Empty struct in C++ does not have zero size

Question

Consider the following code:

#include <stdio.h>

struct Empty1 {
} __attribute__((packed));

struct Empty2 {
        size_t data[0];
} __attribute__((packed));

int main(int argc, char *argv[])
{
        printf("sizeof(Empty1)=%zd\n", sizeof(struct Empty1));
        printf("sizeof(Empty2)=%zd\n", sizeof(struct Empty2));
        return 0;
}

Compiling it and running it with g++ (7.3) prints:

sizeof(Empty1)=1
sizeof(Empty2)=0

Which seems strange to me, since I would expect sizeof(Empty1) to also be 0.

what's more strange to me, however, is that compiling and running the same code with gcc (7.3) prints:

sizeof(Empty1)=0
sizeof(Empty2)=0

The fact that these two structures have different sizes depending on how they are compiled, seems like a bug to me, since it breaks the ABI between programs written in C or C++ that share headers. Not sure what is specified by the spec in this cases, but even as an implementation this looks strange to me.

Let me find the dup for the struct containing the empty array. — YSC, Mar 27 '18 at 10:27
Zero sized arrays are non-standard C or C++, and there are no empty structs in standard C. This question is kind of a mess of tags. — StoryTeller - Unslander Monica, Mar 27 '18 at 10:29
https://stackoverflow.com/questions/33246949/struct-hack-zero-sized-array — YSC, Mar 27 '18 at 10:29

Empty struct in C++ does not have zero size

0 Answers0