std::map 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 (key-value pairs) by using key_comp() to compare the first components of the pairs.

Parameters

(none)

Return value

The value comparision function object.

Complexity

Constant - O(1).

Example

Main.cpp

#include <map>
#include <iterator>
#include <iostream>
#include <utility>
#include <initializer_list>
 
void print(auto const comment, auto const& container)
{
    auto size = std::size(container);
    std::cout << comment << "{ ";
    for (auto const& [key, value]: container)
        std::cout << '{' << key << ',' << value << (--size ? "}, " : "} ");
    std::cout << "}\n";
}
 
int main()
{
    std::map<int, int> x { {1,1}, {2,2}, {3,3} }, y, z;
    const auto w = { std::pair<const int, int>{4,4}, {5,5}, {6,6}, {7,7} };
 
    std::cout << "Initially:\n";
    print("x = ", x);
    print("y = ", y);
    print("z = ", z);
 
    std::cout << "Copy assignment copies data from x to y:\n";
    y = x;
    print("x = ", x);
    print("y = ", y);
 
    std::cout << "Move assignment moves data from x to z, modifying both x and z:\n";
    z = std::move(x);
    print("x = ", x);
    print("z = ", z);
 
    std::cout << "Assignment of initializer_list w to z:\n";
    z = w;
    print("w = ", w);
    print("z = ", z);
}

Output

Initially:
x = { {1,1}, {2,2}, {3,3} }
y = { }
z = { }
Copy assignment copies data from x to y:
x = { {1,1}, {2,2}, {3,3} }
y = { {1,1}, {2,2}, {3,3} }
Move assignment moves data from x to z, modifying both x and z:
x = { }
z = { {1,1}, {2,2}, {3,3} }
Assignment of initializer_list w to z:
w = { {4,4}, {5,5}, {6,6}, {7,7} }
z = { {4,4}, {5,5}, {6,6}, {7,7} }