Do not take address of empty string front

[Jonathan Wakely]

On 22/06/15 16:10 +0100, Jonathan Wakely wrote:
>On 20/06/15 12:59 +0100, Jonathan Wakely wrote:
>>On 20/06/15 12:03 +0200, François Dumont wrote:
>>>Hi
>>>
>>>  2 experimental tests are failing in debug mode because
>>>__do_str_codecvt is sometimes taking address of string front() and
>>>back() even if empty. It wasn't use so not a big issue but it still
>>>seems better to avoid. I propose to rather use string begin() to get
>>>buffer address.
>>
>>But derefencing begin() is still undefined for an empty string.
>>Shouldn't that fail for debug mode too? Why change one form of
>>undefined behaviour that we diagnose to another form that we don't
>>diagnose?
>>
>>It would be better if that function didn't do any work when the input
>>range is empty:
>>
>>--- a/libstdc++-v3/include/bits/locale_conv.h
>>+++ b/libstdc++-v3/include/bits/locale_conv.h
>>@@ -58,6 +58,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>>                   _OutStr& __outstr, const _Codecvt& __cvt, _State& __state,
>>                   size_t& __count, _Fn __fn)
>>   {
>>+      if (__first == __last)
>>+       {
>>+         __outstr.clear();
>>+         return true;
>>+       }
>>+
>>     size_t __outchars = 0;
>>     auto __next = __first;
>>     const auto __maxlen = __cvt.max_length() + 1;
>
>This makes that change, and also moves wstring_convert into the
>ABI-tagged __cxx11 namespace, and fixes a copy&paste error in the
>exception thrown from wbuffer_convert.

This is the equivalent patch for the branch.

Tested powerpc64le-linux, committed to gcc-5-branch.
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