[Bug libstdc++/56437] New: basic_string assumes that allocators are default-constructible

[Bug libstdc++/56437] New: basic_string assumes that allocators are default-constructible

[tneumann at pi3 dot informatik.uni-mannheim.de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56437

             Bug #: 56437
           Summary: basic_string assumes that allocators are
                    default-constructible
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: tneumann@pi3.informatik.uni-mannheim.de


Created attachment 29531
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29531
example

The empty-string optimization of basic_string assumes that allocators are
default constructible. While this used to be the case in C++98, it is no longer
true in C++11, as now allocators are allowed to have state.

Consider the attached example program. Compiling with

g++ -std=c++11 -c t.cpp

produces an error message, even though it should compile fine. The problem is
the the "_S_construct" calls "_Alloc()", which does not exist.

Note that the C++11 standard does not require default constructors. (Section
17.6.3.5, Table 28). In particular, the SimpleAllocator example from Section
17.6.3.5 would trigger the same bug, too.

[Bug libstdc++/56437] basic_string assumes that allocators are default-constructible

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56437

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2013-02-25 00:08:35 UTC ---
This is hardly the only C++11 allocator requirement missing from std::string,
ALL of the new requirements are missing, and unlikely to be implemented until
we switch to a non-COW string implementation.

[Bug libstdc++/56437] basic_string assumes that allocators are default-constructible

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56437

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> 2013-02-25 00:11:16 UTC ---
C++11 allocators are not required to define any of these:

   typedef T* pointer;
   typedef const T* const_pointer;
   typedef T& reference;
   typedef const T& const_reference;
   typedef unsigned size_type;
   typedef int difference_type;
   template <class U> struct rebind { typedef MyAllocator<U> other; };

But if your allocator fails to define any of them then std::string (and some of
the other containers) will fail to compile. This is a known issue.

[Bug libstdc++/56437] basic_string assumes that allocators are default-constructible

[tneumann at pi3 dot informatik.uni-mannheim.de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56437

--- Comment #3 from Thomas Neumann <tneumann at pi3 dot informatik.uni-mannheim.de> 2013-02-25 06:31:43 UTC ---
(In reply to comment #1)
> This is hardly the only C++11 allocator requirement missing from std::string,
> ALL of the new requirements are missing, and unlikely to be implemented until
> we switch to a non-COW string implementation.

Well, but this would be an allocator requirement that is easy to fix. There are
only three places with "_Alloc()" in basic_string.tcc, and only two would have
to be fixed as far as I can see it.

I am not sure yet why the code even tries to construct the allocator, but if
necessary we could use is_default_constructible to figure out of the
constructor is available. I could try to produce a patch myself if you prefer.
But as of new basic_string is quite unusable with stateful allocators.

[Bug libstdc++/56437] basic_string assumes that allocators are default-constructible

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56437

--- Comment #4 from Paolo Carlini <paolo.carlini at oracle dot com> 2013-02-25 11:17:30 UTC ---
Note that all such changes should be applied to ext/vstring (which will become
basic_string in 4.9) not to basic_string. By now the latter is frozen, modulo
serious regressions.

[Bug libstdc++/56437] basic_string assumes that allocators are default-constructible

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56437

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2014-09-19
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org
   Target Milestone|---                         |5.0
     Ever confirmed|0                           |1

[Bug libstdc++/56437] basic_string assumes that allocators are default-constructible

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56437

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed for GCC 5 (when using the new string ABI)