value_compare Class

The latest version of this topic can be found at value_compare Class.

Provides a function object that can compare the elements of a hash_map by comparing the values of their keys to determine their relative order in the hash_map.

Syntax

class value_compare
 : std::public binary_function<value_type, value_type, bool>
{
public:
    bool operator()(
    const value_type& left,
    const value_type& right) const
 {
    return (comp(left.first, right.first));

 }
protected:
    value_compare(const key_compare& c) : comp (c) { }
    key_compare comp;
};

Remarks

The comparison criteria provided by value_compare between value_types of whole elements contained by a hash_map is induced from a comparison between the keys of the respective elements by the auxiliary class construction. The member function operator uses the object comp of type key_compare stored in the function object provided by value_compare to compare the sort-key components of two elements.

For hash_sets and hash_multisets, which are simple containers where the key values are identical to the element values, value_compare is equivalent to key_compare; for hash_maps and hash_multimaps they are not, because the value of the type pair elements is not identical to the value of the element's key.

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.

Example

See the example for hash_map::value_comp for an example of how to declare and use value_compare.

Requirements

Header: <hash_map>

Namespace: stdext

See Also

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