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Read from and Write to a File
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Headers <fstream> Classes fstream Functions void close( ) ostream &flush( ) istream &get(char &ch) bool good( ) const void open(const char *fname, ios::openmode mode = ios::in | ios::out) ostream &put(char ch)
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It is possible to open a file so that it can be used for both input and output This is commonly done when a data file needs to be updated Instead of rewriting the entire file, you can write just a small portion of it This is especially valuable in files that use fixed-length records, because it offers a convenient way to update one record without rewriting the entire file Of course, opening a file for both input and output is useful in other situations, such as when you want to read the contents of a file, modify it, and then write the modified contents back to the same file By using a file opened for input and output, you only need to open and close the file once, thus streamlining your code Whatever your purpose, this recipe shows the basic procedures necessary to read from and write to a file
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To perform both input and output operations on a file involves the following steps: 1 Open the file for both reading and writing by creating an object of type fstream The fstream class inherits both ifstream and ofstream This means that it is capable of both input and output operations 2 Use the output functions defined by ofstream to write to the file The one used by this recipe is put( ) 3 Use the input functions defined by ifstream to read from the file The one used by this recipe is get( )
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4 For many compiler implementations, when switching between input and output, you will need to call seekg( ), seekp( ), or flush( ) This recipe uses flush( ) 5 Close the file 6 Confirm that the I/O operations have been successful This can be done by calling good( ) on the input stream or in a number of other ways
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A general overview of opening and closing a file is found in Opening and Closing a File near the start of this chapter Information relating specifically to fstream is presented here The fstream class inherits the iostream class, which inherits both istream and ostream This makes it able to support both input and output operations Furthermore, all of the techniques described by the preceding recipes, such as reading from and writing to an ifstream or ofstream, apply to fstream The only difference is that fstream supports both read and write To perform input/output operations, you must have an object of type fstream that supports both input and output operations The fstream class uses the header <fstream> and it defines these two constructors: fstream( ) explicit fstream(const char *fname, ios::openmode mode = ios::in | ios::out) The first creates an fstream instance that is not yet linked to a file The second creates an fstream instance and then opens the file specified by fname with the mode specified by mode Notice that mode defaults to both ios::in and ios::out Also, notice that it does not include the ios::binary flag Therefore, by default, the file is opened in text mode To open a file for binary I/O, include the ios::binary flag When a file is opened in text mode, some character translations may occur, such as a newline being replaced by a carriage-return, linefeed sequence Opening a file in binary mode prevents such translations If you use the default constructor, then you will need to link a file to the fstream instance after it is constructed by calling open( ) The version defined by fstream is shown here: void open(const char *fname, ios::openmode mode = ios::in | ios::out) It opens the file specified by fname with the mode specified by mode Notice that, like the fstream constructor, mode defaults to ios::in | ios::out Therefore, the file is automatically opened for read/write operations when mode defaults Before attempting to write to the file, you must confirm that the file has been opened You can do this in various ways The approach used by this recipe is to apply the ! operator to the fstream instance Recall that the ! operator returns the outcome of a call to fail( ) on the stream Therefore, if it returns true, the open operation has failed Once open, you can read from and write to the file using any of the methods provided by istream and ostream, such as get( ), put( ), read( ), and write( ) These methods have been described by the preceding recipes For some compilers, you will need to flush output by calling flush( ) or perform a seek operation by calling seekg( ) or seekp( ) when switching between read and write operations
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