std::basic_filebuf<CharT,Traits>::open

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 )

std::basic_filebuf<CharT, Traits>* open( const std::filesystem::path& p,
                                         std::ios_base::openmode mode )

std::basic_filebuf<CharT, Traits>* open( const std::filesystem::path::value_type* s,
                                         std::ios_base::openmode mode )

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.

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

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.

Parameters

Return value

this on success, a null pointer on failure.

Notes

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

Example

#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);
  fb.close();
 
  // 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