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

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");

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

Found a mistake? Have a question or improvement idea? Let me know.

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