Most efficient way to assign values to maps

Question

Which way to assign values to a map is most efficient? Or are they all optimized to the same code (on most modern compilers)?

   // 1) Assignment using array ind         


        

Answer 1

If there is no object at that key location, then:

std::map::emplace is most efficient. insert is second (but will be extremely close). [] is least efficient.

[], if there is no object there, trivial constructs one. It then calls operator=.

insert does a copy constructor call on the std::pair argument.

However, in the case of maps, map.insert( make_pair( std::move(key), std::move(value) ) ) is going to be close to map.emplace( std::move(key), std::move(value) ).

If there is an object at the key location, then [] will call operator=, while insert/emplace will destroy the old object and create a new one. [] could easily be cheaper in this case.

In the end, it depends on what your operator= vs copy-construct vs trivial-construct vs destructor costs are for your key and value.

The actual work looking things up within the tree structure of the std::map will be so close to identical it isn't funny.