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extern/stdcxx/4.2.1/examples/manual/fmtflags_manip.cpp

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+/**************************************************************************
+ *
+ * fmtflags_manip.cpp - Example program demonstrating the implementation
+ * of a user-defined manipulator for convenient and exception-safe setting
+ * and restoring of stream formatting flags.
+ *
+ * $Id: fmtflags_manip.cpp 648752 2008-04-16 17:01:56Z faridz $
+ *
+ ***************************************************************************
+ *
+ * Licensed to the Apache Software  Foundation (ASF) under one or more
+ * contributor  license agreements.  See  the NOTICE  file distributed
+ * with  this  work  for  additional information  regarding  copyright
+ * ownership.   The ASF  licenses this  file to  you under  the Apache
+ * License, Version  2.0 (the  "License"); you may  not use  this file
+ * except in  compliance with the License.   You may obtain  a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the  License is distributed on an  "AS IS" BASIS,
+ * WITHOUT  WARRANTIES OR CONDITIONS  OF ANY  KIND, either  express or
+ * implied.   See  the License  for  the  specific language  governing
+ * permissions and limitations under the License.
+ *
+ **************************************************************************/
+
+#include <ios>        // for hex
+#include <iomanip>    // for __rw_smanip
+#include <iostream>   // for cout
+
+#include <examples.h>
+
+
+// implementation class of the fmtflags manipulator that saves
+// a stream object's fmtflags state before optionally setting
+// their new value and restores the saved state when destroyed
+// uses ios::basefield as an invalid flags value (it's a valid
+// mask but not a valid flags value)
+class fmtflags_manip
+{
+    std::ios_base           *strm_;
+    std::ios_base::fmtflags  saved_;
+
+public:
+    fmtflags_manip (): strm_ (), saved_ (std::ios_base::basefield) { }
+
+    void operator() (std::ios_base           &strm,
+                     std::ios_base::fmtflags  flags) const {
+
+        const_cast<fmtflags_manip*>(this)->strm_  = &strm;
+        const_cast<fmtflags_manip*>(this)->saved_ = strm.flags ();
+
+        if (flags != std::ios_base::basefield)
+            strm.flags (flags);
+    }
+
+    ~fmtflags_manip () {
+        if (strm_)
+            strm_->flags (saved_);
+    }
+};
+
+
+// saveflags manipulator to temporarily set formatting flags and
+// restore their initial value at the end of an insertion statement
+inline std::__rw_smanip<fmtflags_manip, std::ios_base::fmtflags>
+saveflags (std::ios_base::fmtflags flags = std::ios_base::basefield)
+{
+    typedef std::__rw_smanip<fmtflags_manip, std::ios_base::fmtflags> Manip;
+
+    return Manip (flags);
+}
+
+
+// dummy class whose insertion operator always throws
+struct BadClass { };
+
+
+inline std::ostream&
+operator<< (std::ostream &strm, BadClass&)
+{
+    // inserter that always throws an exception
+    return throw "exception", strm;
+
+}
+
+
+int main ()
+{
+    try {
+        BadClass throw_exception;
+
+        // use the saveflags manipulator to set the formatting flags
+        // for the rest of this insertion statement only and have it
+        // restore the  previous flags at the end of the statement
+        // or on exception
+        std::cout << saveflags (std::cout.hex)
+                  << std::showbase
+                  << "hex 1234: " << 0x1234 << '\n'
+                  << "hex 2345: " << 0x2345 << '\n'
+                  << throw_exception    // inserting will throw
+                  << "never output!";   // this string is never output
+    }
+    catch (...) {
+        // show that default formatting flags (decimal output and no
+        // showbase) have been restored during stack unwinding above
+        std::cout << "dec 3456:   " << 3456 << '\n'
+                  << std::oct
+                  << "oct 4567:   " << 04567 << '\n';
+    }
+
+    // use ordinary manipulators to set hex output with base prefix
+    std::cout << std::hex << std::showbase
+              << "hex 5678: " << 0x5678 << '\n'
+              << "hex 6789: " << 0x6789 << '\n';
+
+    // the same flags set above are still in effect
+    std::cout << "hex 789a: " << 0x789a << '\n';
+
+    return 0;
+}