返回一个int数组

Question

I have created the following program which is supposed to return an int array to the main function, which will then display it on the screen.

#include <iostream.h>
int* returnArray(){
    int* arr;
    arr[0]=1;
    arr[1]=2;
    arr[2]=3;
    return arr;
}
int main(){
    int* res = returnArray();
    for(int i=0; i<3; i++){
        cout<<res[i]<<" ";
    }
    return 0;
}

And i was expecting it to print

1 2 3

but instead, it prints 3 someNumberWhichLooksLikeAPointer 0

Why is that? what can i do to return an int array from my function? Thank you very much!

Answer 1

You forgot to allocate your array:

int* arr = new int[3];

You also need to return it, and free the memory inside main after you finish with the loop in order to avoid a memory leak:

delete[] res;

Although this approach works, it is not ideal. If you have an option of returning a container, say, std::vector<int> it would be a much better choice.

If you must stay with plain arrays, another solution for filling an array inside an API is to pass it in, along with its size:

void fillArray(int *arr, size_t s){
    if (s > 0) arr[0]=1;
    if (s > 1) arr[1]=2;
    if (s > 2) arr[2]=3;
}

int main(){
    int res[3];
    fillArray(res, 3);
    for(int i=0; i<3; i++){
        cout<<res[i]<<" ";
    }
    return 0;
}

Answer 2

You have tagged the question with C++ . You Yous should consider to use the C++ solution: use a vector of int

#include <iostream>
#include <vector>

std::vector<int> returnArray(){
    std::vector<int> arr(3);
    arr[0]=1;
    arr[1]=2;
    arr[2]=3;
    return arr;
}
int main(){
    std::vector<int> res = returnArray();
    for(int i=0; i<3; i++){
        std::cout<<res[i]<<" ";
    }
    return 0;
}