partial_sorting_functor.cpp

#include <algorithm>
#include <iomanip>
#include <iostream>
#include <vector>

template <typename T1, typename T2>
struct Functor {
    T1 T2::*memPointer;
    Functor(T1 T2::*t) : memPointer(t) {}
    bool operator()(const T2& obj1, const T2& obj) {
        return obj1.*memPointer < (obj.*memPointer);
    }
};

struct Foo {
    double x;
    double y;
};

// Utility type deduction factory
template <typename T1, typename T2>
Functor<T1, T2> sort_by(T1 T2::*t) {
    return Functor<T1, T2>(t);
}

int main() {
    std::vector<Foo> v{{9, 89}, {10, 10}, {5, 100}};

    std::cout << "Sorting by the first member x" << std::endl;
    std::sort(v.begin(), v.end(), sort_by(&Foo::x));
    for (auto&& elem : v) {
        std::cout << std::setw(2) << elem.x << " -> " << elem.y << std::endl;
    }

    std::cout << "Sorting by the second member y" << std::endl;
    std::sort(v.begin(), v.end(), sort_by(&Foo::y));
    for (auto&& elem : v) {
        std::cout << std::setw(2) << elem.x << " -> " << elem.y << std::endl;
    }
}