The following code should compute the difference of a map and a vector
std::map<int, int> cursorMap;
QVector<User> userList;
...
std::vector<int> offlineUserIds{};
std::vector<int>::iterator it;
it = std::set_difference(cursorMap.begin(), cursorMap.end(), userList.begin(), userList.end(), offlineUserIds.begin(),
[](const std::pair<int, int> &e, const User &u){ return u.getId() == e.first; });
before invoking set_difference, userList is converted to a std::vector and sorted. The problem is that it gaves me the following errors:
error: cannot convert 'std::pair<const int, int>' to 'int' in assignment
...
error: no match for call to '(TextEditor::updateCursorMap(QVector<User>)::<lambda(const std::pair<int, int>&, const User&)>) (User&, std::pair<const int, int>&)'
{ return bool(_M_comp(*__it1, *__it2)); }
...
note: candidate: 'TextEditor::updateCursorMap(QVector<User>)::<lambda(const std::pair<int, int>&, const User&)>'
...
it = std::set_difference(cursorMap.begin(), cursorMap.end(), userList.begin(), userList.end(), offlineUserIds.begin(), [](const std::pair<int, int> &e, const User &u){ return u.getId() == e.first; });
...
note: no known conversion for argument 1 from 'User' to 'const std::pair<int, int>&'
EDIT:
I tried the following code
std::map<int, int> cursorMap;
QVector<User> userList;
...
std::vector<std::pair<int, int>> offlineUserIds{};
std::vector<std::pair<int, int>>::iterator it;
it = std::set_difference(cursorMap.begin(), cursorMap.end(), userList.begin(), userList.end(), offlineUserIds.begin(), [](const std::pair<int, int> &e, const User &u){ return e.first < u.getId(); });
but now it gives me
note: candidate: 'TextEditor::updateCursorMap(QVector<User>)::<lambda(const std::pair<int, int>&, const User&)>'
it = std::set_difference(cursorMap.begin(), cursorMap.end(), userList.begin(), userList.end(), offlineUserIds.begin(), [](const std::pair<int, int> &e, const User &u){ return e.first < u.getId(); });
...
note: no known conversion for argument 1 from 'User' to 'const std::pair<int, int>&'
EDIT2:
Here is a minimal reproducible example:
#include <iostream>
#include <vector>
#include <map>
#include <algorithm>
class User {
int id;
public:
int getId() const { return id; }
};
int main() {
std::vector<User> newUserList{};
std::map<int, int> cursorMap{};
std::vector<User> userList = std::vector(newUserList.begin(), newUserList.end());
std::sort(userList.begin(), userList.end(), [](const User &u1, const User &u2) { return u1.getId() < u2.getId(); });
std::vector<std::pair<int, int>> offlineUserIds{};
std::vector<std::pair<int, int>>::iterator it;
it = std::set_difference(cursorMap.begin(), cursorMap.end(), userList.begin(), userList.end(), offlineUserIds.begin(), [](std::pair<int, int> e, User u){ return e.first < u.getId(); });
return 0;
}
build output
In file included from C:/Qt/Tools/mingw810_64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_algobase.h:71,
from C:/Qt/Tools/mingw810_64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/char_traits.h:39,
from C:/Qt/Tools/mingw810_64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/ios:40,
from C:/Qt/Tools/mingw810_64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/ostream:38,
from C:/Qt/Tools/mingw810_64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/iostream:39,
from D:\asant\workspace\CLionProjects\untitled\main.cpp:1:
C:/Qt/Tools/mingw810_64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/predefined_ops.h: In instantiation of 'constexpr bool __gnu_cxx::__ops::_Iter_comp_iter<_Compare>::operator()(_Iterator1, _Iterator2) [with _Iterator1 = __gnu_cxx::__normal_iterator<User*, std::vector<User> >; _Iterator2 = std::_Rb_tree_iterator<std::pair<const int, int> >; _Compare = main()::<lambda(std::pair<int, int>, User)>]':
C:/Qt/Tools/mingw810_64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_algo.h:5343:17: required from '_OutputIterator std::__set_difference(_InputIterator1, _InputIterator1, _InputIterator2, _InputIterator2, _OutputIterator, _Compare) [with _InputIterator1 = std::_Rb_tree_iterator<std::pair<const int, int> >; _InputIterator2 = __gnu_cxx::__normal_iterator<User*, std::vector<User> >; _OutputIterator = __gnu_cxx::__normal_iterator<std::pair<int, int>*, std::vector<std::pair<int, int> > >; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<main()::<lambda(std::pair<int, int>, User)> >]'
C:/Qt/Tools/mingw810_64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_algo.h:5447:46: required from '_OIter std::set_difference(_IIter1, _IIter1, _IIter2, _IIter2, _OIter, _Compare) [with _IIter1 = std::_Rb_tree_iterator<std::pair<const int, int> >; _IIter2 = __gnu_cxx::__normal_iterator<User*, std::vector<User> >; _OIter = __gnu_cxx::__normal_iterator<std::pair<int, int>*, std::vector<std::pair<int, int> > >; _Compare = main()::<lambda(std::pair<int, int>, User)>]'
D:\asant\workspace\CLionProjects\untitled\main.cpp:21:188: required from here
C:/Qt/Tools/mingw810_64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/predefined_ops.h:143:18: error: no match for call to '(main()::<lambda(std::pair<int, int>, User)>) (User&, std::pair<const int, int>&)'
{ return bool(_M_comp(*__it1, *__it2)); }
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:/Qt/Tools/mingw810_64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/predefined_ops.h:143:18: note: candidate: 'bool (*)(std::pair<int, int>, User)' <conversion>
C:/Qt/Tools/mingw810_64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/predefined_ops.h:143:18: note: candidate expects 3 arguments, 3 provided
D:\asant\workspace\CLionProjects\untitled\main.cpp:21:156: note: candidate: 'main()::<lambda(std::pair<int, int>, User)>'
it = std::set_difference(cursorMap.begin(), cursorMap.end(), userList.begin(), userList.end(), offlineUserIds.begin(), [](std::pair<int, int> e, User u){ return e.first < u.getId(); });
^
D:\asant\workspace\CLionProjects\untitled\main.cpp:21:156: note: no known conversion for argument 1 from 'User' to 'std::pair<int, int>'
mingw32-make.exe[3]: *** [CMakeFiles\untitled.dir\build.make:82: CMakeFiles/untitled.dir/main.cpp.obj] Error 1
mingw32-make.exe[2]: *** [CMakeFiles\Makefile2:95: CMakeFiles/untitled.dir/all] Error 2
mingw32-make.exe[1]: *** [CMakeFiles\Makefile2:102: CMakeFiles/untitled.dir/rule] Error 2
mingw32-make.exe: *** [Makefile:137: untitled] Error 2
For a reason that is not known to me:
The types Type1 and Type2 must be such that objects of types InputIt1 and InputIt2 can be dereferenced and then implicitly converted to both Type1 and Type2
See cppreference. Last sentence under Parameters -> comp. Highlighted by me.