Defining priority queue inside a class with a custom comparator function

Question

I am trying to define a priority queue with custom comparator as follows:

typedef bool (*comp)(int,int);

bool compare(int exp1,int exp2){
    return (exp1 > exp2);
}

class test{
public:
    priority_queue<int,vector<int>,comp> test_pq(compare); // Gives compilation error
};

int main ()
{
    priority_queue<int,vector<int>,comp> pq(compare); // Compiles perfectly
    return 0;
}

This is the compilation error that shows up

test.cpp:18:47: error: ‘compare’ is not a type
  priority_queue<int,vector<int>,comp> test_pq(compare);
                                               ^

I have also tried declaring another compare function inside the test class with no effect. Why does the priority queue in the main function compile while the one inside the class doesnt? Is defining a dedicated class for comparator the only work around here? Thank you.

magras · Accepted Answer · 2016-09-30 18:22:18Z

3

Your code in test class tries to declare a method test_pq with incorrect signature.

To define member variable you can use curly braces in initialization (C++11 required):

class test{
public:
    priority_queue<int,vector<int>,comp> test_pq{compare};
};

To achieve the same in pre C++11 you need to write custom constructor for test class:

class test
{
public:
    test()
        : test_pq(compare)
    {
        // Constructor code here
    }
private:
    priority_queue<int,vector<int>,comp> test_pq;
};

edited Sep 30, 2016 at 18:22

answered Sep 30, 2016 at 17:39

magras

1,76824 silver badges36 bronze badges

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

3 Comments

ant_1618 Over a year ago

Thank you for the through reply :) But isn't there a way to make it work irrespective of version? Say by shifting the initialisation to the class constructor?

magras Over a year ago

@ant_1618, yes, of course you can do this. I updated answer to include this variant too. But if you already know words initialization and constructor I'd expect that you should already know an answer… =)

ant_1618 Over a year ago

I wasnt aware of cpp's way of initialising member variables implicitly in the constructor with Test() : [initialisation list] { \\ code}. So, I couldnt have come up with this answer :P Now it is clear though :D Thank you :)

Collectives™ on Stack Overflow

Defining priority queue inside a class with a custom comparator function

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related