| I/O manipulators | ||||
| Print functions(C++23) | ||||
| C-style I/O | ||||
| Buffers | ||||
(C++23) | ||||
(C++98/26*) | ||||
(C++20) | ||||
| Streams | ||||
| Abstractions | ||||
| File I/O | ||||
| String I/O | ||||
| Array I/O | ||||
(C++23) | ||||
(C++23) | ||||
(C++23) | ||||
(C++98/26*) | ||||
(C++98/26*) | ||||
(C++98/26*) | ||||
| Synchronized Output | ||||
(C++20) | ||||
| Types | ||||
| Error category interface | ||||
(C++11) | ||||
(C++11) |
| Types and objects | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Defined in header <cstdio> | ||
First, attempts to close the file associated withstream, ignoring any errors. Then, iffilename is not null, attempts to open the file specified byfilename usingmode as if bystd::fopen, and associates that file with the file stream pointed to bystream. Iffilename is a null pointer, then the function attempts to reopen the file that is already associated withstream (it is implementation defined which mode changes are allowed in this case).
Contents |
| filename | - | file name to associate the file stream to |
| mode | - | null-terminated character string determining newfile access mode |
| stream | - | the file stream to modify |
| File access mode string | Meaning | Explanation | Action if file already exists | Action if file does not exist |
|---|---|---|---|---|
| "r" | read | Open a file for reading | read from start | returnNULL and set error |
| "w" | write | Create a file for writing | destroy contents | create new |
| "a" | append | Append to a file | write to end | create new |
| "r+" | read extended | Open a file for read/write | read from start | returnNULL and set error |
| "w+" | write extended | Create a file for read/write | destroy contents | create new |
| "a+" | append extended | Open a file for read/write | write to end | create new |
| File access mode flag"b" can optionally be specified to open a filein binary mode. This flag has no effect on POSIX systems, but on Windows, for example, it disables special handling of'\n' and'\x1A'. On the append file access modes, data is written to the end of the file regardless of the current position of the file position indicator. | ||||
| File access mode flag"x" can optionally be appended to"w" or"w+" specifiers. This flag forces the function to fail if the file exists, instead of overwriting it.(C++17) | ||||
| The behavior is undefined if the mode is not one of the strings listed above. Some implementations define additional supported modes (e.g.Windows). | ||||
stream on success, a null pointer on failure.
std::freopen is the only way to change the narrow/wide orientation of a stream once it has been established by an I/O operation or bystd::fwide.
Microsoft CRT version ofstd::freopen does not support any mode changes whenfilename is a null pointer and treats this as an error (seedocumentation). A possible workaround is the non-standard function_setmode().
The following code redirectsstdout to a file.
#include <cstdio> int main(){std::printf("stdout is printed to console\n");if(std::freopen("redir.txt","w",stdout)){std::printf("stdout is redirected to a file\n");// this is written to redir.txtstd::fclose(stdout);}}
Output:
stdout is printed to console
| opens a file (function)[edit] | |
| closes a file (function)[edit] | |
C documentation forfreopen | |
