std::ifstream::open

Parameters

filename

String with the name of the file to open.
Specifics about its format and validity depend on the library implementation and running environment.

mode

Flags describing the requested i/o mode for the file.
This is an object of the bitmask member type openmode that consists of a combination of the following member constants:

member constant	stands for	access
in *	input	File open for reading: the internal stream buffer supports input operations.
out	output	File open for writing: the internal stream buffer supports output operations.
binary	binary	Operations are performed in binary mode rather than text.
ate	at end	The output position starts at the end of the file.
app	append	All output operations happen at the end of the file, appending to its existing contents.
trunc	truncate	Any contents that existed in the file before it is open are discarded.

These flags can be combined with the bitwise OR operator (|).
* in is always set for ifstream objects (even if explicitly not set in argument mode).
Note that even though ifstream is an input stream, its internal filebuf object may be set to also support output operations.

If the mode has app set, the opening operation fails. It also fails if trunc is set but out is not.

If the mode has both trunc and app set, the opening operation fails. It also fails if trunc is set but out is not.

Return Value

Example

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// print the content of a text file.
#include <iostream>     // std::cout
#include <fstream>      // std::ifstream

int main () {
  std::ifstream ifs;

  ifs.open ("test.txt", std::ifstream::in);

  char c = ifs.get();

  while (ifs.good()) {
    std::cout << c;
    c = ifs.get();
  }

  ifs.close();

  return 0;
}

Data races

Exception safety

See also

ifstream::is_open

Check if a file is open (public member function)

ifstream::close

Close file (public member function)

filebuf::open

Open file (public member function)