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!

Answer 1

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-3×3-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).