c++ function returning incorrect array

Question

I am trying to create a function called getSortedRanks which returns an array. I copied the format for returning arrays from this question Return array in a function but the array is not being returned correctly.

#include <stdlib.h>
#include <stdio.h>
#define familyMembers 4

int *getSortedRanks()
{
    int rankedMembers[familyMembers] = {3,4,2,1};
    return rankedMembers;
}

int main()
{
    int *sortedRanks = getSortedRanks();

    //print the returned array
    for(int i = 0; i < familyMembers; i ++)
    {
        cout << "ranked member is " << sortedRanks[i] << endl;
    }

    return 0;
}

When I run this the output is:

ranked member is 3
ranked member is 0
ranked member is 0
ranked member is 2686744

The first element of the array sortedRanks is always correct but the others are not. How can I correct the way the array is being returned?

stackoverflow.com/q/4643713/1741542, stackoverflow.com/q/7499864/1741542, stackoverflow.com/q/13980362/1741542, stackoverflow.com/q/423186/1741542 should give you a hint. — Olaf Dietsche
– Olaf Dietsche, Commented Dec 12, 2015 at 21:57
Don't return pointers to things that live on the function's stack. — David Schwartz
– David Schwartz, Commented Dec 12, 2015 at 21:58

Community · Accepted Answer · 2017-05-23 12:23:16Z

5

An array with automatic storage duration:

int rankedMembers[familyMembers] = {3, 4, 2, 1};

lives on the stack and gets destroyed after getSortedRanks finishes. The returned pointer is invalidated. Dereferencing it leads to undefined behavior.

You'll either want to:

Allocate the array dynamically (on the heap), as you'll manage its lifetime:
```
int *getSortedRanks()
{
    return new int[familyMembers]{3, 4, 2, 1};
}
```
Don't forget to delete [] it after the use. Using smart pointer will help you with that.

Use std::vector or std::array and return by value:

std::array<int, familyMembers> getSortedRanks()
{
    return {3, 4, 2, 1};
}

(ordered from less to more favorable)

edited May 23, 2017 at 12:23

CommunityBot

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answered Dec 12, 2015 at 21:55

LogicStuff

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2 Comments

Hugh Over a year ago

I get it now. Thanks for the explanation

Peter Over a year ago

The array rankedMembers is not statically allocated. The reason it does not exist after the function returns is that it has automatic storage duration.

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