Example
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
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");
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
