value-compare-class.md

description	title	ms.date	f1_keywords	helpviewer_keywords	ms.assetid
Learn more about: value_compare Class	value_compare Class	11/04/2016	hash_map/std::value_compare	value_compare class	c306c5b9-3505-4357-aa6b-216451b951ed

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.

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
C++ Standard Library Reference