ENH: cmGeneratedFileStream class added to simplify copy-if-different usage on generated files.

Source/cmGeneratedFileStream.h

+/*=========================================================================
+
+  Program:   Insight Segmentation & Registration Toolkit
+  Module:    $RCSfile$
+  Language:  C++
+  Date:      $Date$
+  Version:   $Revision$
+
+Copyright (c) 2001 Insight Consortium
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+ * Redistributions of source code must retain the above copyright notice,
+   this list of conditions and the following disclaimer.
+
+ * Redistributions in binary form must reproduce the above copyright notice,
+   this list of conditions and the following disclaimer in the documentation
+   and/or other materials provided with the distribution.
+
+ * The name of the Insight Consortium, nor the names of any consortium members,
+   nor of any contributors, may be used to endorse or promote products derived
+   from this software without specific prior written permission.
+
+  * Modified source versions must be plainly marked as such, and must not be
+    misrepresented as being the original software.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER AND CONTRIBUTORS ``AS IS''
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE FOR
+ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+=========================================================================*/
+#ifndef cmGeneratedFileStream_h
+#define cmGeneratedFileStream_h
+
+#include "cmStandardIncludes.h"
+
+/** \class cmGeneratedFileStream
+ * \brief Output stream for generated files that does copy-if-different.
+ *
+ * Many files generated by CMake don't change each time they are generated.
+ * This class can be used in place of std::ofstream to open a file and
+ * write output to it.  The class will automatically write output to a
+ * temporary file and copy it over an existing file only if the generated
+ * file has changed.
+ */
+class cmGeneratedFileStream
+{
+public:
+  /**
+   * The constructor takes the name of the file to be generated.  It
+   * automatically generates a name for the temporary file.
+   */
+  cmGeneratedFileStream(const char* name):
+    m_Name(name),
+    m_TempName(m_Name+".tmp"),
+    m_Stream(m_TempName.c_str()),
+    m_Copied(false)
+    {}
+  
+  /**
+   * The destructor ensures that the file has been closed and copied if
+   * it has changed.
+   */
+  ~cmGeneratedFileStream() { this->DoCopy(); }
+  
+  /**
+   * Get the real output stream.
+   */
+  std::ostream& GetStream() { return m_Stream; }
+  
+  /**
+   * Allow a test for whether the file is open.
+   */
+  operator bool() const { return m_Stream; }
+  
+  /**
+   * Close the file stream.  This will cause the copy-if-different to the
+   * real file name to occur.
+   */
+  void close() { this->DoCopy(); }
+  
+private:
+  /**
+   * The name of the real file where output will be copied if it has changed.
+   */
+  std::string m_Name;
+  
+  /**
+   * The name of the temporary file.
+   */
+  std::string m_TempName;
+  
+  /**
+   * The real output stream used to write to the file.
+   */
+  std::ofstream m_Stream;
+  
+  /**
+   * Whether the temporary file has already been copied to the real file.
+   */
+  bool m_Copied;
+
+  /**
+   * Closes the temporary file and does the copy-if-different to the real file.
+   */
+  void DoCopy()
+    {
+    if(!m_Copied)
+      {
+      m_Stream.close();
+      cmSystemTools::CopyFileIfDifferent(m_TempName.c_str(), m_Name.c_str());
+      cmSystemTools::RemoveFile(m_TempName.c_str());
+      m_Copied = true;
+      }
+    }
+};
+
+
+/**
+ * Allow a cmGeneratedFileStream to be used just as a real std::ostream
+ * would be.
+ */
+template <class T>
+std::ostream& operator << (cmGeneratedFileStream& l, const T& r)
+{
+  std::ostream& os = l.GetStream();
+  os << r;
+  return os;
+}
+
+
+
+#endif