STL MAP should use find() or [n] identifier to find element in map?

Question

I am confused which is more efficient?

As we can access map directly, why do we need to use find?

I just need to know which way is more efficient.

Answer 1

Using find means that you don't inadvertently create a new element in the map if the key doesn't exist, and -- more importantly -- this means that you can use find to look up an element if all you have is a constant reference to the map.

That of course means that you should check the return value of find. Typically it goes like this:

void somewhere(const std::map & mymap, K const & key)
{
    auto it = mymap.find(key);
    if (it == mymap.end()) { /* not found! */ }
    else                   { do_something_with(it->second); }
}