Thinking In C++. Volume 2: Practical Programming

The first while loop demonstrates the use of two forms of the get( ) function. The first gets characters into a buffer and puts a zero terminator in the buffer when either sz-1 characters have been read or the third argument (defaulted to '\n') is encountered. The get( ) function leaves the terminator character in the input stream, so this terminator must be thrown away via in.get( ) using the form of get( ) with no argument, which fetches a single byte and returns it as an int. You can also use the ignore( ) member function, which has two default arguments. The first argument is the number of characters to throw away and defaults to one. The second argument is the character at which the ignore( ) function quits (after extracting it) and defaults to EOF^.

Next, you see two output statements that look similar: one to cout and one to the file out. Notice the convenience here; you don’t need to worry about what kind of object you’re dealing with because the formatting statements work the same with all ostream objects. The first one echoes the line to standard output, and the second writes the line out to the new file and includes a line number^.

To demonstrate getline( ), open the file we just created and strip off the line numbers. To ensure the file is properly closed before opening it to read, you have two choices. You can surround the first part of the program with braces to force the out object out of scope, thus calling the destructor and closing the file, which is done here. You can also call close( ) for both files; if you do this, you can even reuse the in object by calling the open( ) member function^.

The second while loop shows how getline( ) removes the terminator character (its third argument, which defaults to '\n') from the input stream when it’s encountered. Although getline( ), like get( ), puts a zero in the buffer, it still doesn’t insert the terminating character^.

This example, as well as most of the examples in this chapter, assumes that each call to any overload of getline( ) will actually encounter a newline character. If this is not the case, the eofbit state of the stream will be set and the call to getline( ) will return false, causing the program to lose the last line of input.

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^.