[boost] sizeof_array?

From: Daniel Frey (daniel.frey_at_[hidden])
Date: 2002-06-24 06:48:52

Hello,

I noticed that boost doesn't have any helper for sizeof_array. The usual
approach is something like

#define sizeof_array( x ) ( sizeof(x) / sizeof(*x) )

The problem is, that it is a potential source of errors if applied to
pointers. Usually, the code that compiles looks like:

     << sizeof_array( s2 ) << endl;

Which outputs

14
1

instead of '14' twice. I therefore wrote a small helper function/class:

#include <boost/type_traits.hpp>
#include <boost/static_assert.hpp>

namespace boost
{
   template< typename array_t > class sizeof_array_t
   {
   private:
      BOOST_STATIC_ASSERT( ::boost::is_array< array_t >::value );
      typedef remove_bounds< array_t >::type element_t;

   public:
      BOOST_STATIC_CONSTANT( std::size_t, value = sizeof( array_t ) /
sizeof( element_t ) );
   };

   template< typename array_t > std::size_t sizeof_array( array_t& )
   {
      return ::boost::sizeof_array_t< array_t >::value;
   }
}

This allows using sizeof_array safely, as it doesn't compile for
non-arrays. The only potential drawback I found is, that it's use is a
bit more complicated for static evaluation and sometimes, you need
typeof (hopefully in the next standard!?). Example usage:

#include <iostream>
using namespace std;

int main()
{
   double x[] = { 1, 2, 3, 4, 5, 6 };
   cout << boost::sizeof_array( x ) << endl;

   typedef int[42] X;
   cout << boost::sizeof_array_t< X >::value << endl;

   // Static use:
   enum {
      V1 = boost::sizeof_array_t< X >::value,
      // Uses gcc-extension '__typeof'
      V2 = boost::sizeof_array_t< __typeof( x ) >::value
   }
}

What's your opinion? Is it worth to be added somewhere (utility.hpp,
static_assert.hpp, ...)? Is the implementation OK? Any feedback is
welcome...

Regards, Daniel

PS: I'm never sure if 'size_t' or 'std::size_t' is correct :)

--
Daniel Frey
aixigo AG - financial training, research and technology
Schloß-Rahe-Straße 15, 52072 Aachen, Germany
fon: [phone], fax: [phone]
eMail: daniel.frey_at_[hidden], web: http://www.aixigo.de