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Totally Immersed: C++ Classes and Object-Oriented Programming
For example, sometimes you want to append data to the end of an existing file rather than
create a new file. In that case you can append data to the end of a file by opening the file in
append mode. To specify append mode, you must use one of the ios class’s open_mode
specifiers in the ofstream constructor when you create the object:
ofstream outfile(“test.dat”, ios::app); // open in append mode
This file will be opened in append mode and any new data written to the file will be written
to the end of the file. There are several specifiers you can use when opening files. Table 4.1
lists the open_mode enumeration’s values and their descriptions.
Table 4.1. ios class open_mode specifiers.
Specifier Description
app The file is opened and any new data will be appended to the end of
the file.
ate Seek to the end of the file when the file is opened.
in The file is opened for input (reading). This is the default for the
ifstream class.
out The file is opened for output (writing). This is the default for the
ofstream class.
binary The file is opened in binary mode. The default is to open the file in
text mode. In text mode, when the file is read, carriage-return/
linefeed (CR/LF) pairs are converted to a single linefeed character
(LF). When the file is written, linefeed characters are converted to
CR/LF pairs before being written to the file. In binary mode no
conversion of CR/LR pairs takes place.
trunc Opens the file and clears the contents. If neither app nor ate are
specified, trunc is the default.
nocreate The file will not be created if it does not exist. Open() will fail if the
file does not exist. Opening an ifstream with this flag can be used
to check for the existence of a file.
noreplace Similar to nocreate except that Open will fail unless either app or ate
is specified.
You can or together two or more of the values listed in Table 4.1 if needed. For example, let’s
say you wanted to open a file in binary mode and that you wanted to append data to the end
of the file. In that case the constructor would look like this:
ofstream outfile(“test.dat”, ios::app | ios::binary);
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