Excerpt from TopCoder article:
The expression
sizeof(data)/sizeof(data[0])returns the size of the arraydata, 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?
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.
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.
c++11 std::extent<decltype(array)>::value
T(&)[N] i.e. & inside brackets as argument? Seeing it first time. What term I should google, any hints?
size(array) is expressing more clearly what it intended and is easier to type than std::extent<decltype(array)>::value.T[N]. Simply writing T[N]& is illegal and T&[N] would be an array of references (which is also illegal). Essentially the reference (or pointer) goes where the name goes but it needs appropriate parenthesis to disambiguate what is meant. Hence T(&)[N]. If it had a name which isn't needed for the function it would be T (&array)[N].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.
std::vectororstd::arrayand you won't care about when that won't work.