为什么std :: map :: insert不能使用键和值而不是std :: pair？

Question

std::map has the member function

template <class P> pair<iterator,bool> insert (P&& val);

which we use eg like so:

my_map.insert(std::make_pair(k, v));

Question is, why isn't there a variant of insert() which just takes a key and a value, ie

pair<iterator,bool> insert (K&& key, V&& value);

(with K and V being the template parameters of the map of course) which we would use eg like so:

my_map.insert(k, v);

That seems to me perfectly reasonable to have.

Answer 1

This was added in C++11 with the emplace members for all containers, which for map has the signature

template< class... Args >
std::pair<iterator,bool> emplace( Args&&... args );

Answer 2

If you're looking for a more obvious syntax, you can perform the same insertion using the key and values as separate entities like this:

my_map[k] = v;

Granted, this is two separate operations. An insert followed by an assignment so it may not be as efficient as a single insert. But then again, maps store store their entries as pairs so a call to an insert(k, v) function would probably wind up creating a pair<k, v> anyway.