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

Question

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Closed 9 years ago.

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Empty class in C++
What is the size of an empty struct in C?

I read somewhere that size of an empty struct in C++ is 1. So I thought of verifying it. Unfortunately I saved it as a C file and used <stdio.h> header and I was surprised to see the output. It was 0.

That means

struct Empty {

};

int main(void)
{
  printf("%d",(int)sizeof(Empty));
}

was printing 0 when compiled as a C file and 1 when compiled as a C++ file. I want to know the reason. I read that sizeof empty struct in c++ is not zero because if the size were 0 two objects of the class would have the same address which is not possible. Where am I wrong?

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