What are the differences between:
sizeof(struct name_of_struct)
vs
sizeof(name_of_struct)
The return values of the subject are the same.
Is there any difference between the two? Even if it is subtle/not important, I'd like to know.
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3 Answers
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;).
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.
2 Comments
Miles Budnek
One additional minor note, the
struct and class keywords are interchangeable in this context. sizeof(struct foo) and sizeof(class foo) would both work. You don't see the latter very often though, since this mainly comes up in the context of interoperating with C code.
eerorika
Furthermore
struct name_of_struct i.e. an elaborated type specifier also declares name_of_struct. This doesn't make a difference in this particular case, since in order to use sizeof, name_of_struct must be defined, and therefore declared prior. But in another case, it can make a difference.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.
Comments
As far as I know: the difference is c style struct notation versus C++ notation.
2 Comments
eerorika
No, they are both C++.
sizeof(name_of_struct) is only C++ i.e. not C, unless name_of_struct is also a typedef.
Tarick Welling
I agree wholeheartedly
