basic_filebuf* open( const char* s, std::ios_base::openmode mode ); | (1) | |
basic_filebuf* open( const std::string& str, std::ios_base::openmode mode ); | (2) | (since C++11) |
basic_filebuf* open( const std::filesystem::path& p, std::ios_base::openmode mode ); | (3) | (since C++17) |
basic_filebuf* open( const std::filesystem::path::value_type* s, std::ios_base::openmode mode ); | (4) | (since C++17) |
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 | (since C++17) |
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.
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::openmode modes |
this
on success, a null pointer on failure.
open()
is typically called through the constructor or the open()
member function of std::basic_fstream
.
#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
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 |
checks if the associated file is open (public member function) |
|
flushes the put area buffer and closes the associated file (public member function) |
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