return reference to temporary when dereferencing map iterator

Question

Consider this code

#include 
#include 

const int& foo(const std::vector& x,unsigned i) {
    auto it = x.be         


        

Answer 1

The problem is that the value_type of std::map<int,int> is not std::pair<int,int>, but std::pair<const int,int>. Then for return *it;, a temporary std::pair<int,int> has to be created and returned. (std::pair<int,int> could be converted from std::pair<const int,int>.) The temporary will be destroyed immediately and left the returned reference dangled.

To sovle the issue you can change the return type to const std::pair<const int,int>& or const std::map<int,int>::value_type &.