SD11 SD11 - 21 days ago 5x
C Question

name of structure as the operand of the sizeof

When you define a structure like below

typedef struct num{

int a;
int b;


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

usr usr

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 ) 

i.e. num is neither a type nor a unary-expression.

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