I/O in C++

Streams

  • Input and output can be thought of as streams of bytes that are handled by an I/O library

  • There are many different kinds of input and output. Some examples include:

    • Data streams (network, files, display devices, etc.)

    • Keyboard interaction

    • GUI interaction

istream and ostream

  • C++ uses the istream type to deal with input and the ostream type for output

  • The ostream type:

    • Turns values into character sequences

    • Sends character sequences to console, file, memory, etc.

    • Internally stores data in a buffer before sending to destination

    • Provides features for formatting

    • An example of an ostream is std::cout

  • The istream type:

    • Gets character sequences from console, file, memory, etc.

    • Turns character sequences into values

    • Provides features for reading output produced by ostreams

File Streams

  • Think of a file as a sequence of bytes

    • The file format is a set of rules that determines what those bytes mean

  • An ostream converts data from memory into streams of bytes and writes them to disk

  • An istream takes a stream of bytes from disk and creates data from it

Opening and Closing a File

Before reading from or writing to a file, you must open a stream for it first.

The C++ standard library privides tools for this:

  • std::ifstream is an istream for reading from a file

  • std::ofstream is an ostream for writing to a file

  • std::fstream is an iostream that can be used for both reading and writing

Here is an example of opening a file for reading:

std::string filename = "test_file.txt";
std::ifstream stream(filename);

if (!stream) {
    std::cout << "ERROR: unable to open file." << std::endl;
}

And here is an example of opening a file for writing:

std::string filename = "test_file.txt";
std::ofstream stream(filename);

if (!stream) {
    std::cout << "ERROR: unable to open file." << std::endl;
}

Notice how similar the two examples are. The check if (!stream) checks if the file was opened properly. You can't open a file stream a second time without closing it first.

Files should always be closed before your program terminates. When a file stream goes out of scope, its associated file is closed automatically. It's often a good idea to explicitly call close()

Reading and Writing a File

Here is an example of reading from a file:

std::string filename = "test_file.txt";
std::ifstream stream(filename);

std::string line;
std::vector<std::string> lines;

if (!stream) {
    std::cout << "ERROR: unable to open file." << std::endl;
}
while (stream >> line) {
    lines.push_back(line);
}

And here is an example of writing:

std::string filename = "test_file.txt";
std::ofstream stream(filename);

if (!stream) {
    std::cout << "ERROR: unable to open file." << std::endl;
}

stream << "Hello world!\n";

I/O Error Handling

An istream has four state-checking functions: good(), eof(), fail(), and bad()

Resources

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