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#include <map>
#include <vector>
#include <iostream>
 
int main() {
 
	std::map<int *, std::vector<double>> m;
	const auto &cm = m;
	std::cout << cm[nullptr].size();
}

prog.cpp: In function 'int main()':
prog.cpp:9:25: error: passing 'const std::map<int*, std::vector<double> >' as 'this' argument discards qualifiers [-fpermissive]
  std::cout << cm[nullptr].size();
                         ^
In file included from /usr/include/c++/5/map:61:0,
                 from prog.cpp:1:
/usr/include/c++/5/bits/stl_map.h:494:7: note:   in call to '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 = int*; _Tp = std::vector<double>; _Compare = std::less<int*>; _Alloc = std::allocator<std::pair<int* const, std::vector<double> > >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = std::vector<double>; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = int*]'
       operator[](key_type&& __k)
       ^

Standard output is empty