c++ - Equality operator (==) unsupported on map iterators for non-copyable maps -

when have map of non-copyable objects, why can't compare iterators using ==? imagine not need copy (or move) actual objects when doing equality testing on iterators.

#include <iostream> #include <utility> #include <map>  using namespace std;  class { private:     int i; public:     a(int i) : i(i) {}     a(a&) = delete;     a(a&& a) : i(a.i) {}     ~a() {}     a& operator=(a&) = delete;      bool operator==(const a& a) const { return == a.i; } };  int main() {     map<int, a> mymap;      map<int, a>::iterator = mymap.find(1);     cout << (it == mymap.end()) << endl; } 

this example fails compile, giving error on line cout.

g++ gives error:

/usr/include/c++/4.8.2/bits/stl_pair.h: in instantiation of ‘struct std::pair<const int, a>’: test2.cpp:24:27:   required here /usr/include/c++/4.8.2/bits/stl_pair.h:127:17: error: ‘constexpr std::pair<_t1, _t2>::pair(const std::pair<_t1, _t2>&) [with _t1 = const int; _t2 = a]’ declared take     const reference, implicit declaration take non-const    constexpr pair(const pair&) = default; 

clang++ gives error:

/usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/4.8.2/../../../../include/c++/4.8.2/bits/stl_pair.h:127:17: error: parameter explicitly-defaulted copy constructor const, member or base requires non-const       constexpr pair(const pair&) = default;                 ^ test2.cpp:24:14: note: in instantiation of template class 'std::pair<const int, a>' requested here         cout << (it == mymap.end()) << endl; 

however, work if map<int, int> used instead of map<int, a>. using const map<int, a> map<int, a>::const_iterator not help.

i tried looking exact signature of map::iterator::operator== on cppreference, (map::iterator bidirectionaliterator, forwarditerator, inputiterator) website vague exact type signatures in concepts.

the problem deleted methods make noncopyable ...

#include <iostream> #include <utility> #include <map>  using namespace std;  class { private:     int i; public:     a(int i) : i(i) {}     // a(a&) = delete; should ...     a(const a&) = delete;     a(a&& a) : i(a.i) {}     ~a() {}     // a& operator=(a&) = delete; should ...     a& operator=(const a&) = delete;      bool operator==(const a& a) const { return == a.i; } };  int main() {     map<int, a> mymap;      map<int, a>::iterator = mymap.find(1);     cout << (it == mymap.end()) << endl; } 

verified gcc/g++ 4.6.3

removing "const" copy constructor , assignment operator caused compiler give same error received.

i have details/references why declared way, copy constructor , assignment operator take const reference (make sense value instance being assigned should not modified method).