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std::basic_filebuf::open(3) C++ Standard Libary 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 argu- ment (mode) determined as follows: mode openmode & ~ate Action if file already Ac- tion if file does exists not exist "r" in Read from start Fail- ure to open "w" out, out|trunc Destroy contents Cre- ate new "a" app, out|app Append to file Cre- ate new "r+" out|in Read from start Error "w+" out|in|trunc Destroy contents Cre- ate new "a+" out|in|app, in|app Write to end Cre- ate new "rb" binary|in Read from start Fail- ure to open "wb" binary|out, binary|out|trunc Destroy contents Cre- ate new "ab" binary|app, binary|out|app Write to end Cre- ate new "r+b" binary|out|in Read from start Error "w+b" binary|out|in|trunc Destroy contents Cre- ate new "a+b" binary|out|in|app, binary|in|app Write to end Cre- ate 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) http://cppreference.com 2022.07.31 std::basic_filebuf::open(3)
NAME | Synopsis | Parameters | Return value | Notes | Example | Output: | See also
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