Are there other ways of passing array in a function?

Question

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passing 2D array to function

My question is related to passing an array as a C++ function argument. Let me show the example first:

void fun(double (*projective)[3])
{
    for(int i=0; i<3; i++)
        for(int j=0; j<3; j++)
        {
            projective[i][j]= i*100+j;
        }
}

int main()
{
    double projective[3][3];     

    fun(projective);
    for(int i=0; i<3; i++)
    {
        cout<<endl;
        for(int j=0; j<3; j++)
            cout<<projective[i][j]<<"   ";
    }


    return 0;
}

In the example, the passing argument for fun is an array, and I was wondering whether there are other ways of passing this argument. Thanks!

score 3 · Accepted Answer · answered Oct 18 '12 at 08:36

fun takes a pointer-to-array-of-3-double and it assumes (relying on the caller) that this points to the first element of an array of at least 3 arrays-of-3-doubles. Which it does, because as you say the argument supplied to the call in main is an array. This immediately decays to a pointer to its first element.

One alternative would be for fun to take a pointer-to-3x3-array-of-double, since it assumes that size anyway and the caller does in fact have such a beast:

void fun(double (*p_projective)[3][3])
{
    for(int i=0; i<3; i++)
        for(int j=0; j<3; j++)
        {
            (*p_projective)[i][j]= i*100+j;
        }
}

Call it with fun(&projective).

score 1 · Answer 2 · answered Oct 18 '12 at 08:32

You cannot pass an array as such, it always decays to a pointer. But you can pass an array if you wrap in it a struct. This works in C and C++. In C++ you can pass a reference to an array. In both cases the array is fixed size.

// as a struct
struct Array
{
  int elems[10];
};

void func(Array a);

// as a reference
void func(int (&a)[10]);

score 1 · Answer 3 · answered Oct 18 '12 at 08:41

If you're so inclined, you can also just use a base pointer and do the offsets yourself, assuming your array was allocate or declared to have dim*dim elements.

void fun(double *projective, size_t dim)
{
    for(size_t i=0; i<dim; i++)
        for(size_t j=0; j<dim; j++)
            projective[i*dim+j] = i*100+j;
}

int main(int argc, char *argv[])
{
    double ar[5*5];
    fun(ar, 5);
    return 0;
}

There are a horde of ways to do this, this is only one, but usually the easiest to wrap my head around (and I usually use std::vector<> for the back-end).

Are there other ways of passing array in a function?

3 Answers3