Fix Efficient access to streambuf buffer (#2361)

microsoft · Dec 17, 2021 · 7c78a42 · 7c78a42
1 parent 2ff8149
commit 7c78a42
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Showing 2 changed files with 32 additions and 17 deletions.
diff --git a/stl/inc/sstream b/stl/inc/sstream
@@ -189,6 +189,7 @@ public:
                 _Mystate &= ~_Allocated;
             }
         }
+
         _Tidy();
         return _Result;
     }
@@ -457,29 +458,27 @@ protected:
 
 #if _HAS_CXX20
     void _Init_string_inplace(_Mystr&& _Str, int _State) {
-        _State |= _From_rvalue;
+        if ((_State & _Noread) && (_State & _Constant)) { // Cannot read / write buffer, do nothing
+            _Seekhigh = nullptr;
+            _Mystate  = _State | _From_rvalue;
+            return;
+        }
 
+        // finite buffer that can be read or written, set it up
         auto [_Ptr, _Size, _Res] = _Str._Release_to_buffer(_Al);
         _Elem* const _Pnew       = _Unfancy(_Ptr);
-        if (_Res != 0 && (_State & (_Noread | _Constant)) != (_Noread | _Constant)) {
-            // finite buffer that can be read or written, set it up
-            _Seekhigh        = _Pnew + _Size;
-            auto _End_buffer = _Pnew + _Res;
+        _Seekhigh                = _Pnew + _Size;
+        auto _Next               = (_State & (_Atend | _Append)) ? _Seekhigh : _Pnew;
+        auto _End_buffer         = _Pnew + _Res;
 
-            _Mysb::setp(_Pnew, (_State & (_Atend | _Append)) ? _Seekhigh : _Pnew, _End_buffer);
-
-            if (_State & _Noread) { // maintain "_Allocated == eback() points to buffer base" invariant
-                _Mysb::setg(_Pnew, nullptr, _Pnew);
-            } else {
-                _Mysb::setg(_Pnew, _Pnew, _Seekhigh);
-            }
-
-            _State |= _Allocated;
+        _Mysb::setp(_Pnew, _Next, _End_buffer);
+        if (_State & _Noread) { // maintain "_Allocated == eback() points to buffer base" invariant
+            _Mysb::setg(_Pnew, nullptr, _Pnew);
         } else {
-            _Seekhigh = nullptr;
+            _Mysb::setg(_Pnew, _Pnew, _Seekhigh);
         }
 
-        _Mystate = _State;
+        _Mystate = _State | _Allocated | _From_rvalue;
     }
 #endif // _HAS_CXX20
 

diff --git a/tests/std/tests/P0408R7_efficient_access_to_stringbuf_buffer/test.cpp b/tests/std/tests/P0408R7_efficient_access_to_stringbuf_buffer/test.cpp
@@ -23,9 +23,25 @@ constexpr char large_string[] =
 template <typename Stream>
 struct test_rvalue {
     void operator()(const string& init_value) {
+        test(init_value, ios_base::in);
+        test(init_value, ios_base::out);
+        test(init_value, ios_base::app);
+        test(init_value, ios_base::ate);
+    }
+
+    void test(const string& init_value, const ios_base::openmode mode) {
         string buffer{init_value};
         size_t res = buffer.capacity();
-        Stream stream{move(buffer)};
+        Stream stream{move(buffer), mode};
+
+        // If the stream cannot be written or read we do nothing
+        const auto buffer_view = stream.rdbuf()->_Get_buffer_view();
+        if (!buffer_view._Ptr) {
+            assert(!(mode & ios_base::in) && !(mode & ios_base::out));
+            assert(buffer == init_value);
+            return;
+        }
+
         assert(stream.view() == init_value);
         assert(stream.str() == init_value);
         assert(stream.view() == init_value);