Chapter 12: File I/O

C++ file I/O is done via streams. The key abstractions are:

std::istream for reading text.

std::ostream for writing text.

std::streambuf for reading or writing characters.

Formatted input uses operator>>.

Formatted output uses operator<<.

Streams use std::locale, e.g., for details of the formatting and for translation between external encodings and the internal encoding.

More on streams: <iostream> Library

Section 12.1: Writing to a file

There are several ways to write to a file. The easiest way is to use an output file stream (ofstream) together with the stream insertion operator (<<):

std::ofstream os("foo.txt"); if(os.is_open()){

os << "Hello World!";

}

Instead of <<, you can also use the output file stream's member function write():

std::ofstream os("foo.txt"); if(os.is_open()){

char data[] = "Foo";

// Writes 3 characters from data -> "Foo". os.write(data, 3);

}

After writing to a stream, you should always check if error state flag badbit has been set, as it indicates whether the operation failed or not. This can be done by calling the output file stream's member function bad():

os << "Hello Badbit!"; // This operation might fail for any reason. if (os.bad())

// Failed to write!

Section 12.2: Opening a file

Opening a file is done in the same way for all 3 file streams (ifstream, ofstream, and fstream).

You can open the file directly in the constructor:

std::ifstream ifs("foo.txt"); // ifstream: Opens file "foo.txt" for reading only.

std::ofstream ofs("foo.txt"); // ofstream: Opens file "foo.txt" for writing only.

std::fstream iofs("foo.txt"); // fstream: Opens file "foo.txt" for reading and writing.

Alternatively, you can use the file stream's member function open():

std::ifstream ifs;

ifs.open("bar.txt"); // ifstream: Opens file "bar.txt" for reading only.

std::ofstream ofs;

ofs.open("bar.txt"); // ofstream: Opens file "bar.txt" for writing only.

std::fstream iofs;

iofs.open("bar.txt"); // fstream: Opens file "bar.txt" for reading and writing.

You should always check if a file has been opened successfully (even when writing). Failures can include: the file doesn't exist, file hasn't the right access rights, file is already in use, disk errors occurred, drive disconnected ...

Checking can be done as follows:

// Try to read the file 'foo.txt'.

std::ifstream ifs("fooo.txt"); // Note the typo; the file can't be opened.

// Check if the file has been opened successfully. if (!ifs.is_open()) {

// The file hasn't been opened; take appropriate actions here. throw CustomException(ifs, "File could not be opened");

}

When file path contains backslashes (for example, on Windows system) you should properly escape them:

// Open the file 'c:\\folder\\foo.txt' on Windows.

std::ifstream ifs("c:\\\\folder\\\\foo.txt"); // using escaped backslashes

Version ≥ C++11

or use raw literal:

// Open the file 'c:\\folder\\foo.txt' on Windows.

std::ifstream ifs(R"(c:\\folder\\foo.txt)"); // using raw literal

or use forward slashes instead:

// Open the file 'c:\\folder\\foo.txt' on Windows. std::ifstream ifs("c:/folder/foo.txt");

Version ≥ C++11

If you want to open file with non-ASCII characters in path on Windows currently you can use non-standard wide character path argument:

// Open the file 'пример\\foo.txt' on Windows.

std::ifstream ifs(LR"(пример\\foo.txt)"); // using wide characters with raw literal

Section 12.3: Reading from a file

There are several ways to read data from a file.

If you know how the data is formatted, you can use the stream extraction operator (>>). Let's assume you have a file named foo.txt which contains the following data:

John Doe 25 4 6 1987

Jane Doe 15 5 24 1976

Then you can use the following code to read that data from the file:

// Define variables. std::ifstream is("foo.txt"); std::string firstname, lastname; int age, bmonth, bday, byear;

//Extract firstname, lastname, age, bday month, bday day, and bday year in that order.

//Note: '>>' returns false if it reached EOF (end of file) or if the input data doesn't

//correspond to the type of the input variable (for example, the string "foo" can't be

//extracted into an 'int' variable).

while (is >> firstname >> lastname >> age >> bmonth >> bday >> byear) // Process the data that has been read.

The stream extraction operator >> extracts every character and stops if it finds a character that can't be stored or if it is a special character:

For string types, the operator stops at a whitespace () or at a newline (\n).

For numbers, the operator stops at a non-number character.

This means that the following version of the file foo.txt will also be successfully read by the previous code:

John

Doe 25

4 6 1987

Jane Doe 15 5 24 1976

The stream extraction operator >> always returns the stream given to it. Therefore, multiple operators can be chained together in order to read data consecutively. However, a stream can also be used as a Boolean expression (as shown in the while loop in the previous code). This is because the stream classes have a conversion operator for the type bool. This bool() operator will return true as long as the stream has no errors. If a stream goes into an error state (for example, because no more data can be extracted), then the bool() operator will return false. Therefore, the while loop in the previous code will be exited after the input file has been read to its end.

If you wish to read an entire file as a string, you may use the following code:

// Opens 'foo.txt'. std::ifstream is("foo.txt"); std::string whole_file;

//Sets position to the end of the file. is.seekg(0, std::ios::end);

//Reserves memory for the file.

whole_file.reserve(is.tellg());

// Sets position to the start of the file. is.seekg(0, std::ios::beg);