std::binary_function – cppreference.com

Original:

binary_function is a base class for creating function objects with two arguments.

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Original:

binary_function does not define operator(); it is expected that derived classes will define this. binary_function provides only three types - first_argument_type, second_argument_type and result_type - defined by the template parameters.

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Original:

Some standard library function object adaptors, such as std::not2, require the function objects they adapt to have certain types defined; std::not2 requires the function object being adapted to have two types named first_argument_type and second_argument_type. Deriving function objects that take two arguments from binary_function is an easy way to make them compatible with those adaptors.

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Original:

binary_function is deprecated in C++11. Its functionality has been made obsolete by std::function and other standard library function objects, which define the necessary types.

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#include <algorithm>
#include <functional>
#include <iostream>
#include <vector>

struct same : std::binary_function<int, int, bool>
{
    bool operator()(int a, int b) const { return a == b; }
};

int main()
{
    std::vector<int> v1{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
    std::vector<int> v2{10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0 };
    std::vector<bool> v3(v1.size());
    
    std::transform(v1.begin(), v1.end(), v2.begin(), v3.begin(), std::not2(same()));
    
    std::cout << std::boolalpha;
    for (std::size_t i = 0; i < v1.size(); ++i)
        std::cout << v1[i] << ' ' << v2[i] << ' ' << v3[i] << '\n';
}

0 10 true
1 9 true
2 8 true
3 7 true
4 6 true
5 5 false
6 4 true
7 3 true
8 2 true
9 1 true