the type of map's key

Just saw a question on STL. The question is "<"does not need to be overloaded when the key of map belongs to certain types. What are thes开发者_JAVA百科e types?

Don't quite understand this question! Thanks for answering.

Assuming the map is instantiated with the default comparator (i.e. as map<Key,Value> with no third argument):

• Built-in numeric types
• Pointers
• Any type for which std::less<Key> has been specialised (as long as the specialisation doesn't require operator<).
• Pedantically, any type which already has an overload of operator<.

For any other key type, the map will try to compare them using an expression like key1 < key2, which will only compile if there is an overload of operator< for the key type.

Basically, primitive types for which < is already defined.