&, _Up*, _Args&& ...) [with _Up = std::pair<const int, Bar>; _Args = {const std::piecewise_construct_t&, std::tuple<const int&>, std::tuple<>}; _Tp = std::_Rb_tree_node<std::pair<const int, Bar> >; std::allocator_traits<std::allocator<_CharT> >::allocator_type = std::allocator<std::_Rb_tree_node<std::pair<const int, Bar> > >]’
/usr/include/c++/6/bits/stl_tree.h:543:32: required from ‘void std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_construct_node(std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Link_type, _Args&& ...) [with _Args = {const std::piecewise_construct_t&, std::tuple<const int&>, std::tuple<>}; _Key = int; _Val = std::pair<const int, Bar>; _KeyOfValue = std::_Select1st<std::pair<const int, Bar> >; _Compare = std::less<int>; _Alloc = std::allocator<std::pair<const int, Bar> >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Link_type = std::_Rb_tree_node<std::pair<const int, Bar> >*]’
/usr/include/c++/6/bits/stl_tree.h:560:4: 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<const int&>, std::tuple<>}; _Key = int; _Val = std::pair<const int, Bar>; _KeyOfValue = std::_Select1st<std::pair<const int, Bar> >; _Compare = std::less<int>; _Alloc = std::allocator<std::pair<const int, Bar> >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Link_type = std::_Rb_tree_node<std::pair<const int, Bar> >*]’
/usr/include/c++/6/bits/stl_tree.h:2149:64: required from ‘std::pair<std::_Rb_tree_iterator<_Val>, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_emplace_unique(_Args&& ...) [with _Args = {const std::piecewise_construct_t&, std::tuple<const int&>, std::tuple<>}; _Key = int; _Val = std::pair<const int, Bar>; _KeyOfValue = std::_Select1st<std::pair<const int, Bar> >; _Compare = std::less<int>; _Alloc = std::allocator<std::pair<const int, Bar> >]’
/usr/include/c++/6/bits/stl_map.h:559:64: required from ‘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>::emplace(_Args&& ...) [with _Args = {const std::piecewise_construct_t&, std::tuple<const int&>, std::tuple<>}; _Key = int; _Tp = Bar; _Compare = std::less<int>; _Alloc = std::allocator<std::pair<const int, Bar> >; 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, Bar> >]’
tes.cpp:24:47: required from here
/usr/include/c++/6/tuple:1590:70: error: use of deleted function ‘Bar::Bar()’
second(std::forward<_Args2>(std::get<_Indexes2>(__tuple2))...)
^
tes.cpp:10:8: note: ‘Bar::Bar()’ is implicitly deleted because the default definition would be ill-formed:
struct Bar
^~~
tes.cpp:10:8: error: no matching function for call to ‘Foo::Foo()’
tes.cpp:5:5: note: candidate: Foo::Foo(int, std::__cxx11::string)
Foo(int a, std::string b) {}
^~~
tes.cpp:5:5: note: candidate expects 2 arguments, 0 provided
How can I create an instance of Bar directly inside the map without the need of a copy- or move-construction of the class Foo?
