How to Correctly Declare a Priority Queue with a Custom Comparator in C ?

Declaring a Priority Queue in C with a Custom Comparator

When working with priority queues in C , one may encounter errors while trying to declare them with custom comparator functions. Let's explore the reasons behind these errors and find out the correct way to declare a priority queue with a custom comparator.

Incorrect Declaration: Causes and Solutions

As you mentioned, trying to declare a priority queue with the following code triggers errors:

<code class="cpp">priority_queue<Node, vector<Node>, Compare> openSet;</code>

The reason for the first error ("Compare is not a type name") is that Compare is expected to be a type, specifically a class that overrides the operator() function. To resolve this, you need to create a class called Compare and overload operator() within it.

The second error ("expected a >'") occurs when the Compare` function is not correctly specified as a type. To fix this, modify the declaration to:

<code class="cpp">priority_queue<Node, vector<Node>, Compare::Compare> openSet;</code>

Here, Compare::Compare explicitly specifies the operator() function within the Compare class.

Alternative Declaration Options

There are alternative ways to declare a priority queue with a custom comparator:

Using std::function:

<code class="cpp">priority_queue<Node, vector<Node>, std::function<bool(Node, Node)>> openSet(Compare);</code>

Using decltype and a Lambda Expression:

<code class="cpp">decltype(Compare) myComparator = Compare;
priority_queue<Node, vector<Node>, decltype(Compare)> openSet(myComparator);</code>

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