Is searching first with std::map::find then using [], to retrieve the value, optimised usually?-CodePudding

I find often this pattern in some codebase I work with:

   std::map<std::string, std::string> mymap;

   ...

   if (mymap.find(key) != mymap.end())
   {
       return mymap[key];
   }

I know this can be replaced by

   std::map<std::string, std::string> mymap;

   ...

   auto it = mymap.find(key) 
   if (it != mymap.end())
   {
       return it->second;
   }

But I was wondering if, in practice, the first version would get optimised to not search twice anyway?

CodePudding user response：

No it will not be optimized. The pattern is too high level.

You could preferentially just do this instead:

 std::map<std::string, std::string> mymap;

   ...

   auto it = mymap.find(key);
   if ( it != mymap.end())
   {
       return it->second;
   }

You can check on the benchmark below:

int findOptim( int key )
{
   auto it = mymap.find(key); 
   if (it != mymap.end())
   {
       return it->second;
   }
   return -1;
}

int findTrivial( int key ) {
    if ( mymap.find(key) != mymap.end() ) {
        return mymap[key];
    }
    return -1;
}

Benchmark: