std::basic_filebuf::open(3)

NAME
std::basic_filebuf::open - std::basic_filebuf::open

Synopsis
   std::basic_filebuf<CharT, Traits>* open( const char* s, (1)
   std::ios_base::openmode mode )
   std::basic_filebuf<CharT, Traits>* open( const std::string& str, (2) (since C++11)
   std::ios_base::openmode mode )
   std::basic_filebuf<CharT, Traits>* open( const
   std::filesystem::path& p, (3) (since C++17)
   std::ios_base::openmode mode )
   std::basic_filebuf<CharT, Traits>* open( const
   std::filesystem::path::value_type* s, (4) (since C++17)
   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.
   Overload (4) is only provided if std::filesystem::path::value_type is (since C++17)
   not char.
   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 Action if file does
                        exists 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.

Parameters
   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.

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

Example
// Run this code
 #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';
   }
 }

Output:
 Read back from file: 3.14

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