I/O Stream Concepts and Stream Library Structure

A program creates a stream object: * When a program exchanges information with the external environment, there are two objects: one is the object in the program, and the other is the file object. * A stream is an abstraction that is responsible for establishing connections between data producers and data consumers and managing data flow.

Specify that this stream object establishes a connection with a certain file object. The program operates the stream object, and the object acts on the connected file object through the file system.

Operating systems treat keyboards, screens, printers, and communication ports as extended files, and this processing is implemented through the operating system’s device drivers. These devices are equivalent to disk files.

Read operations in the general sense are called extraction (from the stream) in stream data objects, and write operations are called insertion (into the stream).

I/O Stream Class List

Output Streams

The three most important output streams: * ostream * ofstream * ostringstream

Predefined output stream objects: * cout standard output * cerr standard error output, no buffering, content sent to it is immediately output. * clog similar to cerr, but with buffering, output when buffer is full.

Constructing Output Stream Objects

Common methods for constructing file output streams: 1. Use default constructor, then call open member function.

1 2	ofstream myFile; //Define a static file output stream object myFile.open("filename"); //Open file, establish connection between stream object and file

2. Specify filename when calling constructor:

1	ofstream myFile("filename");

3. Can specify mode when constructing object or opening file with open

1	ofstream myFile("filename", ios_base::out \| ios_base::binary);

4. Can also use the same stream object to open different files sequentially (only one is open at a time)

File Output Stream Member Functions

Three types: * Member functions equivalent to manipulators. * Member functions that perform unformatted write operations. * Other member functions that modify stream state and are different from manipulators or insertion operators.

File output stream member functions * open function * Associates the stream with a specific disk file. * Need to specify opening mode. * put function * Writes a character to the output stream. * write function * Writes a block of content from memory to a file output stream * seekp and tellp functions * Operate the internal pointer of the file stream * close function * Closes the disk file associated with a file output stream * Error handling functions * Error handling when writing to a stream

Output to Text Files

Standard output device display is treated as a text file by the system, so we use output to standard device as an example to introduce text file output format control.

Insertion Operator

Insertion (<<) operator: Pre-designed for all standard C++ data types, used to transfer bytes to an output stream object.

Manipulators

Insertion operators work with manipulators to control output format. * Many manipulators are defined in the ios_base class (such as hex()) and <iomanip> header file (such as setprecision()). * Control output width: Place setw manipulator in the stream or call width member function to specify output width for each item. >setw and width only affect the immediately following output item, but other stream format manipulators remain effective until changed. * dec, oct and hex manipulators set the default base for input and output

Setting Width

width source code setw source code

width member function is declared in iostream. If using setw or any other manipulator with parameters, must include iomanip

Alignment

Set alignment source code

setiosflags Manipulator

In this program, left alignment is set by using the setiosflags manipulator with parameters. setiosflags is defined in the iomanip header file. * Parameter ios_base::left is a static constant of ios_base, so the ios_base:: prefix must be included when referencing. * Here we need to use the resetiosflags manipulator to turn off the left alignment flag. setiosflags is different from width and setw, its effect is persistent until resetiosflags is used to restore the default value. * The parameter of setiosflags is the format flag value of the stream, which can be combined using the bitwise OR (|) operator.

Precision

The default value for floating-point output precision is 6, for example: 3466.98. * To change precision: setprecision manipulator (defined in iomanip header file) * If fixed or scientific is not specified, the precision value represents the number of significant digits. * If ios_base::fixed or ios_base::scientific is set, the precision value represents the number of digits after the decimal point.

Control output precision - without specifying fixed or scientific Control output precision - specifying fixed Control output precision - specifying scientific

Output to Binary Files

Binary File Stream

Use the mode parameter in the ofstream constructor to specify binary output mode; construct a stream in the usual way, then use the setmode member function to change the mode after the file is opened; complete output through the binary file output stream object.

#include <fstream>
using namespace std;
struct Date { 
int mon, day, year; 
};
int main() {
	Date dt = { 6, 10, 92 };
	ofstream file("date.dat", ios_base::binary);
	file.write(reinterpret_cast<char *>(&dt),sizeof(dt));
	file.close();
	return 0;
}

The write function does not stop when encountering null characters, so it can write complete class structures. This function takes two parameters: a char pointer (actual address) and the number of bytes to write. Note that reinterpret_cast is needed to explicitly convert the object’s address to char * type.

Output to Strings

Using strings as the target of output streams can achieve the function of converting other data types to strings.

String Output Stream (ostringstream)

Used to construct strings

Functions: * Supports all operations of ofstream class except open and close * str function can return the currently constructed string

Typical applications * Convert numerical values to strings

Using ostringstream to convert numerical values to strings

Input Streams

Important input stream classes: * istream class is most suitable for sequential text mode input. cin is its instance. * ifstream class supports disk file input. * istringstream

Constructing Input Stream Objects

If a filename is specified in the constructor, the file is automatically opened when constructing the object.
1
ifstream myFile("filename");

Use the open function to open the file after calling the default constructor.

1 2	ifstream myFile;//Create a file stream object myFile.open("filename"); //Open file "filename"

Can specify mode when opening file

1	ifstream myFile("filename", ios_base::in \| ios_base::binary);

Using Extraction Operator for Input from Text Files

Extraction operator (>>) is pre-designed for all standard C++ data types. It is the easiest way to get bytes from an input stream object. Many manipulators in the ios class can be applied to input streams. But only a few have actual impact on input stream objects, the most important being the base manipulators dec, oct and hex.

open function associates the stream with a specific disk file.
get function is very similar to the extraction operator (>>), the main difference is that get function includes whitespace characters when reading data. (Introduced in Chapter 6)
getline function reads multiple characters from the input stream and allows specifying input termination characters. After reading is complete, the termination character is removed from the read content. (Introduced in Chapter 6)
read member function reads bytes from a file to a specified memory area, with the length parameter determining the number of bytes to read. Reading ends when encountering end of file or end of file marker character in text mode files.
seekg function is used to set the pointer for reading data position in file input stream.
tellg function returns the current position of the file read pointer.
close function closes the disk file associated with a file input stream.

Examples

Read a binary record from file to a structure Use seekg function to set position pointer Read a file and display positions of 0 elements in it

Input from Strings (istringstream)

Used to read data from strings, set the string to read in the constructor.

Functions * Supports all operations of ifstream class except open and close

Typical applications * Convert strings to numerical values

Input/Output Streams

Two Important Input/Output Streams

An iostream object can be a source or destination of data. * Two important I/O stream classes are derived from iostream: fstream and stringstream * These classes inherit the functionality of the istream and ostream classes described earlier.

fstream Class

fstream class supports disk file input and output. * If you need to read from and write to a specific disk file in the same program, you can construct an fstream object. * An fstream object is a single stream with two logical sub-streams, one for input and the other for output.

stringstream Class

stringstream class supports string-oriented input and output * Can be used for alternating read and write operations on the same string content, also composed of two logical sub-streams.