hash<T>



Categories: containers, functors	Component type: type

Description

The function object hash<T> is a Hash Function; it is used as the default hash function by all of the Hashed Associative Containers that are included in the STL.

The hash<T> template is only defined for template arguments of type char*, const char*, crope, wrope, and the built-in integral types. [1] If you need a Hash Function with a different argument type, you must either provide your own template specialization or else use a different Hash Function.

Example

int main()
{
  hash<const char*> H;
  cout << "foo -> " << H("foo") << endl;
  cout << "bar -> " << H("bar") << endl;
}

Definition

Defined in the headers hash_map and hash_set, and in the backward-compatibility headers hash_map.h and hash_set.h. This class is an SGI extension; it is not part of the C++ standard.

Template parameters

Parameter	Description	Default
`T`	The argument type. That is, the type of object that is being hashed.

Model of

Hash Function

Type requirements

T must be a type for which a specialization of hash has been defined. The STL defines the following specializations:

char*
const char*
crope
wrope
char
signed char
unsigned char
short
unsigned short
int
unsigned int
long
unsigned long

Public base classes

None.

Members

Member	Where defined	Description
`size_t operator()(const T& x)`	Hash Function	Returns `x`'s hash value.

New members

All of hash's members are defined in the Hash Function requirements. Hash does not introduce any new members.

Notes

[1] Technically, what this means is that the actual template hash<T> is an empty class; the member function operator() is defined only in the various specializations.