toString for printing array is not working C++

Question

I have a int array. Size of this array is different is main and in toString method.

void toString(int arr[]){
    int size = sizeof(arr)/sizeof(arr[0]);
    cout<<"Size in toString  "<<size<<endl;
    cout<<"{";
    for(int i=0; i<size; i++){
        cout<<arr[i]<<", ";
    }
    cout<<"}"<<endl;
}

int main(){
    int input_arr[] = {1, 2, 3, 4};
    int size_in_main = sizeof(input_arr)/sizeof(input_arr[0]);
    cout<<"Size in main      "<<size_in_main<<endl;
    toString(input_arr);
    return 0;
}

Output:

Size in main      4
Size in toString  2
{1, 2, }

Why is it print different array size?

Another possible duplicate: https://stackoverflow.com/a/968112/1632887 — seleciii44, Oct 29 '17 at 06:26
@manni66 I am using `using namespace std;` in my code so i guess `std::array` is not required. — Rahul, Oct 29 '17 at 14:24
std::array != []. http://en.cppreference.com/w/cpp/container/array — , Oct 29 '17 at 17:09

score 1 · Accepted Answer · answered Oct 29 '17 at 06:02

1

A pointer passed as an argument does not have size information. You have to add size as an argument.

answered Oct 29 '17 at 06:02

Joe Sonderegger

784
5
15

But why is it always showing size 2. Why not some random number.? – Rahul Oct 29 '17 at 06:09
I would have expected 1. No idea. – Joe Sonderegger Oct 29 '17 at 06:13
1

A possible explanation may be that : A pointer on a 64-bit system gets allocated 8 bytes and arr[0] will be of size 4 so 8/4=2. – Amit Singh Oct 29 '17 at 06:25

Richard Denton · Answer 2 · 2017-10-29T07:14:13.357

1

In C++, passing an array to a procedure or function call won't create a local copy of the data inside of the array, but rather provide a pointer to the array of data (pass by reference). Because of this, sizeof(arr) will return the size of 'int *', not 'int []'.

This can be resolved by adding a size parameter to your function or procedure that represents the size of the array like so.

void toString(int arr[], int arr_size){
    int size = arr_size/sizeof(arr[0]);
    cout<<"Size in toString  "<<size<<endl;
    cout<<"{";
    for(int i=0; i<size; i++){
        cout<<arr[i]<<", ";
    }
    cout<<"}"<<endl;
}

Which could then be called like this

toString(input_arr, sizeof(input_arr));

edited Oct 29 '17 at 07:14

answered Oct 29 '17 at 06:16

Richard Denton

982
2
7
13

You mean `int size = arr_size/sizeof(arr[0]);` – haronaut Oct 29 '17 at 06:23
Just realised I pasted the wrong line - thanks – Richard Denton Oct 29 '17 at 07:15

toString for printing array is not working C++

2 Answers2