How to pass array reference from function?

Question

I want to pass value of array in function by reference

My first try with variable (it works well):

#include <iostream>
using namespace std;

int input(int &);
int main()
{
    int a;
    input(a);
    cout << "the value : " << a << endl;
    system("pause");
    return 0;
}

int input(int &a)
{
    cin >> a;   
}

But when i change a to a[] like in the following code, it doesn't work

#include <iostream.h>
using namespace std;

int input(int &[]);
int main()
{
    int a[2], i;
    input(a);
    for(i=0;i<=2;i++)
    {
        cout << a << endl;
    }

    system("pause");
    return 0;
}

int input(int &a[])
{
    int i;
    for(i=0;i<=2;i++)
    {
        cin >> a[i];
    }  
}

At least the array parameter in the function declaration must match the exact length of what's passed to it: `int input(int &a[2])`, you are trying to pass two parameters, where only one was declared: `input(a, 2);` — πάντα ῥεῖ, May 10 '14 at 13:48
You have to call your method as `input(a)` in the second case — Ashalynd, May 10 '14 at 13:49
You should say what "not working" means - did you get a compiler error? runtime error? Crash? Looks to me like you should get a compiler error. — Bryan, May 10 '14 at 13:50
You should strongly consider using `std::vector` instead. Much easier to use and no need to worry about size. — crashmstr, May 10 '14 at 13:53

Vlad from Moscow · Answer 1 · 2014-05-10T14:02:14.307

This record

int &[]

denotes an array of references that is forbided in C++.

Declare the function the following way

void input( int ( & )[2] );

Also you are using invalid range of indeces. The valid range is [0, 1]. So the function will look as

void input( int ( &a )[2] );
{
   for ( size_t i = 0; i < 2; i++ ) std::cin >> a[i]; 
}

Also you could define the function the following way

void input( int a[], size_t n );
{
   for ( size_t i = 0; i < n; i++ ) std::cin >> a[i]; 
}

and call it as

input( a, 2 );

herohuyongtao · Answer 2 · 2014-05-10T13:56:54.203

2

It should be:

void input(int (&a)[2]);

or simply

void input(int (&)[2]);

But in function definition, you need also to give the array name:

void input(int (&a)[2]) {...}

You need to use (&array) to clarify to the compiler that you want a reference to an array, rather than the (invalid) array of references int &array[2];.

And call it like this:

input(a);

edited May 10 '14 at 13:56

answered May 10 '14 at 13:51

herohuyongtao

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score 1 · Answer 3 · answered May 10 '14 at 13:49

The syntax for returning and taking array references is:

template<typename T, size_t N>
T(&do_nothing(T(&arg)[N]))[N]
{
    return arg;
}

Or, more simply with a templated type alias:

template<typename T, size_t N> using arrayref = T(&)[N];

template<typename T, size_t N>
arrayref<T, N> do_nothing(arrayref<T, N> arg)
{
    return arg;
}

When you have defined the previous type alias template, it becomes very simple to work with array references, e.g. This is how you would declare a function that takes a reference to an array of two integers:

void give_me_two_ints(arrayref<int, 2> two_ints);

To be used like:

int a[] = {1,2,3};
for(auto const& i : do_nothing(a)) std::cout << i << "\n";

How to pass array reference from function?

3 Answers3