Is accessing a static struct::var without an object undefined behaviour?

Question

According to Compiler Explorer, only some will build this code:

struct s { static int i; };

int main( int argc, char *argv[] )
{
  s::i = 1;
  return 0;
}

Most newer C++ compilers fail at linking.

GCC 4.7.2 works, 4.7.3 and newer fail
Clang 3.2 works, 3.4 and newer fail
msvc works with all versions

So, is this undefined behaviour or should this work?

Does this answer your question? [static variable link error](https://stackoverflow.com/questions/9282354/static-variable-link-error) Basically the same problem. — Lukas-T, Sep 07 '20 at 20:32
Note that undefined behaviour is unrelated to compilation or linking but occurrs at runtime. Also accessing static member without an object is not UB ... that's why `static` exists in the first place. — Lukas-T, Sep 07 '20 at 20:33
@churill - UB has very much to do with both compilation and linking. The standard describes the behavior of the implementation, that includes translation too. If a program violates the ODR, it's UB. The manifestation can be a linker error or even appearing to "work" because something was optimized away anyway. — StoryTeller - Unslander Monica, Sep 07 '20 at 20:46
@StoryTeller-UnslanderMonica That seems logical, though I never viewed it this way. Thanks, for clarifaing. — Lukas-T, Sep 08 '20 at 05:12
I didn't see this as a declaration and not a definition for s::i - thanks for that. But something must have changed in the language since old compiler suits seem to happily build this code. Has there been a language clarification? (That's why i thought of UB here) — dee, Sep 09 '20 at 19:44

score 4 · Answer 1 · answered Sep 07 '20 at 20:32

This code violates the One Definition Rule (ODR), which requires a single definition of every entity that is used in the program.

There is no definition of s::i in the program, but you are using it, and so the code violates the ODR. Any violation of the ODR makes the code ill-formed, no diagnostic required. This means the compiler can do anything it wants, including rejecting the code, or compiling it and producing an executable program (which could do anything it wants).

score 4 · Answer 2 · edited Sep 07 '20 at 21:26

4

should this work?

It is not guaranteed to work.

So, is this undefined behaviour

Technically, the program is ill-formed (no diagnostic required). The distinction is largely irrelevant to the programmer, and it is safe to assume that it means same as undefined behaviour.

This is same for non-member variables as well. An equivalent non-member example is:

extern int global_variable;

edited Sep 07 '20 at 21:26

463035818_is_not_an_ai

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answered Sep 07 '20 at 20:33

eerorika

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Thanks for the example. I didn't see this as just being a declaration since some compiler versions build it without error. – dee Sep 09 '20 at 19:51
@dee Unfortunately, C++ is a language where lack of an error doesn't mean that the program is correct. – eerorika Sep 09 '20 at 19:52
So... UB (ex- or implicit)? – dee Sep 09 '20 at 19:53
@dee See the answer. – eerorika Sep 09 '20 at 19:58
I guess I'm too fixated on "UB"... – dee Sep 09 '20 at 20:02

Is accessing a static struct::var without an object undefined behaviour?

2 Answers2