Although sizeof is also a keyword, it is a compile-time operator that determines the size, in bytes, of a variable or data type, including classes, structures, and unions. If used with a type, the type name must be enclosed by parentheses.
For most 32-bit compilers, the following example prints the number 4:
int x; cout << sizeof x;
For C99, sizeof is evaluated at runtime when it is applied to a variable-length array.