std::map<Key,T,Compare,Allocator>::value_comp

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std::map::value_compare value_comp() const;

Returns a function object that compares objects of type std::map::value_type (key-value pairs) by using key_comp to compare the first components of the pairs.

Parameters

(none)

Return value

The value comparison function object.

Complexity

Constant.

Example

Run this code
#include <cassert>
#include <iostream>
#include <map>
 
// 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::map<int, char, ModCmp> cont;
    cont = { { 1, 'a' }, { 2, 'b' }, { 3, 'c' }, { 4, 'd' }, { 5, 'e' } };
 
    auto comp_func = cont.value_comp();
 
    const std::pair<int, char> val = { 100, 'a' };
 
    for (auto it : cont) {
        bool before = comp_func(it, val);
        bool after = comp_func(val, it);
 
        std::cout << '(' << it.first << ',' << it.second;
        if (!before && !after)
            std::cout << ") equivalent to key " << val.first << '\n';
        else if (before)
            std::cout << ") goes before key " << val.first << '\n';
        else if (after)
            std::cout << ") goes after key " << val.first << '\n';
        else
            assert(0); // Cannot happen
    }
}

Output: 

(1,a) goes before key 100
(2,b) goes before key 100
(3,c) equivalent to key 100
(4,d) goes after key 100
(5,e) goes after key 100

See also

 returns the function that compares keys
(public member function)
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