Help about sorting vectors with custom comparator

[panecasareccio]

Dear all,

I am trying to sort a vector of object by using std::sort in combination wigh my own comparator.

I have tried the following code, which does not compile, and I do not understand why.

#include<stdio.h>
#include<vector>
#include<algorithm>

template<class T>
class component
{
public:
	int number;
	T value;
	component(int n, T v) : number(n), value(v) {}
};

template<class T>
class compare
{
public:
	compare(){};
	bool operator() (const component<T>& c1, const component<T>& c2)
	{
	return (c1.value < c2.value);
	}
};

int main()
{
	std::vector<component<float> > v;
	v.push_back(component<float>(22, 5.2));
	v.push_back(component<float>(18, 3.2));
	v.push_back(component<float>(22, 5.9));
	v.push_back(component<float>(18, 3.6));

        //this is the line that does not compile
	std::sort(v.begin(), v.end(), compare<float>);

	return 0;
}

Th error message I get is this: "expected primary-expression before ')' token". What is a primary expression?


I thank you all,
Panecasareccio.

[Catfish666]

You need to instantiate your functors.

int main()
{
    std::vector<component<float> > v;
    v.push_back(component<float>(22, 5.2));
    v.push_back(component<float>(18, 3.2));
    v.push_back(component<float>(22, 5.9));
    v.push_back(component<float>(18, 3.6));

    // compare<float> cf_functor;
    //std::sort(v.begin(), v.end(), cf_functor);

    std::sort(v.begin(), v.end(), compare<float>());

/*
    for (const component<float> &c: v)
        std::cout << c.number << ' ' << c.value << '\n';

    std::cout << std::endl;
*/
    return 0;
}

[Smac89]

The sorting needs an actual object or function to use. As you can see this is the syntax for providing a custom compare method:

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

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

In your code you have tried to call sort that has these parameters:

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

Which is what is causing compiler errors