Name of structure as the operand of the sizeof

Question

When you define a structure like below

typedef struct num {
   int a; 
   int b; 
} A;  

Then sizeof(A) is valid, but sizeof(num) isn't. Could anyone please explain the reason why the sizeof operator doesn't accept num ?

Answer 1

Because num on its own doesn't constitute a type in C (only struct num or its typedef A constitutes the type ). So, sizeof fails as its operand is not in either form of that it's defined for:

sizeof unary-expression
sizeof ( type-name ) 

ie num is neither a type nor a unary-expression .

So, sizeof(struct num) or sizeof(A) is fine but sizeof(num) isn't.