Re-write of the Pop() method

June 30, 2023

Under Go’s installation, they have an example of priority queue at container/heap/example_pq_test.go
I am pasting the contents of the entire file so that I can ask about the Pop() method.

// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// This example demonstrates a priority queue built using the heap interface.
package heap_test

import (
    "container/heap"
    "fmt"
)

// An Item is something we manage in a priority queue.
type Item struct {
    value    string // The value of the item; arbitrary.
    priority int    // The priority of the item in the queue.
    // The index is needed by update and is maintained by the heap.Interface methods.
    index int // The index of the item in the heap.
}

// A PriorityQueue implements heap.Interface and holds Items.
type PriorityQueue []*Item

func (pq PriorityQueue) Len() int { return len(pq) }

func (pq PriorityQueue) Less(i, j int) bool {
    // We want Pop to give us the highest, not lowest, priority so we use greater than here.
    return pq[i].priority > pq[j].priority
}

func (pq PriorityQueue) Swap(i, j int) {
    pq[i], pq[j] = pq[j], pq[i]
    pq[i].index = i
    pq[j].index = j
}

func (pq *PriorityQueue) Push(x any) {
    n := len(*pq)
    item := x.(*Item)
    item.index = n
    *pq = append(*pq, item)
}

func (pq *PriorityQueue) Pop() any {
    old := *pq
    n := len(old)
    item := old[n-1]
    old[n-1] = nil  // avoid memory leak
    item.index = -1 // for safety
    *pq = old[0 : n-1]
    return item
}

// update modifies the priority and value of an Item in the queue.
func (pq *PriorityQueue) update(item *Item, value string, priority int) {
    item.value = value
    item.priority = priority
    heap.Fix(pq, item.index)
}

// This example creates a PriorityQueue with some items, adds and manipulates an item,
// and then removes the items in priority order.
func Example_priorityQueue() {
    // Some items and their priorities.
    items := map[string]int{
        "banana": 3, "apple": 2, "pear": 4,
    }

    // Create a priority queue, put the items in it, and
    // establish the priority queue (heap) invariants.
    pq := make(PriorityQueue, len(items))
    i := 0
    for value, priority := range items {
        pq[i] = &Item{
            value:    value,
            priority: priority,
            index:    i,
        }
        i++
    }
    heap.Init(&pq)

    // Insert a new item and then modify its priority.
    item := &Item{
        value:    "orange",
        priority: 1,
    }
    heap.Push(&pq, item)
    pq.update(item, item.value, 5)

    // Take the items out; they arrive in decreasing priority order.
    for pq.Len() > 0 {
        item := heap.Pop(&pq).(*Item)
        fmt.Printf("%.2d:%s ", item.priority, item.value)
    }
    // Output:
    // 05:orange 04:pear 03:banana 02:apple
}

What harm may come or is there a fallacy if I had the Pop() method as below(without creating a deep copy of the original slice)

func (pq *PriorityQueue) Pop2() any {
    n := len(*pq)
    item := (*pq)[n-1]
    (*pq)[n-1] = nil  // avoid memory leak
    item.index = -1 // for safety
    *pq = (*pq)[: n-1]
    return item
}

I believe in the original Pop() method, this line creates a deep copy(allocates a new underlying array) for the slice old := *pq. Is that true?