std::basic_filebuf<CharT,Traits>::seekoff

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std::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 recent 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.

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

Return value

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

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

Example

Run this code
#include <iostream>
#include <fstream>
#include <locale>
 
template<typename CharT>
int get_encoding(const std::basic_istream<CharT>& stream)
{
    using Facet = std::codecvt<CharT, char, std::mbstate_t>;
    return std::use_facet<Facet>(stream.getloc()).encoding();
}
 
int main()
{
    // prepare a 10-byte file holding 4 characters ("zß水𝄋") in UTF-8
    std::ofstream("text.txt") << "\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 "
              << get_encoding(f1) << '\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 "
              << get_encoding(f2) << '\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

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 55 C++98 seekoff returned an undefined
invalid stream position on failure 		pos_type(off_type(-1))
is returned on failure

See also

 invokes seekoff()
(public member function of std::basic_streambuf<CharT,Traits>)
[virtual]
 repositions the file position, using absolute addressing
(virtual protected member function)
moves the file position indicator to a specific location in a file
(function)
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