declaring a priority_queue in c++ with a custom comparator

问题

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?

回答1:

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;
}

回答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);

回答3:

The third template parameter must be a class who has operator()(Node,Node) overloaded. So you will have to create a class this way:

class ComparisonClass {
    bool operator() (Node, Node) {
        //comparison code here
    }
};

And then you will use this class as the third template parameter like this:

priority_queue<Node, vector<Node>, ComparisonClass> q;

回答4:

Answering your question directly:

I'm trying to declare a priority_queue of nodes, using bool Compare(Node a, Node b) as the comparator function

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

The compiler is telling you exactly what's wrong: Compare is not a type name, but an instance of a function that takes two Nodes and returns a bool.
What you need is to specify the function pointer type:
std::priority_queue<Node, std::vector<Node>, bool (*)(Node, Node)> openSet(Compare)

回答5:

One can also use a lambda function.

auto Compare = [](Node &a, Node &b) { //compare };
std::priority_queue<Node, std::vector<Node>, decltype(Compare)> openset(Compare);

回答6:

prefer struct, and it's what std::greater do

struct Compare {
  bool operator()(Node const&, Node &) {}
}

来源：https://stackoverflow.com/questions/16111337/declaring-a-priority-queue-in-c-with-a-custom-comparator

标签

c++

std

priority-queue