[libcxx] r320604 - [libcxx] Fix basic_stringbuf constructor

[Zhihao Yuan via cfe-commits]

Author: lichray
Date: Wed Dec 13 10:12:55 2017
New Revision: 320604

URL: http://llvm.org/viewvc/llvm-project?rev=320604&view=rev
Log:
[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

Modified:
    libcxx/trunk/include/sstream
    libcxx/trunk/test/std/input.output/string.streams/stringbuf/stringbuf.cons/default.pass.cpp

Modified: libcxx/trunk/include/sstream
URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/sstream?rev=320604&r1=320603&r2=320604&view=diff
==============================================================================
--- libcxx/trunk/include/sstream (original)
+++ libcxx/trunk/include/sstream Wed Dec 13 10:12:55 2017
@@ -243,7 +243,6 @@ basic_stringbuf<_CharT, _Traits, _Alloca
     : __hm_(0),
       __mode_(__wch)
 {
-    str(string_type());
 }
 
 template <class _CharT, class _Traits, class _Allocator>

Modified: libcxx/trunk/test/std/input.output/string.streams/stringbuf/stringbuf.cons/default.pass.cpp
URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/test/std/input.output/string.streams/stringbuf/stringbuf.cons/default.pass.cpp?rev=320604&r1=320603&r2=320604&view=diff
==============================================================================
--- libcxx/trunk/test/std/input.output/string.streams/stringbuf/stringbuf.cons/default.pass.cpp (original)
+++ libcxx/trunk/test/std/input.output/string.streams/stringbuf/stringbuf.cons/default.pass.cpp Wed Dec 13 10:12:55 2017
@@ -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();
+    }
 }