std::basic_filebuf<CharT, Traits>* open( const char* s,
                                         std::ios_base::openmode mode )
std::basic_filebuf<CharT, Traits>* open( const std::string& str,
                                         std::ios_base::openmode mode )
(2) (since C++11)
std::basic_filebuf<CharT, Traits>* open( const std::filesystem::path& p,
                                         std::ios_base::openmode mode )
(3) (since C++17)
std::basic_filebuf<CharT, Traits>* open( const std::filesystem::path::value_type* s,
                                         std::ios_base::openmode mode )
(4) (since C++17)

Opens the file with the given name (s, p.c_str() (since C++17) or str.c_str(), depending on the overload). openmode values may be written as, e.g., std::ios::out|std::ios::app.

Overload (4) is only provided if std::filesystem::path::value_type is not char.

(since C++17)

The file is opened as if by calling std::fopen with the second argument (mode) determined as follows:

mode openmode & ~ate Action if file already exists Action if file does not exist
"r" in Read from start Failure to open
"w" out, out|trunc Destroy contents Create new
"a" app, out|app Append to file Create new
"r+" out|in Read from start Error
"w+" out|in|trunc Destroy contents Create new
"a+" out|in|app, in|app Write to end Create new
"rb" binary|in Read from start Failure to open
"wb" binary|out, binary|out|trunc Destroy contents Create new
"ab" binary|app, binary|out|app Write to end Create new
"r+b" binary|out|in Read from start Error
"w+b" binary|out|in|trunc Destroy contents Create new
"a+b" binary|out|in|app, binary|in|app Write to end Create new

If openmode is not one of the modes listed, the open() fails.

If the open operation succeeds and openmode & std::ios_base::ate != 0 (the ate bit is set), repositions the file position to the end of file, as if by calling std::fseek(file, 0, SEEK_END), where file is the pointer returned by calling fopen. If the repositioning fails, calls close() and returns a null pointer to indicate failure.

If the associated file was already open, returns a null pointer right away.


s, str, p - the file name to open; s must point to a null-terminated string
openmode - the file opening mode, a binary OR of the std::ios_base modes

Return value

this on success, a null pointer on failure.


open() is typically called through the constructor or the open() member function of std::basic_fstream.


#include <fstream>
#include <iostream>
int main() {
  std::string filename = "Test.b";
  std::filebuf fb;
  // prepare a file to read
  double d = 3.14;
  if (!fb.open(filename, std::ios::binary|std::ios::out)) {
    std::cout << "Open file " << filename << " for write failed\n";
    return 1;
  fb.sputn(reinterpret_cast<char*>(&d), sizeof d);
  // open file for reading
  double d2 = 0.0;
  if (!fb.open(filename, std::ios::binary|std::ios::in)) {
    std::cout << "Open file " << filename << " for read failed\n";
    return 1;
  auto got = fb.sgetn(reinterpret_cast<char*>(&d2), sizeof d2);
  if (sizeof(d2) != got) {
    std::cout << "Read of " << filename << " failed\n";
  } else {
    std::cout << "Read back from file: " << d2 << '\n';


Read back from file: 3.14

See also

checks if the associated file is open
(public member function)
flushes the put area buffer and closes the associated file
(public member function)

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