[Bug c++/80542] Warn about accidental copying of data in range based for

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80542

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2017-04-27
     Ever confirmed|0                           |1
           Severity|normal                      |enhancement

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
That's what 'auto' is for :-)

This could be helpful though. Code that intentionally wants to perform a
conversion to a different type doesn't need to use a const-reference, it could
just do:

  for (std::pair<std::string, std::string> s : m)

That means there's a simple way (using fewer tokens!) to avoid the warning if a
conversion is really wanted.

It could be useful in other contexts too, this has the same problem:

  const std::pair<std::string, std::string>& obj = *m.begin();

And again, if the conversion is intended there is no reason to use a reference
bound to a temporary instead of:

  const std::pair<std::string, std::string> obj = *m.begin();

(People who prefer to former need to learn to trust copy elision).

In other contexts we probably wouldn't want to warn:

  void foo(const std::pair<std::string, std::string>&);
  foo(*m.begin());

This performs the same conversion but it's probably not possible to tell if a
warning is relevant.