std::basic_filebuf<CharT,Traits>::open
std::ios_base::openmode mode );
std::ios_base::openmode mode );
If the associated file was already open (is_open() != false), returns a null pointer right away.
Otherwise, opens the file with the given name (s, p.c_str()(since C++17) or str.c_str(), depending on the overload). std::ios_base::openmode values may be written as, e.g., std::ios_base::out | std::ios_base::app .
Overload (4) is only provided if std::filesystem::path::value_type
is not char.
The file is opened as if by calling std::fopen with the second argument (file access mode) determined by the result of mode & ~std::ios_base::ate as follows, open()
fails if the result is not some combination of flags shown in the table:
mode & ~std::ios_base::ate | std::fopen access mode |
Action if file already exists | Action if file does not exist | |||||
---|---|---|---|---|---|---|---|---|
binary | in | out | trunc | app | noreplace (since C++23) | |||
- | + | - | - | - | - | "r" | Read from start | Failure to open |
+ | + | - | - | - | - | "rb" | ||
- | + | + | - | - | - | "r+" | Error | |
+ | + | + | - | - | - | "r+b" | ||
- | - | + | - | - | - | "w" | Destroy contents | Create new |
- | - | + | + | - | - | |||
+ | - | + | - | - | - | "wb" | ||
+ | - | + | + | - | - | |||
- | + | + | + | - | - | "w+" | ||
+ | + | + | + | - | - | "w+b" | ||
- | - | + | - | - | + | "wx" | Failure to open | Create new |
- | - | + | + | - | + | |||
+ | - | + | - | - | + | "wbx" | ||
+ | - | + | + | - | + | |||
- | + | + | + | - | + | "w+x" | ||
+ | + | + | + | - | + | "w+bx" | ||
- | - | + | - | + | - | "a" | Write to end | Create new |
- | - | - | - | + | - | |||
+ | - | + | - | + | - | "ab" | ||
+ | - | - | - | + | - | |||
- | + | + | - | + | - | "a+" | ||
- | + | - | - | + | - | |||
+ | + | + | - | + | - | "a+b" | ||
+ | + | - | - | + | - |
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 std::fopen . If the repositioning fails, calls close() and returns a null pointer to indicate failure.
[edit] Parameters
[edit] Return value
this on success, a null pointer on failure.
[edit] Notes
open()
is typically called through the constructor or the open()
member function of std::basic_fstream .
[edit] 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'; }
Output:
Read back from file: 3.14
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
---|---|---|---|
LWG 596 | C++98 | open() could not open files in append mode
|
can open in append mode |