C++ Priority queue with custom type and comparator not working

Question

I'm trying to use the C++ STL priority queue with a custom type and comparator, but no matter how I put it, I keep getting an error.

Does anyone know what the issue might be? I'm trying to copy the syntax from documentation but nothing has been working...

The custom type is a pointer to the ListNode class used in LeetCode:

 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */

Within my class, I have a static comparison function:

static bool compare(ListNode* n1, ListNode* n2) {
    return n1->val < n2->val;
}

And I am trying to initialize the priority queue like this:

priority_queue<ListNode*, vector<ListNode*>, decltype(compare)> pq(compare);

But I keep getting an error, saying:

In file included from prog_joined.cpp:1:
In file included from ./precompiled/headers.h:55:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/queue:64:
/usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_queue.h:485:18: error: data member instantiated with function type 'bool (ListNode *, ListNode *)'
      _Compare   comp;
                 ^
Line 137: Char 73: note: in instantiation of template class 'std::priority_queue<ListNode *, std::vector<ListNode *, std::allocator<ListNode *>>, bool (ListNode *, ListNode *)>' requested here
        priority_queue<ListNode*, vector<ListNode*>, decltype(compare)> pq(compare);
                                                                        ^
1 error generated.

Thanks!

songyuanyao · Accepted Answer · 2021-01-14 05:15:19Z

6

You should specify a function pointer type, but not a function type as the template argument for priority_queue.

Change

priority_queue<ListNode*, vector<ListNode*>, decltype(compare)> pq(compare);

to

priority_queue<ListNode*, vector<ListNode*>, decltype(compare)*> pq(compare);
//                                                            ^

answered Jan 14, 2021 at 5:15

songyuanyao

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3 Comments

Jake Sanders Over a year ago

Thank you very much! Just to understand it better, why does it have to be a function pointer?

songyuanyao Over a year ago

@JakeSanders Function pointers could be passed to priority_queue, which stores it as object and uses it to compare elements. While functions can't. In priority_queue<ListNode*, vector<ListNode*>, decltype(compare)*> pq(compare);, compare decays to function pointer and then gets passed to the constructor of priority_queue, same as priority_queue<ListNode*, vector<ListNode*>, decltype(compare)*> pq(&compare);.

douyu Over a year ago

priority_queue<ListNode*, vector<ListNode*>, decltype(&compare)> pq(compare) also works.

Yiğit · Accepted Answer · 2021-01-14 05:20:48Z

2

Instead of declaring and defining a static function, you can define a lambda.

auto compare = [](ListNode* n1, ListNode* n2) {
    return n1->val < n2->val;
};

Since the type of lambda is resolved automatically by the compiler, then you would not need to think about things like a parameter's type being a function pointer or a function etc.

decltype(...) will resolve to a callable object anyways.

answered Jan 14, 2021 at 5:20

Yiğit

556 bronze badges

3 Comments

Jake Sanders Over a year ago

Would I still need to use "decltype" and everything if I do it this way? How would I declare the priority queue then?

Yiğit Over a year ago

priority queue initialization becomes correct if you use an auto lambda, instead of writing a function. if you use a declared/defined function with decltype, what you need to tell the priority queue is you are passing a function pointer (which is callable).

Yiğit Over a year ago

you do not need to specify the last two types. they would work with vector and an auto lambda by default.

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