In fact, in C having a struct without members is undefined behavior: > If the struct-declaration-list does not contain any named members, either > directly or via an anonymous structure or anonymous union, the behavior is > undefined.
However, gcc allows this as an extension, and clang has apparently followed that: https://gcc.gnu.org/onlinedocs/gcc/Empty-Structures.html -Dimitry > On 20 Feb 2025, at 12:03, David Chisnall <thera...@freebsd.org> wrote: > > No, that’s always been the case in C++. It comes from the rule that two > allocations must have unique addresses. If a structure could have size zero, > an array of these structures would have size zero and the two elements in the > array would have the same address. Similarly, two struct fields could have > the same address, which breaks other bits of the language (pointers to > members would compare equal when they should not). > > C++20 introduced the no_unique_address attribute. This allows you to embed a > struct in another and, if the child struct has no fields, then its address is > not required to be unique and you are explicitly saying that you won’t do any > of the things where this would be a problem. > > This lets you do things like: > > ```c++ > template<typename Embedded=void> > struct SomeStructThatMayContainAnother > { > // Normal fields go here > > [[no_unique_address]] > std::conditional_t<std::is_same_v<Embedded, void>, struct {}, Embedded> > embeddedStruct; > > // More fields maybe here > }; > ``` > > In this case, `embeddedStruct` will not add any space to the parent struct if > the template parameter is `void`. > > David > >> On 20 Feb 2025, at 09:40, Poul-Henning Kamp <p...@phk.freebsd.dk> wrote: >> >> Is this a bug ? >> >> >> critter phk> cat /tmp/_.c >> >> #include <stdio.h> >> struct foo { >> }; >> >> int >> main(int argc, char **argv) >> { >> struct foo bar; >> printf("%jd %jd\n", sizeof(struct foo), sizeof(bar)); >> return (0); >> } >> critter phk> cc -o /tmp/a.out /tmp/_.c >> critter phk> /tmp/a.out >> 0 0 >> critter phk> c++ -o /tmp/a.out /tmp/_.c >> c++: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is >> deprecated [-Wdeprecated] >> critter phk> /tmp/a.out >> 1 1 >> >> -- >> Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 >> p...@freebsd.org | TCP/IP since RFC 956 >> FreeBSD committer | BSD since 4.3-tahoe >> Never attribute to malice what can adequately be explained by incompetence. >> >> > >
