Why sizeof int is wrong, while sizeof(int) is right?

Question

We know that sizeof is an operator used for calculating the size of any datatype and expression, and when the operand is an expression, the parentheses can be o

Answer 1

There are two ways to use the sizeof operator in C. The syntax is this:

C11 6.5.3 Unary operators
...
sizeof unary-expression
sizeof ( type-name )

Whenever you use a type as operand, you must have the parenthesis, by the syntax definition of the language. If you use sizeof on an expression, you don't need the parenthesis.

The C standard gives one such example of where you might want to use it on an expression:

sizeof array / sizeof array[0]

However, for the sake of consistency, and to avoid bugs related to operator precedence, I would personally advise to always use () no matter the situation.