[Boost-bugs] [Boost C++ Libraries] #11150: string_ref::to_string is necessarily UB for a default-constructed string_ref, despite stating no preconditions

Subject: [Boost-bugs] [Boost C++ Libraries] #11150: string_ref::to_string is necessarily UB for a default-constructed string_ref, despite stating no preconditions
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2015-03-28 18:22:43

#11150: string_ref::to_string is necessarily UB for a default-constructed
string_ref, despite stating no preconditions
-------------------------------+---------------------------
 Reporter:  Tomalak Geret'kal  |      Owner:  no-maintainer
     Type:  Bugs               |     Status:  new
Milestone:  To Be Determined   |  Component:  utility
  Version:  Boost 1.57.0       |   Severity:  Problem
 Keywords:                     |
-------------------------------+---------------------------
 Consider:

 {{{
 boost::string_ref foo;
 auto bar = foo.to_string();
 }}}

 As currently specified (`to_string` is not documented but in the code it
 seems to share the semantics of explicit string conversion, which is
 defined by the reference material, N3442), this invokes
 `std::string::string(nullptr, 0)`, which has UB because one of the
 preconditions of that `std::string` constructor is that the pointer not be
 `nullptr` or `NULL`... and default-constructing `boost::string_ref` indeed
 sets the relevant pointer to `NULL`.

 This has been fixed in newer versions of the proposal, e.g. by N3762
 (seemingly accidentally):

 > Returns: `basic_string<charT, traits, Allocator>(str.begin(), str.end(),
 a)`

 ''Boost.StringRef'' should be updated to make the same change, otherwise
 invoking `.to_string()` on a default-constructed `boost::to_string` has
 undefined behaviour, despite there being no preconditions specified on
 `.to_string()`.

 Alternatively it could be documented that `.to_string()`'s precondition is
 that the `string_ref` actually refers to a string.


 (Thanks to Mgetz and Luc Danton on Stack Overflow for help identifying
 this bug.)

 (I tried to link to references but Trac thought I was a spammer and the
 Captcha did not show up.)

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/11150>
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