[libcxx] Fix basic_stringbuf constructor

[miyuki]

Summary

[libcxx] Fix basic_stringbuf constructor

The C++ Standard [stringbuf.cons]p1 defines the effects of the basic_stringbuf
constructor that takes ios_base::openmode as follows:

Effects: Constructs an object of class basic_stringbuf, initializing the
base class with basic_streambuf(), and initializing mode with which.
Postconditions: str() == "".

The default constructor of basic_streambuf shall initialize all its
pointer member objects to null pointers [streambuf.cons]p1.

Currently libc++ calls "str(string_type());" in the aforementioned constructor
setting basic_streambuf's pointers to a non-null value.

This patch removes the call (note that the postcondition str() == ""
remains valid because __str_ is default-initialized) and adds a test checking
that the basic_streambuf's pointers are null after construction.

Thanks Mikhail Maltsev for the patch.

[miyuki]

Ping

[mclow.lists]

This looks good to me; with a couple of nits.

test/std/input.output/string.streams/stringbuf/stringbuf.cons/default.pass.cpp

26

Not zero, please. nullptr or NULL .
nullptr would be better, but we can't assume c++11 here, so I guess it's NULL.

[miyuki]

Modified the test to use NULL instead of literal zero.

[miyuki]

Use diff with context.

[mclow.lists]

LGTM. Do you need someone to commit it?

[miyuki]

Yes, I don't have write access.

include/sstream

virtual pos_type seekpos(pos_type __sp,
                              ios_base::openmode __wch = ios_base::in | ios_base::out);
 };
 
 template <class _CharT, class _Traits, class _Allocator>
 basic_stringbuf<_CharT, _Traits, _Allocator>::basic_stringbuf(ios_base::openmode __wch)
     : __hm_(0),
       __mode_(__wch)
 {
-    str(string_type());
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 basic_stringbuf<_CharT, _Traits, _Allocator>::basic_stringbuf(const string_type& __s,
                              ios_base::openmode __wch)
     : __str_(__s.get_allocator()),
       __hm_(0),
       __mode_(__wch)

test/std/input.output/string.streams/stringbuf/stringbuf.cons/default.pass.cpp

 // template <class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> >
 // class basic_stringbuf
 
 // explicit basic_stringbuf(ios_base::openmode which = ios_base::in | ios_base::out);
 
 #include <sstream>
 #include <cassert>
 
+template<typename CharT>
+struct testbuf
+    : std::basic_stringbuf<CharT>
+{
+    void check()
+    {
+        assert(this->eback() == NULL);
+        assert(this->gptr() == NULL);
+        assert(this->egptr() == NULL);
+        assert(this->pbase() == NULL);
+        assert(this->pptr() == NULL);
+        assert(this->epptr() == NULL);
+    }
+};
+
 int main()
 {
     {
         std::stringbuf buf;
         assert(buf.str() == "");
     }
     {
         std::wstringbuf buf;
         assert(buf.str() == L"");
     }
+    {
+        testbuf<char> buf;
+        buf.check();
+    }
+    {
+        testbuf<wchar_t> buf;
+        buf.check();
+    }
 }