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_countof Macro

Visual Studio 2010

Compute the number of elements in a statically-allocated array.

Copy 
_countof( 
   array
);
array

The name of an array.

The number of elements in the array.

Ensure that array is actually an array, not a pointer. In C, _countof will produce erroneous results if array is a pointer. In C++, _countof will fail to compile if array is a pointer.

Macro

Required header

_countof

<stdlib.h>
Copy 
// crt_countof.cpp
#define _UNICODE
#include <stdio.h>
#include <stdlib.h>
#include <tchar.h>

int main( void )
{
   _TCHAR arr[20], *p;
   printf( "sizeof(arr) = %d bytes\n", sizeof(arr) );
   printf( "_countof(arr) = %d elements\n", _countof(arr) );
   // In C++, the following line would generate a compile-time error:
   // printf( "%d\n", _countof(p) ); // error C2784 (because p is a pointer)

   _tcscpy_s( arr, _countof(arr), _T("a string") );
   // unlike sizeof, _countof works here for both narrow- and wide-character strings
}

sizeof(arr) = 40 bytes
_countof(arr) = 20 elements
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Re: What is the C++ version doing
There is a good explanation here: http://social.msdn.microsoft.com/Forums/en/vclanguage/thread/252b4be3-8adf-49e4-a240-93ccb91849f2 $0$0 $0
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What is the C++ version doing?

Can anyone help explain in detail how the C++ version works?

template <typename _CountofType, size_t _SizeOfArray>
char (*__countof_helper(UNALIGNED _CountofType (&_Array)[_SizeOfArray]))[_SizeOfArray];
#define _countof(_Array) (sizeof(*__countof_helper(_Array)) + 0)

Some things I don't get:

  • What is the type of __countof_helper?
  • What is the * in sizeof(*__countof_helper(_Array)) doing? Knowing the type of __countof_helper will probably help here.
  • What is the + 0 doing?
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