How to return a local array from a C/C++ function?

27 Dec, 2020
Consider the below C++ program. Is it right way of returning array from a function? 
C
C++
#include <stdio.h> int* fun(){    int arr[100];     /* Some operations on arr[] */    arr[0] = 10;    arr[1] = 20;     return arr;} int main(){    int* ptr = fun();    printf("%d %d", ptr[0], ptr[1]);    return 0;}
#include <iostream>using namespace std; int* fun(){    int arr[100];     /* Some operations on arr[] */    arr[0] = 10;    arr[1] = 20;     return arr;} int main(){    int* ptr = fun();    cout << ptr[0] << " " << ptr[1];    return 0;}
Warning:  
In function 'int* fun()':
6:8: warning: address of local variable 'arr' returned [-Wreturn-local-addr]
    int arr[100];
        ^




 
10 20




 
The above program is WRONG. It may produce values 10 20 as output or may produce garbage values or may crash. The problem is, we return address of a local variable which is not advised as local variables may not exist in memory after function call is over.So in simple words, Functions can’t return arrays in C. However, inorder to return the array in C by a function, one of the below alternatives can be used. Following are some correct ways of returning array: 
Using Dynamically Allocated Array : Dynamically allocated memory (allocated using new or malloc()) remains their until we delete it using delete or free(). So we can create a dynamically allocated array and we can delete it once we come out of function. 
C
C++
#include <stdio.h> int* fun(int *arr){    /* Some operations on arr[] */    arr[0] = 10;    arr[1] = 20;     return arr;} int main(){    int arr[100];    int* ptr = fun(arr);    printf("%d %d", ptr[0], ptr[1]);    return 0;}
#include <iostream>using namespace std; int* fun(){    int* arr = new int[100];     /* Some operations on arr[] */    arr[0] = 10;    arr[1] = 20;     return arr;} int main(){    int* ptr = fun();    cout << ptr[0] << " " << ptr[1];    delete[] ptr; //allocated memory must be deleted    return 0;}
10 20




 
Using static array: Lifetime of a static variable is throughout the program. So we can always create a local static array and return it. 
C
C++
#include <stdio.h> int* fun(){    static int arr[100];     /* Some operations on arr[] */    arr[0] = 10;    arr[1] = 20;     return arr;} int main(){    int* ptr = fun();    printf("%d %d", ptr[0], ptr[1]);    return 0;}
#include <iostream>using namespace std; int* fun(){    static int arr[100];     /* Some operations on arr[] */    arr[0] = 10;    arr[1] = 20;     return arr;} int main(){    int* ptr = fun();    cout << ptr[0] << " " << ptr[1];    return 0;}
10 20




 
Using struct: We can wrap array in a structure/class and return an instance of the struct/class. The reason for this work is, array members of structures are deeply copied. In below program deep copy happens when we returned instance is copied in main. 
C
C++
#include <stdio.h> struct arrWrap {    int arr[100];}; struct arrWrap fun(){    struct arrWrap x;     x.arr[0] = 10;    x.arr[1] = 20;     return x;} int main(){    struct arrWrap x = fun();    printf("%d %d", x.arr[0], x.arr[1]);    return 0;}
#include <iostream>using namespace std; struct arrWrap {    int arr[100];}; struct arrWrap fun(){    struct arrWrap x;     x.arr[0] = 10;    x.arr[1] = 20;     return x;} int main(){    struct arrWrap x = fun();    cout << x.arr[0] << " " << x.arr[1];    return 0;}
10 20




 
RishabhPrabhu
richardurban
cpp-array
cpp-structure
C++
CPP
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