make_pair does not work with string, works with string&

Question

counts[k] = make_pair<string, int>(s, count_inversion(pos, 0, pos.size()));

Whenever make_pair is used like make_pair<string,int>(s,i) it gives an error:

error: no matching function to call for 'make_pair(std::__cxx11::string&, int)'
note: cannot convert 's' (type 'std::__cxx11::string {aka std::__cxx11::basic_string<char>}') to type 'std::__cxx11::basic_string<char>&&'

However make_pair<string&,int>(s,i) works fine. Can somebody explain this. If the variable s is changed after make_pair call in the latter case, will it affect the pair?

Answer 1

std::make_pair is intended to be used without explicitly specifying template arguments. And since C++11, it uses perfect forwarding, which makes it very hard (and even more pointless) to supply template arguments manually.

Use std::make_pair when you want to deduce the types in the pair from the types of the arguments to make_pair.

Use std::pair<T, U> when you want explicit control of the types in the pair.

Notice that in your case, calling make_pair(s, count_inversion(pos, 0, pos.size())) will actually deduce the templte parameters to string& and whatever the return type of count_inversion is. This doesn't mean the pair will contain a reference, it's just perfect forwarding in action. So the produced pair will contain a copy of s and will not be affected by future changes to s.

Using std::pair<string&, int> would be different and would indeed give you a live reference to s in the pair.