Question

A while ago, I had a discussion with a colleague about how to insert values in STL maps. I preferred map[key] = value; because it feels natural and is clear to read whereas he preferred map.insert(std::make_pair(key, value)).

I just asked him and neither of us can remember the reason why insert is better, but I am sure it was not just a style preference rather there was a technical reason such as efficiency. The SGI STL reference simply says: "Strictly speaking, this member function is unnecessary: it exists only for convenience."

Can anybody tell me that reason, or am I just dreaming that there is one?

Answer 1

When you write

map[key] = value; 

there’s no way to tell if you replaced the value for key, or if you created a new key with value.

map::insert() will only create:

using std::cout; using std::endl; typedef std::map<int, std::string> MyMap; MyMap map; // ... std::pair<MyMap::iterator, bool> res = map.insert(MyMap::value_type(key,value)); if ( ! res.second ) {     cout << "key " <<  key << " already exists "          << " with value " << (res.first)->second << endl; } else {     cout << "created key " << key << " with value " << value << endl; } 

For most of my apps, I usually don’t care if I’m creating or replacing, so I use the easier to read map[key] = value.