Hacker News Reader

Comment by johnydepp | Hacker News Reader

do you realize this would crash with empty array: #define countof(X) (sizeof(X) / sizeof((X)[0]))

No, because the expression is not evaluated at any time. Only the type of the expression is used.

A much better implementation would be

    using std::begin, std::end;
    #define countof(x) (end(x) - begin(x))

(except for the double evaluation, but this is better as a template function instead of a macro anyway) because it'll work on any type that supports random-access iterators which include arrays but also std::array and vector.

There is such a function in the STL already.

http://en.cppreference.com/w/cpp/iterator/distance

Does this work at compile-time?

One of the uses for countof is (somewhat ironically) doing a enum->string table where you want to enforce the table gets manually updated properly:

    enum Color { red, green, blue, num_colors };
    const char *color_string[] = { "red", "green", "blue" };

    static_assert(countof(color_string) == num_colors);

No, it doesn't, which is why I said this is better implemented with a template :)

Corrected below, leaving here for posterity and to make sure this conversation isn't confusing in the future. The only thing I'll note is that you get a compile error on a zero length array, so the OP of this chain turns out right in a way heh.

Start ignoring here: Are you suggesting end() and begin() can be called on an array? As far as I know they can't. Apparently you can deduce array length in a constexpr function now in C++11 (which I only learned just now but also couldn't get working quickly, so have some salt with that), but before that arrays always degrade to pointers when passed as function arguments so there's (afaik) no way to extract their length from their type...

Before the array decays to a pointer, you can get its length.

Here is the implementation of std::begin for arrays in libc++: https://github.com/llvm-mirror/libcxx/blob/60d223df071f6e3d4...

Quite right, I'm apparently rusty on my stdlib knowledge. Didn't know about the reference trick.

Yes you can call std::begin() and std::end() on an array.

    int a[] = {4,5,6};
    for(int i : a)
        ;

That is how the above works.

1) Don't use empty arrays. 2) So add a proper version to the language which works under all cases.

Like a std::vector ?

std::array has size() without any runtime overhead.

I tried it and it works for me (gcc 4.8.4), but I couldn't tell you why.

Reply on news.ycombinator.com