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# std::basic_filebuf::seekoff

From cppreference.com
< cpp‎ | io‎ | basic filebuf

protected: virtual pos_type seekoff( off_type off, std::ios_base::seekdir dir, std::ios_base::openmode which = std::ios_base::in | std::ios_base::out ); |  |

Repositions the file pointer, if possible, to the position that corresponds to exactly `off` characters from beginning, end, or current position of the file (depending on the value of `dir`.

If the associated file is not open (is_open()==false, fails immediately.

If the multibyte character encoding is state-dependent (`codecvt::encoding()` returned -1) or variable-length (`codecvt::encoding()` returned ​0​) and the offset `off` is not ​0​, fails immediately: this function cannot determine the number of bytes that correspond to `off` characters.

If `dir` is not std::basic_ios::cur or the offset `off` is not ​0​, and the most resent operation done on this filebuf object was output (that is, either the put buffer is not empty, or the most recently called function was overflow()), then calls std::codecvt::unshift to determine the unshift sequence necessary, and writes that sequence to the file by calling overflow().

Then converts the argument `dir` to a value `whence` of type int as follows:

value of `dir` | value of `whence`
std::basic_ios::beg | SEEK_SET
std::basic_ios::end | SEEK_END
std::basic_ios::cur | SEEK_CUR

Then, if the character encoding is fixed-width (`codecvt::encoding()` returns some positive number `width`, moves the file pointer as if by std::fseek(file, width*off, whence).

Otherwise, moves the file pointer as if by std::fseek(file, 0, whence).

The `openmode` argument, required by the base class function signature, is usually ignored, because `std::basic_filebuf` maintains only one file position.

### [edit] Parameters

off | - | relative position to set the position indicator to.
dir | - | defines base position to apply the relative offset to. It can be one of the following constants:
Constant | Explanation
`beg` | the beginning of a stream
`end` | the ending of a stream
`cur` | the current position of stream position indicator
which | - | defines which of the input and/or output sequences to affect. It can be one or a combination of the following constants:
Constant | Explanation
`in` | affect the input sequence
`out` | affect the output sequence

### [edit] Return value

A newly constructed object of type pos_type which stores the resulting file position, or pos_type(off_type(-1)) on failure.

### [edit] Notes

`seekoff()` is called by std::basic_streambuf::pubseekoff, which is called by std::basic_istream::seekg, std::basic_ostream::seekp, std::basic_istream::tellg, and std::basic_ostream::tellp

### [edit] Example

Run this code

    #include <iostream>
    #include <fstream>
    #include <locale>
    int main()
    {
        // prepare a 10-byte file holding 4 characters in UTF8
        std::ofstream("text.txt") << u8"z\u00df\u6c34\U0001d10b"; // or u8"zß水𝄋"
                                               // or "\x7a\xc3\x9f\xe6\xb0\xb4\xf0\x9d\x84\x8b";

        // open using a non-converting encoding
        std::ifstream f1("text.txt");
        std::cout << "f1's locale's encoding() returns "
                  << std::use_facet<std::codecvt<char, char, std::mbstate_t>>(f1.getloc()).encoding() << '\n'
                  << "pubseekoff(3, beg) returns " << f1.rdbuf()->pubseekoff(3, std::ios_base::beg) << '\n'
                  << "pubseekoff(0, end) returns " << f1.rdbuf()->pubseekoff(0, std::ios_base::end) << '\n';;

        // open using UTF-8
        std::wifstream f2("text.txt");
        f2.imbue(std::locale("en_US.UTF-8"));
        std::cout << "f2's locale's encoding() returns "
                  << std::use_facet<std::codecvt<wchar_t, char, std::mbstate_t>>(f2.getloc()).encoding() << '\n'
                  << "pubseekoff(3, beg) returns " << f2.rdbuf()->pubseekoff(3, std::ios_base::beg) << '\n'
                  << "pubseekoff(0, end) returns " << f2.rdbuf()->pubseekoff(0, std::ios_base::end) << '\n';

    }

Output:

    f1's locale's encoding() returns 1
    pubseekoff(3, beg) returns 3
    pubseekoff(0, end) returns 10
    f2's locale's encoding() returns 0
    pubseekoff(3, beg) returns -1
    pubseekoff(0, end) returns 10

### [edit] See also

pubseekoff | invokes seekoff() (public member function of `std::basic_streambuf`) [edit]
seekpos [virtual] | repositions the file position, using absolute addressing (virtual protected member function) [edit]
fseek | moves the file position indicator to a specific location in a file (function) [edit]

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