hash_compare Class


For the latest documentation on Visual Studio 2017, see Visual Studio 2017 Documentation.

For the latest documentation on Visual Studio 2017, see hash_compare Class on docs.microsoft.com. The template class describes an object that can be used by any of the hash associative containers — hash_map, hash_multimap, hash_set, or hash_multiset — as a default Traits parameter object to order and hash the elements they contain.

class hash_compare { Traits comp; public: const size_t bucket_size = 4; const size_t min_buckets = 8; hash_compare(); hash_compare(Traits pred); size_t operator()(const Key& key) const; bool operator()( const Key& key1, const Key& key2) const; };

Each hash associative container stores a hash traits object of type Traits (a template parameter). You can derive a class from a specialization of hash_compare to selectively override certain functions and objects, or you can supply your own version of this class if you meet certain minimum requirements. Specifically, for an object hash_comp of type hash_compare<Key, Traits>, the following behavior is required by the above containers:

  • For all values key of type Key, the call hash_comp( key) serves as a hash function, which yields a distribution of values of type size_t. The function supplied by hash_compare returns key.

  • For any value key1 of type Key that precedes key2 in the sequence and has the same hash value (value returned by the hash function), hash_comp( key2, key1) is false. The function must impose a total ordering on values of type Key. The function supplied by hash_compare returns comp( key2, key1) , where comp is a stored object of type Traits that you can specify when you construct the object hash_comp. For the default Traits parameter type less<Key>, sort keys never decrease in value.

  • The integer constant bucket_size specifies the mean number of elements per "bucket" (hash-table entry) that the container should try not to exceed. It must be greater than zero. The value supplied by hash_compare is 4.

  • The integer constant min_buckets specifies the minimum number of buckets to maintain in the hash table. It must be a power of two and greater than zero. The value supplied by hash_compare is 8.

In Visual C++ .NET 2003, members of the <hash_map> and <hash_set> header files are no longer in the std namespace, but rather have been moved into the stdext namespace. See The stdext Namespace for more information.

See examples for hash_map::hash_map, hash_multimap::hash_multimap, hash_set::hash_set, and hash_multiset::hash_multiset, for examples of how to declare and use hash_compare.

Header: <hash_map>

Namespace: stdext

Thread Safety in the C++ Standard Library
Standard Template Library