The keyword _Countof determines
the number of elements of an array operand.
Its syntax is similar to sizeof.
The operand must be
a parenthesized complete array type name
or an expression of such a type.
For example:
int a[n]; _Countof (a); // returns n _Countof (int [7][3]); // returns 7
The result of this operator is an integer constant expression, unless the array has a variable number of elements. The operand is only evaluated if the array has a variable number of elements. For example:
_Countof (int [7][n++]); // integer constant expression _Countof (int [n++][7]); // run-time value; n++ is evaluated