Passing arrays by reference in C++

Question

Possible Duplicate:
What is useful about a reference-to-array parameter?
How do I use arrays in C++?

I am trying to pass the arrays by reference. The problem is I am getting errors for passing these arrays.

error C2664: 'InitializeArrays' : cannot convert parameter 1 from 'int [64]' to 'int (&)[]'

Here is the code:

void InitializeS(int (&s)[], int (&BeforeDecimal1)[]);

int main()
{
   int BeforeDecimal[128],s[128];

   InitializeS(s,BeforeDecimal);

   return 0;
}

void InitializeS(int (&s)[], int (&BeforeDecimal1)[])
{
    for(int i=0;i<128;i++)
    {
        s[i]=0;
        BeforeDecimal1[i]=0;
    }
}

What am I doing wrong?

Look [this useful FAQ](http://stackoverflow.com/questions/4810664/how-do-i-use-arrays-in-c). — PaperBirdMaster, Dec 21 '12 at 10:31
You probably want `std::vector` instead of C style int arrays. — fredoverflow, Dec 21 '12 at 10:32
But sometimes I pass arrays of different sizes to this function, thats why I didn't put specific size in there — Alfred, Dec 21 '12 at 10:32
Then you should write a function template taking the size as a template parameter. — fredoverflow, Dec 21 '12 at 10:33
@Alfred, and that is exactly why you have a bug in `InitializeS` when you overwrite your `s` array which has 50 elements while you write 128 elements to it :) Yes, I know it's an example, but that is essentially the reason why you need size — Zdeslav Vojkovic, Dec 21 '12 at 10:35
@Alfred: Sometimes I have people of varying sizes in my car so I just took out the seats. — Lightness Races in Orbit, Dec 21 '12 at 10:45

score 4 · Accepted Answer · answered Dec 21 '12 at 10:32

4

In C++ there is unlike as in C no concept of compatible types and the type T[] is unrelated to T[N]. You need to make the reference have a size and the size must equal the one of the array you pass.

C++ also bans references to arrays without bounds as function parameter types (perhaps for this reason).

answered Dec 21 '12 at 10:32

Johannes Schaub - litb

496,577
130
894
1,212

It is an array without size. Incomplete type – Johannes Schaub - litb Dec 21 '12 at 10:54
Are you a full time programmer? Man how do you keep all this stuff in mind.I forget things when I don't work in C++ for a while – Alfred Dec 21 '12 at 11:04
@JohannesSchaub-litb: That is better :) – Lightness Races in Orbit Dec 21 '12 at 11:32
@Alfred: Everybody has an area of expertise! – Lightness Races in Orbit Dec 21 '12 at 11:33

Lightness Races in Orbit · Answer 2 · 2012-12-21T11:33:00.657

Why yours doesn't work

The "array" function argument syntax void foo(int array[]) is misleading in that it has nothing to do with arrays. It is syntactic "sugar" (read: horribly legacy confusion that we're stuck with) for void foo(int* array).

As such, void foo(int (&array)[]) doesn't do what you think it does. It's not even equivalent to void foo(int*& array). In fact, the syntax simply doesn't exist.

Passing arrays by reference

Real arrays have dimensions and these dimensions are part of the static type (and everywhere that you see int[] as a "type" is either using it as an incomplete type, or is merely syntactic sugar for something else), so you will need to state the dimension with the type:

void foo(int (&array)[5])

or use templates:

template <size_t N>
void foo(int (&array)[N])

What if I can't do that?

If you don't know the dimension at compile-time then you are SOL and will have to use a std::vector instead.

...unless you declare `extern int a[];` in which case `a` really *is* an array and *not* a pointer. — fredoverflow, Dec 21 '12 at 11:11
@FredOverflow: Okay, so `int[]` exists as an _incomplete_ type. — Lightness Races in Orbit, Dec 21 '12 at 11:32

score 0 · Answer 3 · answered Dec 21 '12 at 10:43

As FredOverflow suggests, you need to use templates to implement this functionality.

Here is a way to implement it:

template<int S1, int S2> void InitializeS(int(&)[S1], int(&)[S2]);

int main()
{
   int BeforeDecimal[100],s[50];

   InitializeS(s,BeforeDecimal);

   return 0;
}

template<int S1, int S2> void InitializeS(int (&s)[S1], int (&BeforeDecimal1)[S2])
{
    for(int i=0;i<128;i++)
    {
        s[i]=0;
        BeforeDecimal1[i]=0;
    }
}

Note that this only works if the size of the arrays is known at compile-time.

Passing arrays by reference in C++

3 Answers3

Why yours doesn't work

Passing arrays by reference

What if I can't do that?