What is the difference between the index overloaded operator and the insert method call for std::map?
ie:
some_map["x"] = 500;
vs.
some_map.insert(pair<std::string, int>("x", 500));
What is the difference between the index overloaded operator and the insert method call for std::map?
ie:
some_map["x"] = 500;
vs.
some_map.insert(pair<std::string, int>("x", 500));
The insert method inserts into the map, while the overloaded index operator will return the element with the key key_value if it is in the map, if it is not already in the map then it will insert it.
I believe insert() will not overwrite an existing value, and the result of the operation can be checked by testing the bool value in the iterator/pair value returned
The assignment to the subscript operator [] just overwrites whatever's there (inserting an entry if there isn't one there already)
Either of the insert and [] operators can cause issues if you're not expecting that behaviour and don't accommodate for it.
Eg with insert:
std::map< int, std::string* > intMap;
std::string* s1 = new std::string;
std::string* s2 = new std::string;
intMap.insert( std::make_pair( 100, s1 ) ); // inserted
intMap.insert( std::make_pair( 100, s2 ) ); // fails, s2 not in map, could leak if not tidied up
and with [] operator:
std::map< int, std::string* > intMap;
std::string* s1 = new std::string;
std::string* s2 = new std::string;
intMap[ 100 ] = s1; // inserted
intMap[ 100 ] = s2; // inserted, s1 now dropped from map, could leak if not tidied up
I think those are correct, but haven't compiled them, so may have syntax errors
For a map
, the former (operator[]
) expression will always replace the value part of the key-value pair with the new supplied value. A new key-value pair will be inserted if one doesn't already exist.
In contrast, insert
will only insert a new key-value pair if a key-value pair with the supplied key part does not already exist in the map.
In addition to the fact that map::operator[]
will replace an existing value is that operator[]
map::will create and add to the map a default existing value to replace before the replacement occurs (the map::operator[]()
call has to return a reference to something). For items that are expensive to create this could be a performance issue.
See "Item 24: Choose carefully between map::operator[]
and map::insert
when efficiency is important" in Scott Meyers' Effective STL.