Question

I am trying to implement the Merge sort algorithm:

#include <list>
#include <functional>
#include <iterator>
#include <iostream>
#include <random>

template <typename TIterator, typename TObject>
void mergeSort(const TIterator& begin, const TIterator& end,
               std::function<bool (const TObject& left,
                                   const TObject& right)> criterium)
{
    //...
}

bool compare(int a, int b)
{
    return a < b;
}

int main(int argc, char** argv)  // And now to test the algorithm
{
    std::list<int> container;
    for (int i = 0; i < 100; ++i)
        container.push_back(random() % 20);

    mergeSort(container.begin(), container.end(), compare);

    for (auto it = container.begin(); it != container.end(); ++it)
        std::cout << (*it) << std::endl;

    return 0;
}

This program does not compile:

error: no matching function for call to 
mergeSort(std::list<int>::iterator, std::list<int>::iterator, bool (&)(int, int))

candidate is:

template<class TIterator, class TObject> 
void mergeSort(const TIterator&, const TIterator&, 
std::function<bool(const TObject&, const TObject&)>)

at global scope

I know that I messed something simple in my declaration but I cannot figure it out.

Answer 1

Your function compare isn’t a std::function but your mergeSort expects one.
Moreover, you should not pass const_iterator to your function because it’s supposed to modify the array.

If you change your code and use this:

std::function<bool(const int&, const int&)> myCompare = compare;
mergeSort(container.begin(), container.end(), myCompare);

It works (see http://ideone.com/7FdKTP).

In my opinion, it’s easier to implement comparators as structs with an operator(). This way, you pass an object instead of a function, which is easier to manage.