Thinking in C++ Vol 2 - Practical Programming
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Open modes

You can control the way a file is opened by overriding the constructor s default arguments. The following table shows the flags that control the mode of the file:

Flag	Function
ios::in	Opens an input file. Use this as an open mode for an ofstream to prevent truncating an existing file.
ios::out	Opens an output file. When used for an ofstream without ios::app, ios::ate or ios::in, ios::trunc is implied.
ios::app	Opens an output file for appending only.
ios::ate	Opens an existing file (either input or output) and seeks to the end.
ios::trunc	Truncates the old file if it already exists.
ios::binary	Opens a file in binary mode. The default is text mode.

You can combine these flags using a bitwise or operation.

The binary flag, while portable, only has an effect on some non-UNIX systems, such as operating systems derived from MS-DOS, that have special conventions for storing end-of-line delimiters. For example, on MS-DOS systems in text mode (which is the default), every time you output a newline character ('\n'), the file system actually outputs two characters, a carriage-return/linefeed pair (CRLF), which is the pair of ASCII characters 0x0D and 0x0A. Conversely, when you read such a file back into memory in text mode, each occurrence of this pair of bytes causes a '\n' to be sent to the program in its place. If you want to bypass this special processing, you open files in binary mode. Binary mode has nothing whatsoever to do with whether you can write raw bytes to a file you always can (by calling write( )) . You should, however, open a file in binary mode when you ll be using read( ) or write( ), because these functions take a byte count parameter. Having the extra '\r' characters will throw your byte count off in those instances. You should also open a file in binary mode if you re going to use the stream-positioning commands discussed later in this chapter.

You can open a file for both input and output by declaring an fstream object. When declaring an fstream object, you must use enough of the open mode flags mentioned earlier to let the file system know whether you want to input, output, or both. To switch from output to input, you need to either flush the stream or change the file position. To change from input to output, change the file position. To create a file via an fstream object, use the ios::trunc open mode flag in the constructor call to do both input and output.

Thinking in C++ Vol 2 - Practical Programming
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