priority_queue with custom comparator as a class method

Question

i have code like this

priority_queue<int, std::vector<int>, decltype(&VD::CompareByDistance)> pqDistances(&VD::CompareByDistance);

where

class VD 
{
    ...
    bool CompareByDistance(int indexLeft, int indexRight) const;
};

But i get error

error C2064: term does not evaluate to a function taking 2 arguments

How can i pass class member as a compator, the thing is that i want comparator to access its instance fields. Thanks.

Btw, is this comparison function really non-const? That does not feel very idiomatic. — Baum mit Augen
– Baum mit Augen ♦, Commented Jun 15, 2015 at 13:34

Baum mit Augen · Accepted Answer · 2015-06-15 13:58:03Z

1

bool CompareByDistance(int indexLeft, int indexRight);

takes three arguments: The this-pointer and the both ints.

Besides this being the wrong number of arguments for the priority_queue template parameter: What instance of VD do you expect this non-static method to be called on?

A workaround for this would be "carrying" the instance with you like this:

VD v;
auto comp = [&](int i1, int i2) {return v.CompareByDistance(i1, i2);}

This comp object would now be a valid template argument for your queue. For example:

struct A{
    bool compare (int, int) const {return true;}
};

int main()
{
    A a;
    auto comp = [&](int i, int i2) {return a.compare(i, i2);};
    std::priority_queue<int, std::vector<int>, decltype(comp)> q (comp);
}

edited Jun 15, 2015 at 13:58

answered Jun 15, 2015 at 13:28

Baum mit Augen♦

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6 Comments

Yola Over a year ago

i have the same error. The lambda captures this, and it is a problem

Baum mit Augen Over a year ago

@Yola Did you forget to pass the comp to the constructor of the queue? Works for me. (Live)

KABoissonneault Over a year ago

Is the 'this' pointer actually used in CompareByDistance though? Otherwise we could simply make it static

Baum mit Augen Over a year ago

@KABoissonneault " the thing is that i want comparator to access its instance fields." So I guess that won't work.

KABoissonneault Over a year ago

@BaummitAugen Alright, missed that line. Unrelated, but I don't recommend using generalized lambda capture. The capture list should be [&v] or [&a] instead.

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