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closed Need help understanding classes, objects and shared_ptr in C++?

+2 votes
asked Oct 20, 2021 by Areeb Sherjil (2,000 points)
closed Oct 25, 2021 by Admin
Hi,

Please can anyone advise me on the following code which is based on a linked list?

This is a sample code that computes the largest object in the class Truckload:

#include "RandomBoxes.h"
#include "Truckload.h"
#include<iostream>

int main()
{
    Truckload load1;  // Create an empty list

    // Add 12 random Box objects to the list
    const size_t boxCount{ 12 };
    for (size_t i{}; i < boxCount; ++i)
        load1.addBox(randomSharedBox()); // random function that generates boxes

    std::cout << "The first list:\n";
    load1.listBoxes();

    // Copy the truckload
    Truckload copy{ load1 };
    std::cout << "The copied truckload:\n";
    copy.listBoxes();

    // Find the largest Box in the list
    SharedBox largestBox{ load1.getFirstBox() };

    SharedBox nextBox{ load1.getNextBox() };
    while (nextBox) // how does this work?!?
    {
        if (nextBox->compare(*largestBox) > 0)
            largestBox = nextBox;
        nextBox = load1.getNextBox();
    }

    std::cout << "\nThe largest box in the first list is ";
    largestBox->listBox();

From the truckload.cpp file the getFirstBox function code is:

SharedBox Truckload::getFirstBox()
{
    // Return m_head's box (or nullptr if the list is empty)
    m_current = m_head;
    return m_current ? m_current->m_box : nullptr;
}

Question: how does the while loop work? Especially the part about 'while(nextBox)' ?
closed with the note: answered

1 Answer

+1 vote
answered Oct 20, 2021 by Peter Minarik (58,190 points)
selected Oct 21, 2021 by Areeb Sherjil
 
Best answer

You have shared a partial code. We do not have the headers RandomBoxes.h and Truckload.h, so one has to make assumptions here.

My assumption is that the type SharedBox is an alias to std::shared_ptr<Box>, where Box is a class that forms the load.

typedef std::shared_ptr<Box> SharedBox;

In the loop, getNextBox() returns a SharedBox and when there is no more Box in the list, it probably returns a null pointer. A null pointer interpreted as a logical expression is false. A non-null pointer interpreted as a logical expression is true.

Therefore

SharedBox nextBox = load1.getNextBox();
while (nextBox)
{
    // ...
}

means: repeat the loop as long as we have a nextBox.

commented Oct 21, 2021 by Areeb Sherjil (2,000 points)
I couldn't really share all the code as there isn't enough space.
But thanks tho!
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