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

From cppreference.com
< cpp‎ | container‎ | map
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.

Contents

[edit] Parameters

(none)

[edit] Return value

The value comparison function object.

[edit] Complexity

Constant.

[edit] Example

#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

[edit] See also

returns the function that compares keys
(public member function) [edit]