std::map::insert change in C++17

Question

I see that the insert method of std::map and std::unordered_map is going to change from

template<class P> std::pair<iterator,bool> insert(P&& value); (C++11)

to

std::pair<iterator,bool> insert(value_type&& value);  (C++17)

However, for these containers, value_type is std::pair<A const, int> . Two questions here:

1. Why this change? What is the upside?
2. How is this going to work to move a key on insertion? The C++11 version accepts anything (the constraint on P is default_constructible<value_type, P&&> ), then std::pair<A, int> - which is most of the time the type of this argument as it is the one returned by std::make_pair - and can call the move constructor of A . But in the C++17 version, this argument is casted to value_type , where A is const, then non-movable. A has to be copied, if I am not overlooking something. Or does C++17 change anything on that side too?

Thanks!

Answer 1

An additional non-template overload to insert was added in C++17.

Such an overload has the advantage that it permits .insert( { {key}, {value, args} } ) syntax -- {} based construction. template arguments cannot be passed {} based construction instructions without an explicit type.