Reading and Writing Files

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If you’ve used the C run-time library file-handling functions, MFC reading and writing operations will appear familiar. This article describes reading directly from and writing directly to a CFile object. You can also do buffered file I/O with the CArchive class.

To read from and write to the file

• Use the Read and Write member functions to read and write data in the file.

  -or-

• The Seek member function is also available for moving to a specific offset within the file.

Read takes a pointer to a buffer and the number of bytes to read and returns the actual number of bytes that were read. If the required number of bytes could not be read because end-of-file (EOF) is reached, the actual number of bytes read is returned. If any read error occurs, an exception is thrown. Write is similar to Read, but the number of bytes written is not returned. If a write error occurs, including not writing all the bytes specified, an exception is thrown. If you have a valid CFile object, you can read from it or write to it as shown in the following example:

char      szBuffer[256]; 
UINT      nActual = 0; 
CFile myFile;

myFile.Write( szBuffer, sizeof( szBuffer ) ); 
myFile.Seek( 0, CFile::begin );
nActual = myFile.Read( szBuffer, sizeof( szBuffer ) ); 

Note   You should normally carry out input/output operations within a try/catch exception handling block. For more information, see Exception Handling Syntax.