Ill-Advised C++ Rant, Part 2

Post by stormbrew » Mon, Mar 07, 2016, 3:00 AM UTC

tines wrote: post #4

johnydepp wrote: post #2

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…

Post by hendzen » Mon, Mar 07, 2016, 3:05 AM UTC

stormbrew wrote: post #11

tines wrote: post #4

Earlier quoted context omitted.

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…

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 functio…

Post by onedognight » Mon, Mar 07, 2016, 3:08 AM UTC

stormbrew wrote: post #11

tines wrote: post #4

Earlier quoted context omitted.

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…

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 functio…

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

Post by kayamon » Mon, Mar 07, 2016, 3:10 AM UTC

tines wrote: post #4

johnydepp wrote: post #2

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…

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

    static_assert(countof(color_string) == num_colors);

Post by tines » Mon, Mar 07, 2016, 3:19 AM UTC

kayamon wrote: post #17

tines wrote: post #4

Earlier quoted context omitted.

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…

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);