Re: [g++] make_pair inlining failure?

Marc Glisse <marc.glisse@xxxxxxxx> · Mon, 28 Nov 2011 15:42:09 +0100 (CET)

On Mon, 28 Nov 2011, John Doe wrote:

Currently I am participating in a project where a large number of
objects are inserted into a std::map. Recently someone replaced all calls of
  map_instance.insert( std::make_pair(key, value) );
with
  map_instance.insert( MapType::value_type(key, value) );
claiming it would be faster.

A classic. insert takes a pair<const Key,Value> whereas make_pair produces 
a pair<Key,Value>, so the first thing the compiler has to do is convert 
one into the other, ie copy. The set of circumstances where a compiler is 
allowed to elide a copy is extremely restrictive.

With C++11 you could use emplace instead.

--
Marc Glisse