[ub] Proposal: make self-initialized references ill-formed (C++17?)

Jens Maurer Jens.Maurer at gmx.net
Sun Sep 21 14:27:43 CEST 2014


On 09/18/2014 09:42 PM, Richard Smith wrote:
> On 13 September 2014 06:52, John Zwinck <jzwinck at gmail.com <mailto:jzwinck at gmail.com>> wrote:
> 
>     I recently happened upon some code which, boiled down to its essence, was like this:
> 
>     |  for (int ii = 0; ii < 1; ++ii)
>       {
>         const std::string& str = str; // !!
>         std::cout << str << std::endl;
>       }|
> 
>     My to my surprise, this code compiled (and produced a segfault at runtime).  I say surprise because I had all warnings enabled (as errors) in GCC 4.7 and 4.9, yet there was no complaint.  I got a good answer from Jonathan Wakely (http://stackoverflow.com/a/25720743/4323) explaining why GCC failed to catch it, but this got me thinking: why does C++ allow this at all?
> 
>     So, a proposal: perhaps in C++17 we could declare that self-initialized references are ill-formed.  I did consider whether this might impact existing code; the only use case that came to mind might be SFINAE, though I surely have never seen it used that way.
> 
> 
> FYI, this is core issue 504:
> 
> http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#504

... which doesn't necessarily mean it will be automatically addressed by CWG
in the near future.

Feel free to write a short paper suggesting specific wording changes to address
this issue.  (Or, if just 1-2 sentence, send your wording changes to Mike Miller
if you won't attend WG21 meetings in person.)

Jens



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