Problems sorting an array of std::reference_wrapper, referencing vectors

Question

I am a bit stuck on this, I'm using a std::array to store reference wrappers to vectors. I was trying to sort these by the vector's size using std::sort but am unable for reasons I am not quite sure of, even after reading the compiler errors. Will I have to use another sort function because it appears that std::sort implementations use operations that are unavailable for use on reference wrappers.

Thank you :)

Compiler explorer version

    #include <iostream>
    #include <algorithm>
    #include <cstdio>
    #include <vector>
    #include <array>
    #include <string>
    #include <sstream>
    #include <utility>
    
    
    void findSum(const std::vector<int>& vec1, const std::vector<int>& vec2, const std::vector<int>& vec3)
    {    
        std::array<std::reference_wrapper<const std::vector<int>>, 3> vecRefs{vec1, vec2, vec3};
        
        std::sort(std::cbegin(vecRefs), std::cend(vecRefs), [](const auto vecA, const auto vecB) -> bool {
                return vecA.get().size() > vecB.get().size(); //descending order
            });
    
        //vecRefs should now be storing reference wrappers to the vectors with sizes in descending order
    
        //print longest vec first, shortest vec last
        for (const auto vec : vecRefs)
        {
            for (const auto val : vec.get())
            {
                std::cout << val << ' ';
            }
            std::cout << '\n';
        }
    
        //TODO: rest of function(the bit before this can be made into a function)
    }

You are passing const_iterator to sort. That can't work. — Werner Henze
– Werner Henze, Commented Mar 20, 2021 at 15:29
BTW you are missing the <functional> include for ref_wrapper — OznOg
– OznOg, Commented Mar 20, 2021 at 15:34
Thanks all, I was just unable to properly understand the error messages. — VevsVire
– VevsVire, Commented Mar 20, 2021 at 15:35

cigien · Accepted Answer · 2021-03-20 18:18:09Z

2

It's because you are using std::cbegin (const begin) and std::cend (const end) in std::sort.
It means, that you can't change the order of your array!
Just replace it with std::begin and std::end like this:

std::sort(std::begin(vecRefs), std::end(vecRefs), [](const auto vecA, const auto vecB) -> bool {
        return vecA.get().size() > vecB.get().size();
        });

edited Mar 20, 2021 at 18:18

cigien

61.2k11 gold badges86 silver badges124 bronze badges

answered Mar 20, 2021 at 15:31

Volodymyr Osadchuk

3724 silver badges12 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Problems sorting an array of std::reference_wrapper, referencing vectors

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related