CODEWITHSUNDEEP
c++heappriority-queue
shawtynerd
shawtynerd

Reputation: 23

C++ Priority Queue Declaration

I'm looking at a declaration of a priority queue in C++ but I don't quite understand it.

priority_queue<string, vector<string>,function<bool(string,string)>>
min_heap([] (const string& a,const string& b) {return a.size()>=b.size();});

What is the purpose of the brackets []? Is that part of the function definition?

Also, would I be able to implement the same thing by using a bool operator?

Upvotes: 1

Views: 182

Answers (1)

Brian Bi
Brian Bi

Reputation: 119641

The entire expression

[] (const string& a,const string& b) {return a.size()>=b.size();}

is a lambda expression. It is an object of an unnamed class that can be called with two string arguments, a and b, and returns bool.

You could also write such a class yourself (in that case, it would have a name):

struct Comp {
    bool operator()(const string& a, const string& b) {
        return a.size() > b.size();
    }
};
priority_queue<string, vector<string>, Comp> min_heap;
// a value of type Comp will be value-initialized

Note that the comparator must be a strict weak ordering, so you must use > instead of >=.

If performance matters, it is better to define a named class, because the function call operator can be inlined.

Upvotes: 1

Related Questions