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Re: Struct assignment

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On 20/01/13 03:25, Eric Sosman wrote:
> On 1/19/2013 9:18 AM, Russell Shaw wrote:
>> On 20/01/13 00:27, Eric Sosman wrote:
>>> [...]
>>> Have you ever run into the (in)famous "struct declaration
>>> in parameter list" problem (FAQ Question 2.5)? That's very much
>>> like what's happening here: Having written a type declaration
>>> without a name (tag) or alias (typedef), you have no way to
>>> designate that same type again.
>>
>> Why should it matter that the second struct is "declared" in a separate
>> translation unit? To be able to do the assignment, you have to #include
>> that second declaration in to the first file anyway.
>
> The point of separate compilation is that the compilations
> are, well, separate. The compiler processes one file on Tuesday
> and the other on Wednesday, perhaps even on different machines.
> The two declarations are not visible to the compiler at the same
> time, so the compiler has no way to connect them. The compiler
> cannot even tell that the declarations were #include'd from the
> same file -- or, quite likely, from distinct files with identical
> content (Tuesday's compilation on Machine A, Wednesday's on
> Machine B with files copied via FTP, say).
>
> That's why there's a special dispensation for textually
> similar type declarations in different translation units: Without
> such a rule, separate compilation simply wouldn't work.
>
> Within a single translation unit, though, things are different.
> Two "anonymous" type declarations are treated as distinct types,
> even if they bear a strong resemblance. Yes, this can lead to a
> paradox, where two types in Module X are mutually incompatible,
> but are both compatible with a single type in Module Y. Don't
> get too hung up over it; just laugh it off and maybe use it as
> a trick question to win bets down at the nerd bar.
>
>> It's easy to write a compiler to do that assignment deterministically,
>> so why's it not allowed?
>
> I'm not entirely sure what you mean by this -- but if "it's
> easy," go ahead and do it. Get the gcc source, modify the compiler
> to treat duplicated anonymous declarations the way you want, try it
> out on assorted code bases, and see where it leads. If the outcome
> is good, your modification can be "prior art" to support a proposal
> to the next revision of the Standard.

I've half done a compiler, so i should know why things are done the way they 
are. Testing what gcc dis-allows is only a secondary way of reading between the 
lines of the standard. Even if the struct assignment is easy to do, i'll only do 
it if the standard allows it.

>> p.6.2.7 describes the compatability for structs in separate translation
>> units, but says nothing (ie, does not forbid) about assignments in the
>> same translation unit.
>
> 6.2.7 doesn't allow or forbid any kind of assignment; the
> string "assign" does not even appear there.
>
> Assignment is covered in 6.5.16, and in 6.5.16.1p1 (second
> bullet) you'll find the requirement that struct assignment must
> be between compatible types. *Then* we go back to 6.2.7 to read
> the rules on compatibility, and learn that identical anonymous
> struct types are compatible only if declared in separate
> translation units. Your pair are not in separate translation
> units so that language doesn't apply; the structs are therefore
> incompatible and do not satisfy the "shall" of 6.5.16.1.

The problem is that if you #include the declaration of an anonymous struct sa 
from a separate translation unit, then the compiler complains about sb = sa.

If you *don't* #include the declaration of an anonymous struct sa from a 
separate translation unit, then the compiler complains that sa isn't declared.

In what use-case can sb = sa be done without complaint?

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Struct assignment Russell Shaw <rjshawN_o@s_pam.netspace.net.au> - 2013-01-19 23:09 +1100
  Re: Struct assignment Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-01-19 08:27 -0500
    Re: Struct assignment Russell Shaw <rjshawN_o@s_pam.netspace.net.au> - 2013-01-20 01:18 +1100
      Re: Struct assignment "BartC" <bc@freeuk.com> - 2013-01-19 14:59 +0000
      Re: Struct assignment Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-01-19 11:25 -0500
        Re: Struct assignment Russell Shaw <rjshawN_o@s_pam.netspace.net.au> - 2013-01-20 12:46 +1100
          Re: Struct assignment Bart van Ingen Schenau <bart@ingen.ddns.info.invalid> - 2013-01-20 09:20 +0000
          Re: Struct assignment Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-01-20 09:08 -0500
            Re: Struct assignment Russell Shaw <rjshawN_o@s_pam.netspace.net.au> - 2013-01-21 12:02 +1100
              Re: Struct assignment Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-01-20 22:08 -0500
                Re: Struct assignment James Kuyper <jameskuyper@verizon.net> - 2013-01-20 23:21 -0500
              Re: Struct assignment James Kuyper <jameskuyper@verizon.net> - 2013-01-21 08:23 -0500
                Re: Struct assignment Russell Shaw <rjshawN_o@s_pam.netspace.net.au> - 2013-01-22 13:33 +1100
                Re: Struct assignment Russell Shaw <rjshawN_o@s_pam.netspace.net.au> - 2013-01-23 19:53 +1100
      Re: Struct assignment Shao Miller <sha0.miller@gmail.com> - 2013-01-19 11:45 -0500
        Re: Struct assignment Russell Shaw <rjshawN_o@s_pam.netspace.net.au> - 2013-01-20 13:21 +1100
          Re: Struct assignment Shao Miller <sha0.miller@gmail.com> - 2013-01-20 01:06 -0500
            Re: Struct assignment Russell Shaw <rjshawN_o@s_pam.netspace.net.au> - 2013-01-20 18:03 +1100
  Re: Struct assignment glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-19 19:18 +0000
    Re: Struct assignment James Kuyper <jameskuyper@verizon.net> - 2013-01-19 15:22 -0500
      Re: Struct assignment Shao Miller <sha0.miller@gmail.com> - 2013-01-19 15:47 -0500
      Re: Struct assignment Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-01-19 17:56 -0500
        Re: Struct assignment Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-20 12:12 -0800
  Re: Struct assignment falk@rahul.net (Edward A. Falk) - 2013-01-19 23:41 +0000
    Re: Struct assignment glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-20 01:01 +0000
    Re: Struct assignment James Kuyper <jameskuyper@verizon.net> - 2013-01-19 20:53 -0500
      Re: Struct assignment Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-20 12:03 -0800

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