std::map default value

Question

Is there a way to specify the default value std::map\'s operator[] returns when an key does not exist?

Answer 1

The value is initialized using the default constructor, as the other answers say. However, it is useful to add that in case of simple types (integral types such as int, float, pointer or POD (plan old data) types), the values are zero-initialized (or zeroed by value-initialization (which is effectively the same thing), depending on which version of C++ is used).

Anyway, the bottomline is, that maps with simple types will zero-initialize the new items automatically. So in some cases, there is no need to worry about explicitly specifying the default initial value.

std::map map;
typedef char *P;
char *p = map[123],
    *p1 = P(); // map uses the same construct inside, causes zero-initialization
assert(!p && !p1); // both will be 0

See Do the parentheses after the type name make a difference with new? for more details on the matter.