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| Categories: containers, functors | Component type: type |
Description
The function object hash<T> is a HashFunction; 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*, Rope, Rope, 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
HashFunction
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*
-
Rope
-
Rope
-
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) | HashFunction | Returns x's hash value. |
New members
All of hash's members are defined in the HashFunction 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.
See also
HashedAssociativeContainer, HashFunction
