sizeof empty structure is 0 in C and 1 in C++ why? [duplicate]

Answer 1

You cannot have an empty structure in C. It is a syntactic constraint violation. However gcc permits an empty structure in C as an extension. Furthermore the behaviour is undefined if the structure does not have any named member because

C99 says :

If the struct-declaration-list contains no named members, the behavior is undefined.

So

struct Empty {}; //constraint violation

struct Empty {int :0 ;}; //no named member, the behaviour is undefined.

And yes size of an empty struct is C++ cannot be zero :)

Answer 2

Here is a wonderful article describing why this occurs, and more pertinently, a (safe) way around it :)

http://www.cantrip.org/emptyopt.html

Answer 3

There are several good reasons. Among others, this is to ensure that pointer arithmetics over pointers to that structure don't lead to an infinite loop. More information:

http://bytes.com/topic/c/insights/660463-sizeof-empty-class-structure-1-a