Calculation of Array size using sizeof()

Question

Excerpt from TopCoder article:

The expression sizeof(data)/sizeof(data[0]) returns the size of the array data, but only in a few cases, so don’t use it anywhere except in such constructions.(C programmers will agree with me!)

To get array size, I've been using this expression sizeof(data)/sizeof(data[0]) all the time for all primitive types.

Does anyone know about any such case in which the above expression should not be used?

Answer 1

If data were declared like so:

int *data;

And then space for it allocated like so:

data = malloc( NUM_ELEMENTS * sizeof(int) );

Then your technique will not work, because sizeof(data) is the size of a pointer, not the content of the array.

Answer 2

The sizeof approach compiles but doesn't work when giving it a pointer or an array of indeterminate size. Just use the proper C++ approach:

template <typename T, std::size_t N>
constexpr std::size_t size(T(&)[N]) {
    return N;
}

Using size() on an array works correctly. It will be a compile-time error to use it in a case where it is not applicable, e.g., on a pointer.

Answer 3

The sizeof technique work correction for static array, it will not have any issue. As mentioned in the above for dynamic array and pointer data it will not work correctly.