std::multiset value_comp() method

since C++98

// Const version only
value_compare value_comp() const;

Returns a function object that compares objects of type value_type by comparing the first components of the pairs, same as key_comp().

Parameters

(none)

Return value

The value comparison function object.

Complexity

Constant - O(1).

Exceptions

(none)

Notes

Feature testing macro: __cpp_lib_generic_associative_lookup (for overloads (3) and (4)).

Example

Main.cpp

#include <cassert>
#include <iostream>
#include <set>
 
// Example module 97 key compare function
struct ModCmp {
    bool operator()(const int lhs, const int rhs) const
    {
        return (lhs % 97) < (rhs % 97);
    }
};
 
int main()
{
    std::multiset<int, ModCmp> cont{1, 2, 3, 4, 5};
 
    // Same behaviour as key_comp()
    auto comp_func = cont.value_comp();
 
    const int val = 100;
 
    for (int key : cont) {
        bool before = comp_func(key, val);
        bool after = comp_func(val, key);
        if (!before && !after)
            std::cout << key << " equivalent to key " << val << '\n';
        else if (before)
            std::cout << key << " goes before key " << val << '\n';
        else if (after)
            std::cout << key << " goes after key " << val << '\n';
        else
            assert(0); // Cannot happen
    }
}

Possible output

1 goes before key 100
2 goes before key 100
3 equivalent to key 100
4 goes after key 100
5 goes after key 100