insert vs emplace vs operator[] in c++ map

Question

I\'m using maps for the first time and I realized that there are many ways to insert an element. You can use emplace(), operator[] or insert(

Answer 1

Emplace: Takes advantage of the rvalue reference to use the actual objects that you have already created. This means that no copy or move constructor is called, good for LARGE objects! O(log(N)) time.

Insert: Has overloads for standard lvalue reference and rvalue reference, as well as iterators to lists of elements to insert, and "hints" as to the position an element belongs. The use of a "hint" iterator can bring the time insertion takes down to contant time, otherwise it is O(log(N)) time.

Operator[]: Checks to see if the object exists, and if it does, modifies the reference to this object, otherwise uses the provided key and value to call make_pair on the two objects, and then does the same work as the insert function. This is O(log(N)) time.

make_pair: Does little more than make a pair.

There was no "need" for adding emplace to the standard. In c++11 I believe the && type of reference was added. This removed the necessity for move semantics, and allowed optimization of some specific type of memory management. In particular, the rvalue reference. The overloaded insert(value_type &&) operator does not take advantage of the in_place semantics, and is therefore much less efficient. While it provides the capability of dealing with rvalue references, it ignores their key purpose, which is in place construction of objects.