_countof Macro

Computes the number of elements in a statically allocated array.

#define _countof(array) (sizeof(array) / sizeof(array[0]))

array
The name of an array.

The number of elements in the array, expressed as a size_t.

_countof is implemented as a function-like preprocessor macro. The C++ version has extra template machinery to detect at compile time if a pointer is passed instead of a statically declared array.

Ensure that array is actually an array, not a pointer. In C, _countof produces erroneous results if array is a pointer. In C++, _countof fails to compile if array is a pointer. An array passed as a parameter to a function decays to a pointer, which means that within the function, you can't use _countof to determine the extent of the array.

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

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

sizeof Operator