Why values of array cannot be printed using foreach loop in called function when array is passed as argument in caller function? [duplicate]

Question

I am trying to print values of array in called function using foreach loop but facing compilation error. Using c++11 compiler in linux platform and using VIM editor.

Tried using C-style for loop and it worked, when the size is passed from calling function

#include <iostream>
using namespace std;

void call(int [], int);

int main()
{
    int arr[] = {1,2,3,4,5};
    int size = sizeof(arr)/sizeof(arr[0]);
    call(arr,size);
}

void call(int a[],int size)
{    
    for(int i =0 ; i<size; i++)
        cout << a[i];
}

For-each loop used in the below code, which fails to compile.

#include <iostream>
using namespace std;

void call(int []);

int main()
{
    int arr[] = {1,2,3,4,5};
    call(arr);
}

void call(int a[])
{
    for ( int x : a ) 
        cout << x << endl;
}

For-each loop in C++11 expects to know the size of array to iterate ? If so, how it will be helpful with respect to traditional for loop. Or something wrong i coded here?

I would look forward for your help. Thanks in advance.

I'm sure someone will come with a suitable duplicate sooner or later. Until then, when used as an argument int a[] will be treated as int* a, and passing the array will make it decay to a pointer to its first element (i.e. you pass &arr[0]). That is, all you have is a pointer to the first element. — Some programmer dude
– Some programmer dude, Commented Apr 29, 2019 at 13:59
std::vector is your go to for these kinds of things. You may consider std::array. What you probably do not want is a C-style array, since it doesn't provide the nice things that you get with C++ such as supporting C++ for-each loop. — Eljay
– Eljay, Commented Apr 29, 2019 at 14:10
Thanks Ejay, my question was specific to for-each loop in c++11. — Vishwas
– Vishwas, Commented Apr 29, 2019 at 14:13

mch · Accepted Answer · 2019-04-29 14:00:09Z

5

Because int a[] as function parameter is not an array, it is the same as writing int *a.

You can pass the array by reference to make it work:

template <size_t N> void call(int (&a)[N])

working example: https://ideone.com/ZlEMHC

template <size_t N> void call(int (&a)[N])
{
    for ( int x : a ) 
        cout << x << endl;
}

int main()
{
    int arr[] = {1,2,3,4,5};
    call(arr);
}

answered Apr 29, 2019 at 14:00

mch

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