Repositions the file pointer, if possible, to the position that corresponds to exactlyoff characters from beginning, end, or current position of the file (depending on the value ofdir).

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 offsetoff is not​0​, fails immediately: this function cannot determine the number of bytes that correspond tooff characters.

Ifdir is notstd::basic_ios::cur or the offsetoff 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 wasoverflow()), then callsstd::codecvt::unshift to determine the unshift sequence necessary, and writes that sequence to the file by callingoverflow().

Then converts the argumentdir to a valuewhence of typeint as follows:

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

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

Theopenmode argument, required by the base class function signature, is usually ignored, becausestd::basic_filebuf maintains only one file position.

[edit]Parameters

[edit]Return value

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

[edit]Notes

seekoff() is called bystd::basic_streambuf::pubseekoff, which is called bystd::basic_istream::seekg,std::basic_ostream::seekp,std::basic_istream::tellg, andstd::basic_ostream::tellp.

[edit]Example

#include <fstream>#include <iostream>#include <locale> template<typename CharT>int get_encoding(conststd::basic_istream<CharT>& stream){using Facet=std::codecvt<CharT,char,std::mbstate_t>;returnstd::use_facet<Facet>(stream.getloc()).encoding();} int main(){// prepare a 10-byte file holding 4 characters ("zß水𝄋") in UTF-8std::ofstream("text.txt")<<"\x7a\xc3\x9f\xe6\xb0\xb4\xf0\x9d\x84\x8b"; // open using a non-converting encodingstd::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-8std::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';}

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

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit]See also

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