C++ ifstream Class

❮ fstream classes

// Create a text string, which is used to output the text file
string myText;

// Read from the text file
ifstream MyReadFile("filename.txt");

// Use a while loop together with the getline() function to read the file line by line
while (getline (MyReadFile, myText)) {
  // Output the text from the file
  cout << myText;
}

// Close the file
MyReadFile.close(); 

Definition and Usage

The ifstream class (short for "input file stream") is used to read from files.

The ifstream class is defined in the <fstream> header file.

To open a file, pass the file path into the constructor:

ifstream MyReadFile("filename.txt");

The ifstream class has many ways to read data from a file. An easy way is to use the getline() function to read all of the characters up to the next line break and write them into a string.

string myText;
getline(MyReadFile, myText);
cout << myText;

File Reading Functions

File reading functions extract characters from a file and move the file pointer.

get()

The get() method reads a single character from a file and returns its ASCII value as an int value. Convert it to a char type to see the character. The file pointer is moved to the next character in the file.

char myChar = MyReadFile.get();
cout << myChar;

The get(destination, size, delimiter) method writes up to size characters to the destination with data read from the file. It stops reading as soon as it reaches a line break, end of file, or an optional character given by the delimiter parameter. The value written in destination always ends with a \0 null terminating character. This method moves the file pointer to the line break or delimiter where it stopped reading.

char destination[20];
MyReadFile.get(destination, 20);
cout << destination << "\n";

// Stop reading when a '.' is found
MyReadFile.get(destination, 20, '.');
cout << destination << "\n";

getline()

The getline(destination, size, delimiter) method is the same as the get(destination, size, delimiter) method, except that the line break or delimiter is discarded and the file pointer is moved to the character that follows it.

char destination[20];
MyReadFile.getline(destination, 20);
cout << destination << "\n";

// Stop reading when a '.' is found
MyReadFile.getline(destination, 20, '.');
cout << destination << "\n";

There is a similar getline(stream, destination, delimiter) function which reads all of the characters up to the next line break (or optional delimiter) from the file specified by the ifstream object in the stream parameter and writes them into the string specified by destination.

string destination;
getline(MyFile, destination);
cout << destination << "\n";

// Stop reading when a '.' is found
getline(MyFile, destination, '.');
cout << destination << "\n";

read()

The read(destination, n) method reads n characters from the file and writes them into the char array specified by the destination parameter. Unlike other functions, it does not stop reading at line breaks and it does not add a null terminating character to the data.

char destination[20];
MyReadFile.read(destination, 19);
destination[20] = '\0'; // Make sure it ends with a null terminating character
cout << destination << "\n";

peek()

The peek() method reads a single character from a file and returns its ASCII value as an int value. Convert it to a char type to see the character. Unlike the get() method, this method does not move the file pointer.

char myChar = MyReadFile.peek();
cout << myChar;

gcount()

The gcount() method returns the number of characters extracted from the file by most recently called file reading method.

char destination[20];
MyReadFile.getline(destination, 20);
cout << MyReadFile.gcount() << "\n";

File Handling Functions

File handling functions open, close and navigate files.

open()

The open(filepath) method opens the file at the path specified by filepath. If a file is already open then this method has no effect.

ifstream MyReadFile;
MyReadFile.open("filename.txt");

is_open()

The is_open() method returns true if a file is open and false if there is no file open.

ifstream MyReadFile;
cout << MyReadFile.is_open(); << "\n"; // Displays 0 because the file is not open
MyReadFile.open("filename.txt");
cout << MyReadFile.is_open(); << "\n"; // Displays 1 because the file is open

close()

The close() method closes a file. It is good to close a file when you are finished working with it to free up resources.

MyReadFile.close();

rdbuf()

The rdbuf() method returns a pointer to the internal filebuf object which directly handles the file.

filebuf * buf = MyReadFile.rdbuf();

unget()

The unget() method moves the file pointer back by one character.

char myChar = MyReadFile.get();
cout << myChar << "\n";
MyReadFile.unget();
myChar = MyReadFile.get();
cout << myChar;

seekg()

The seekg(position) method moves the file pointer to a specified position relative to the beginning of the file.

MyReadFile.seekg(6)

The seekg(position, origin) method moves the file pointer to a specified position in the file relative to an origin. The origin has three possible values:

• ifstream::beg - The position is relative to the beginning of the file.
• ifstream::cur - The position is relative to the current file position.
• ifstream::end - The position is relative to the end of the file.

MyReadFile.seekg(6, ifstream::beg);
cout << MyReadFile.tellg(); << "\n";
MyReadFile.seekg(-3, ifstream::cur);
cout << MyReadFile.tellg(); << "\n";
MyReadFile.seekg(-4, ifstream::end);
cout << MyReadFile.tellg(); << "\n";

tellg()

The tellg() method returns the current position of the file pointer in the file.

cout << MyReadFile.tellg();

The Extraction Operator

The >> extraction operator reads a number of characters from the current position in the file, interprets them and writes the interpreted value into a variable. Then the file pointer is moved to the next character which has not yet been read. The way that the characters are interpreted depends on the data type of the variable.

MyReadFile >> variable

It can also be used multiple times to read parts of a file one after another.

MyReadFile >> variable1 >> variable2 >> variable3

The >> extraction operator starts by skipping over whitespace characters (spaces, tabs and line breaks) until it reaches the first character that is not whitespace. After that, it follows the rules shown in the following table based on the data type of the variable.

Manipulators

A manipulator can be used in place of a variable. When manipulators are used they change how data is interpreted by the ifstream object. The effect of a manipulator remains until another manipulator changes it.

The following table has a list of manipulators that the ifstream object can use.

bool myBool;
int myInt;

// Interpret character sequences "true" and "false" as boolean values
MyFile >> boolalpha >> myBool;

// Revert to reading booleans normally
MyFile >> noboolalpha;

// Read hexadecimal numbers from the file and interpret them as integers
MyFile >> hex >> myInt;

// Revert to reading integers normally
MyFile >> dec;

❮ fstream classes