Make a vector of comparator functions

Question

My problem is to test an implementation of quicksort with different comparator functions: std::less and std::greater. But I do not want to copy-paste testing code that differs just in two comparators, so I would like to put them into a vector (or maybe something else?) and iterate over them.

To simplify this post, lets say I would like to write a loop over a vector of two functions that get 0 and 1 as their arguments and output a boolean. Here is my take at that:

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

int main() {
    auto fs = std::vector<>{std::less<int>{}, std::greater<int>{}};

    for (auto f: fs) {
        std::cout << f(0, 1) << " ";
    } std::cout << std::endl;
}

My g++ 6.1.1 compiler rightfully complains that I have not specified the template arguments for the vector. I have been trying things like std::function<bool(int, int)> and others with no luck.

Could you tell me how to fix this piece of code?

Update: The exact error I am getting:

% g++ -std=c++14 -Wall deleteme.cpp && ./a.out
deleteme.cpp: In function ‘int main()’:
deleteme.cpp:6:27: error: wrong number of template arguments (0, should be at least 1)
     auto fs = std::vector<>{std::less<int>{}, std::greater<int>{}};
                           ^
In file included from /usr/include/c++/6.1.1/vector:64:0,
                 from deleteme.cpp:2:
/usr/include/c++/6.1.1/bits/stl_vector.h:214:11: note: provided for ‘template<class _Tp, class _Alloc> class std::vector’
     class vector : protected _Vector_base<_Tp, _Alloc>
           ^~~~~~
deleteme.cpp:8:18: error: unable to deduce ‘auto&&’ from ‘fs’
     for (auto f: fs) {
                  ^~

Can you add your compilation error? – JVApen Aug 21 '16 at 11:22 — JVApen, Aug 21 '16 at 11:22

score 2 · Accepted Answer · edited May 23 '17 at 10:28

2

Only from C++17, template arguments of the constructor can be used to deduce the template arguments of the type, so you will have to write std::vector<std::function<bool(int,int)>> instead of std::vector<>.

Please note that std::function has a performance overhead compared to calling functions directly, so you might want to check out variadic template arguments (and swallowing) for getting the last few percentages

edited May 23 '17 at 10:28

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answered Aug 21 '16 at 11:26

JVApen

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Thank you for your reply. I have added `std::function` back into vector and it worked. I could swear it did not work 30 mins ago. – alisianoi Aug 21 '16 at 11:30
Could you please elaborate on your second paragraph about `std::greater`, `std::greater{}` and `std::greater()`? For me, the first is just a type, the second and the third are calling default constructors for the `struct std::greater` and do not differ. Do they differ? – alisianoi Aug 21 '16 at 11:32
Sorry, my bad, that is wrong. I'm used to work with a codebase which has similar concepts as the STL and sometimes mix them up. std::greater is indeed a struct, not a free function. – JVApen Aug 21 '16 at 11:33

score 0 · Answer 2 · answered Aug 22 '16 at 01:34

As you use different types, you might use tuple, and iterate over tuple:

namespace detail {
    template <typename F, typename TUPLE, std::size_t ... Is>
    void run_for_each(F&& f, TUPLE&& t, std::index_sequence<Is...>)
    {
        const int dummy[] = {0, (f(std::get<Is>(std::forward<TUPLE>(t))), void(), 0)...};
        static_cast<void>(dummy); // Avoid warning for unused variable
    }
}

template <typename F, typename TUPLE>
void run_for_each(F&& f, TUPLE&& t)
{
     detail::run_for_each(std::forward<F>(f),
                          std::forward<TUPLE>(t),
                          std::make_index_sequence<
                              std::tuple_size<std::decay_t<TUPLE>>::value>());
}

And then

int main() {
    auto t = std::make_tuple(std::less<int>{}, std::greater<int>{});

    run_for_each([](auto&&f) {std::cout << f(0, 1) << " ";}, t);
    std::cout << std::endl;
}

Demo

Make a vector of comparator functions

2 Answers2