Equivalents to MSVC's _countof in other compil

问题:

Are there any builtin equivalents to _countof provided by other compilers, in particular GCC and Clang? Are there any non-macro forms?

回答1:

Using C++11, the non-macro form is:

char arrname[5];
size_t count = std::extent< decltype( arrname ) >::value;

And extent can be found in the type_traits header.

Or if you want it to look a bit nicer, wrap it in this:

template < typename T, size_t N >
size_t countof( T ( & arr )[ N ] )
{
    return std::extent< T[ N ] >::value;
}

And then it becomes:

char arrname[5];
size_t count = countof( arrname );

char arrtwo[5][6];
size_t count_fst_dim = countof( arrtwo );    // 5
size_t count_snd_dim = countof( arrtwo[0] ); // 6

Edit: I just noticed the "C" flag rather than "C++". So if you're here for C, please kindly ignore this post. Thanks.

回答2:

I'm not aware of one for GCC, but Linux uses GCC's __builtin_types_compatible_p builtin to make their ARRAY_SIZE() macro safer (it'll cause a build break if applied to a pointer):

/* &a[0] degrades to a pointer: a different type from an array */
#define __must_be_array(a) \
 BUILD_BUG_ON_ZERO(__builtin_types_compatible_p(typeof(a), typeof(&a[0])))

#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr))

Note: I think the BUILD_BUG_ON_ZERO() macro has a misleading name (it causes a build failure if the expression is not zero and returns 0 otherwise):

/* Force a compilation error if condition is true, but also produce a
   result (of value 0 and type size_t), so the expression can be used
   e.g. in a structure initializer (or where-ever else comma expressions
   aren't permitted). */
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))

I think the naming for this macro comes from looking at it in two parts: BUILD_BUG_ON is what the macro does when the expression is true, and ZERO is the value 'returned' by the macro (if there's not a build break).

回答3:

This?

#define _countof(a) (sizeof(a)/sizeof(*(a)))

回答4:

Update: C++ 17 support std::size() (defined in header <iterator>)

You can use boost::size() instead:

#include <boost/range.hpp>

int my_array[10];
boost::size(my_array);