c++ custom compare function for std::sort()

Question

I want to create custom compare function for std::sort(), to sort some key-value pairs std::pair

Here is my function

 template <typename K, typename V>
 int comparePairs(const void* left, const void* right){
        if((((pair<K,V>*)left)->first) <= (((pair<K,V>*)right)->first))
            return 1;
        else 
            return -1;
    }

Then, inside some class I have vector of pairs class member:

vector<pair<K,V>> items;  

And some method for sort this vector by keys, using std::sort()

std::sort(items.begin(), items.end(), comparePairs<K,V>);

I have compilation errors within , which said

"cannot convert parameter number from 'std::pair<_Ty1,_Ty2>' to 'const void*'"

. What is a mistake?

Answer 1

Look here: http://en.cppreference.com/w/cpp/algorithm/sort.

It says:

template< class RandomIt, class Compare >
void sort( RandomIt first, RandomIt last, Compare comp );

• comp - comparison function which returns ?true if the first argument is less than the second. The signature of the comparison function should be equivalent to the following: bool cmp(const Type1 &a, const Type2 &b);

Also, here's an example of how you can use std::sort using a custom C++14 polymorphic lambda:

std::sort(std::begin(container), std::end(container),
          [] (const auto& lhs, const auto& rhs) {
    return lhs.first < rhs.first;
});