Use map::find
:
auto it = myMap.find(key);
if (it != myMap.end())
{
// use it->second
}
else
{
// not found
}
This part was easy. The harder problem is when you want to look up if an element exists and return it if it does, but otherwise insert a new element at that key, all without searching the map twice. For that you need to use lower_bound
followed by hinted insertion.