Karl Giesing Karl Giesing - 11 months ago 47
C++ Question

C++: getting the row size of a multidimensional array passed to a function

I'm trying to write a function that will print out the contents of a multidimensional array. I know the size of the columns, but not the size of the rows.

EDIT: Since I didn't make this clear, the arrays passed to this function are NOT dynamically allocated. The sizes are known at compile time.

I am testing it using a 3x2 array. Here is the function as it stands:

void printArrays(int array1[][2], int array2[][2]) {
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 2; j++) {
cout << "\narray1[" << i << "][" << j << "] = "
<< setfill('0') << setw(2) << array1[i][j]
<< "\tarray2[" << i << "][" << j << "] = "
<< setfill('0') << setw(2) << array2[i][j];

Obviously, this only works if I know the size of "i" is 3 (it is in this case). Ideally, however, I would like the function to work no matter what the size of the first dimension.

I thought I would be able to do this using the sizeof() function, e.g.

int size = sizeof(array1);

... and do some math from there.

Here's the odd part. If I use the sizeof() function inside the array, it returns a value of 4. I can use pointer notation to dereference the array:

int size = sizeof(*array1);

... but this actually returns a value of 8. This is odd, because the total size should be rows(which = 3) * columns(= 2) * sizeof(int)(= 4), or 24. And, indeed, this is the result, when I use sizeof(*array1) outside of the function.

Does anyone know what is going on here? More importantly, does anyone have a solution?

Answer Source

The answer is that you can not do this. You must pass the number of rows as an argument to the function, or use an STL container such as std::vector or std::array.

sizeof is computed compile time; sizeof is never useful in determining dynamic size of objects in C/C++. You (yourself, the programmer) can always calculate sizeof(x) just from looking at code and header files since sizeof counts the number of bytes used to represent the object. sizeof(*array1) will always be 8 since array1[i] is an array of two ints and 4==sizeof(int). When you declare int array1[][2] this is equivalent to int *array1[2]. That is, array1 is a pointer to arrays of two integers. sizeof(array1) is therefore 4 bytes, since it takes 4 bytes on your machine to represent a pointer.