table of contents
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 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)
2022.07.31 | http://cppreference.com |