[libcxx] Fix basic_stringbuf constructor

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.

Reviewers: EricWF, mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D40707

llvm-svn: 320604

libcxx/include/sstream

@@ -243,7 +243,6 @@ basic_stringbuf<_CharT, _Traits, _Allocator>::basic_stringbuf(ios_base::openmode
     : __hm_(0),
       __mode_(__wch)
 {
-    str(string_type());
 }
 
 template <class _CharT, class _Traits, class _Allocator>

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

@@ -17,6 +17,21 @@
 #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()
 {
     {
@@ -27,4 +42,12 @@ int main()
         std::wstringbuf buf;
         assert(buf.str() == L"");
     }
+    {
+        testbuf<char> buf;
+        buf.check();
+    }
+    {
+        testbuf<wchar_t> buf;
+        buf.check();
+    }
 }