A C++ macro to compute the number of elements in an array.

array_size.h

/*
ARRAY_SIZE(a)
A macro to compute the number of elements in an array a.

Features & considerations:
- Will not compile if a is not an array (error message is not elegant).
- Able to be used with non-const a.
- Happens to produce very few (false-positive) compile-time warnings with msvc 19 when 
  the result is used in signed and truncating arithmetic - unlike std::size()
  (see also https://t.co/NvhjvMUhA1)
- Avoids these msvc compiler bugs:
  https://developercommunity.visualstudio.com/content/problem/410617/sizeofchar-1-evaluates-to-0xffffffff.html
  https://developercommunity.visualstudio.com/content/problem/410675/lambda-implicit-variable-capture-fails-near-value.html
- A revision of a pre-existing implementation used extensively in a large legacy codebase.
  This version is written to avoid changes to existing code. YMMV :)
- Valid for C++ 98, 11, 14, and 17.
*/

namespace Detail
{
  // No matching overloaded function found? Typically that error is produced when calling 
  // ARRAY_SIZE(a) where a is not an array.  
  template <typename T, size_t N> char ArrayHelper(const T(&)[N]); 
}

// Returns the number of elements in the array 'a'
// Fails to compile unless 'a' is actually an array.
#define ARRAY_SIZE(a) (sizeof(a) / sizeof(*a) * sizeof(Detail::ArrayHelper(a)))