Difference between sizeof(struct name_of_struct) vs sizeof(name_of_struct)?

struct name_of_struct refers unambiguously to a struct/class tagged name_of_struct whereas name_of_struct may be a variable or a function name.

For example, in POSIX, you have both struct stat and a function named stat. When you want to refer to the struct type, you need the struct keyword to disambiguate (+ plain C requires it always -- in plain C, regular identifiers live in a separate namespace from struct tags and struct tags don't leak into the regular identifier namespace like they do in C++ unless you explicitly drag them there with a typedef as in typedef struct tag{ /*...*/ } tag;).

Example:

struct  foo{ char x [256];};
void (*foo)(void);

int structsz(){ return sizeof(struct foo); } //returns 256
int ptrsz(){ return sizeof(foo); } //returns typically 8 or 4

If this seems confusing, it basically exists to maintain backwards compatibility with C.

And, of course,

sizeof name_of_struct

is a valid expression only when name_of_struct is an object (not just a type); writing

sizeof(struct name_of_struct)

will not compile unless name_of_struct is already defined (complete type) to be a struct (or class). But wait, there's more! If both exist then

sizeof(struct name_of_struct) 

will get you the size of the type, not the object, whereas both

sizeof(name_of_struct)

and

(sizeof name_of_struct)

will get you the size of the object.

As far as I know: the difference is c style struct notation versus C++ notation.