declaring a priority_queue in c++ with a custom comparator

I'm trying to declare a priority_queue of nodes, using bool Compare(Node a, Node b) as the comparator function (which is outside the node class).

What I currently have is:

priority_queue<Node, vector<Node>, Compare> openSet;

For some reason, I'm getting Error: "Compare" is not a type name

Changing the declaration to priority_queue <Node, vector<Node>, bool Compare>

gives me Error: expected a '>'

I've also tried:

priority_queue<Node, vector<Node>, Compare()> openSet;
priority_queue<Node, vector<Node>, bool Compare()> openSet;
priority_queue<Node, vector<Node>, Compare<Node, Node>> openSet; 

How should I correctly declare my priority_queue?

Solution 1:

Note - You may also want to check other answers, especially the one with decltype and lambda

You should declare a class Compare and overload operator() for it like this:

class Foo
{

};

class Compare
{
public:
    bool operator() (Foo, Foo)
    {
        return true;
    }
};

int main()
{
    std::priority_queue<Foo, std::vector<Foo>, Compare> pq;
    return 0;
}

Or, if you for some reasons can't make it as class, you could use std::function for it:

class Foo
{

};

bool Compare(Foo, Foo)
{
    return true;
}

int main()
{
    std::priority_queue<Foo, std::vector<Foo>, std::function<bool(Foo, Foo)>> pq(Compare);
    return 0;
}

Solution 2:

The accepted answer makes you believe that you must use a class or a std::function as comparator. This is not true! As cute_ptr's answer shows, you can pass a function pointer to the constructor. However, the syntax to do so is much simpler than shown there:

class Node;
bool Compare(Node a, Node b);

std::priority_queue<Node, std::vector<Node>, decltype(&Compare)> openSet(Compare);

That is, there is no need to explicitly encode the function's type, you can let the compiler do that for you using decltype.

This is very useful if the comparator is a lambda. You cannot specify the type of a lambda in any other way than using decltype. For example:

auto compare = [](Node a, Node b) { return a.foo < b.foo; }
std::priority_queue<Node, std::vector<Node>, decltype(compare)> openSet(compare);