How to use CFile::Duplicate ?

Posted on 1998-10-02
Last Modified: 2013-11-20
How to use CFile::Duplicate ?
I always fail to use it.
Any sample code would be appreciated.
Thanx in advance
Question by:nightingale
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Expert Comment

ID: 1322867
CFile *fl=m_File.Duplicate();


Author Comment

ID: 1322868

Yes , I wrote the same code.

After that I wrote

delete  fl;

The error report the file fl with no name

Would you mind writing me in detail?

Expert Comment

ID: 1322869
CFile::Duplicate is equivalent to the runtime library _dup (look this up in the help it has a nice example)

The reason for using this is to re-assign one of the standard file handles (stdin, stdout, stderr) to a new CFile variable.

For any other file you shouldn't call Duplicate.  You should simply copy the pointer

CFile * pFile2 = &m_File ;

// Now work with pFile2

Remember though if you do this, you aren't duplicating the fact that the file is already open, just the pointer to the same C++ object.  Therefore you should only work with one pointer/C++ object to the file at a time.

// this is okay
CFile * pFile2 = &m_File ;
pFile2->Flush() ;
pFile2->Close() ;

// this is okay
CFile * pFile2 = &m_File ;
m_File.Flush() ;
m_File.Close() ;

// this is NOT okay
pFile2->Flush() ;
m_File.Flush() ; // <--- equivalent to flushing the same file twice
m_File.Close() ;
pFile2->Close() ; // <-- now closing the same file for a second time, arghh!!;

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Author Comment

ID: 1322870

Luckily to hear from you, Answers2000
Thank for your patient, and explaining so carefully

something still unclear is

I donot understand why  you said

this is to re-assign one of the standard file handles (stdin, stdout, stderr) to a new CFile variable.

There is no document show that

//I want to write such code which finish COPY command

      char * pFileName1="c:\\temp\\qq1";
      char * pFileName2="c:\\temp\\qq2";
      CFile *p1,*p2;
      p1 =new CFile(pFileName1, CFile::modeRead);
      p2= p1 ->Duplicate();             <--------------does not work
      delete p2;  //because in CFile:Duplicate , CFile* pFile = new CFile(hFileNull);
      p1 -> Close();

// instead of wrote code like that

TCHAR sBuffer[1024];                <-------------- works fine
while (sActual=p1->Read(sBuffer,1023)
    p2 -> Write(sBuffer, sActual);


Here is prototype of CFile:Duplicate()
CFile* CFile::Duplicate() const
      ASSERT(m_hFile != (UINT)hFileNull);

      CFile* pFile = new CFile(hFileNull);
      HANDLE hFile;
      if (!::DuplicateHandle(::GetCurrentProcess(), (HANDLE)m_hFile,
            ::GetCurrentProcess(), &hFile, 0, FALSE, DUPLICATE_SAME_ACCESS))
            delete pFile;
      pFile->m_hFile = (UINT)hFile;
      ASSERT(pFile->m_hFile != (UINT)hFileNull);
      pFile->m_bCloseOnDelete = m_bCloseOnDelete;
      return pFile;

//Doc in Help
virtual CFile* Duplicate( ) const;
throw( CFileException );

Return Value

A pointer to a duplicate CFile object.


Constructs a duplicate CFile object for a given file.
 This is equivalent to the C run-time function _dup.

Thanx, Waiting you


Accepted Solution

Answers2000 earned 50 total points
ID: 1322871
In my docs for CFile::Duplicate (MS VC v5) it says _dup and CFile::Duplicate are equivalent.

The code you list for CFile::Duplicate calling DuplicateHandle proves this.

Why would this matter ?
Because if you consider a (for example) command line process there are some standard file handles (stdin, stdout, stderr).  This are normally hooked into certain output directions (the console, the keyboard and the console respectively).  These file handles can be redirected for command line programs using the various piping and redirection symbols (e.g. |, >, <, >>).

CFile::Duplicate and _dup C function are used to do this kind of redirection even when you get a command line which doesn't tell you to redirect.

Why would you want to do this ?

Consider a program which takes a command line option and can output to several places.  Where the output is redirected may have to be calculated at runtime (e.g. depending on the result of some calculation).  The user will not know how to redirect from the command line, so the program will have decide for itself, hence CFile::Duplicate.

Now what about copying a file ?

Look at CopyFile and CopyFileEx APIs (Okay to call these from an MFC program)

BOOL bWorked = CopyFile( "C:\OLDFILE.TXT", "C:\NEWFILE.TXT", FALSE ) ;

The last param of CopyFile says what to do if the output file already exists (whether it should be overwritten or the CopyFile should fail).

Now the point to remember for a CFile object is that it does NOT represent a File on your disk or else where, it simply represents the "Access Mechanism" to a file.  By duplicating  this "Access Mechanism" in an arbitary way you are confusing the operating system (and hence also the runtime library including MFC).

If you consider my previous explanation, and think of CFile objects as "Access Mechanisms to Files" rather than the files themselves, this should make sense

Author Comment

ID: 1322872
Very much Thanks
I just want to use Duplicate to copy file from URL

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