Создание массива функторов для сравнения по полям структуры

field_comparer.cpp

#include <functional>
#include <algorithm>
#include <iostream>
#include <utility>
#include <string>
#include <vector>
#include <assert.h>

template <class type>
using compare     = std::function<bool(const type&, const type&)>;

template<
        class classT,
        class memberT,
        class member_ptrT = memberT classT::*,
        class cmpT = compare<memberT>
        >
compare<classT> make_cmp(
        memberT classT::*member,
        cmpT cmp = std::less<memberT>()
        ) {
    return  [cmp, member](const classT& a, const classT& b) -> bool {
            return cmp(a.*member, b.*member);
    };
}

template< class classT >
struct cmp {
    using cmpT = compare < classT > ;
    template< class memberT, class cmp_funcT = compare<memberT>>
    static cmpT make(memberT classT::*member, cmp_funcT cmp_func = std::less<memberT>()) {
        return  [cmp_func, member](const classT& a, const classT& b) -> bool {
            return cmp_func(a.*member, b.*member);
        };
    }
};

struct test_idx {
    char one;
    int two;
    std::string three;
    test_idx* four;
};
struct test_idx2 {
    char one;
    int two;
    std::string three;
    test_idx* four;
};
namespace type_checks {
    static_assert(
        std::is_same <
        decltype(make_cmp(&test_idx::one)),
        decltype(cmp<test_idx>().make(&test_idx::one)) > ::value,
        "Разные типы");
}
int main() {
    using namespace std;
    vector<test_idx> a{
        {'a',  100500, "hello", nullptr},
        {'z', 42, "a hell", (test_idx*)(0xdead)}
    };
    vector<test_idx2> b{
            { 'a', 100500, "hello", nullptr },
            { 'z', 42, "a hell", (test_idx*)(0xdead) }
    };
    vector<compare<test_idx>>
        idx_a{
        make_cmp(&test_idx::one),
        make_cmp(&test_idx::two),
        make_cmp(&test_idx::three),
        make_cmp(&test_idx::four)
    };
    vector<compare<test_idx2>>
        idx_b{
        make_cmp(&test_idx2::one),
        make_cmp(&test_idx2::two),
        make_cmp(&test_idx2::three),
        make_cmp(&test_idx2::four)
    };
    for (const auto& cmp : idx_a) {
		std::sort(a.begin(), a.end(), cmp);
		for (const auto& i : a)
			cout << '{'
			<< i.one << ", "
			<< i.two << ", "
			<< i.three << ", "
			<< i.four << "} ";
		cout << endl;
    }
    cout << endl;
    for (const auto& cmp : idx_b) {
        std::sort(b.begin(), b.end(), cmp);
		for (const auto& i : b)
			cout << '{'
			<< i.one << ", "
			<< i.two << ", "
			<< i.three << ", "
			<< i.four << "} ";
		cout << endl;
    }
    return 0;
}