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std::basic_...buf::seekoff(3) C++ Standard Libarystd::basic_...buf::seekoff(3) NAME std::basic_filebuf::seekoff - std::basic_filebuf::seekoff Synopsis 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 cor- responds 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 immedi- ately. If the multibyte character encoding is state-dependent (codecvt::en- coding() returned -1) or variable-length (codecvt::encoding() returned 0) and the off- set 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 over- flow()), then calls std::codecvt::unshift to determine the unshift sequence neces- sary, and writes that sequence to the file by calling overflow(). Then converts the argument dir to a value whence of type int as fol- lows: 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 signa- ture, is usually ignored, because std::basic_filebuf maintains only one file posi- tion. Parameters off - relative position to set the position indicator to. defines base position to apply the relative offset to. It can be one of the following constants: dir - Constant Explanation beg the beginning of a stream end the ending of a stream cur the current position of stream position indicator defines which of the input and/or output sequences to af- fect. It can be one or a combination of the following constants: which - 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 result- ing 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::ba- sic_istream::tellg, and std::basic_ostream::tellp 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"z" // 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::mb- state_t>>(f1.getloc()).encoding() << '\n' << "pubseekoff(3, beg) returns " << f1.rdbuf()->pub- seekoff(3, std::ios_base::beg) << '\n' << "pubseekoff(0, end) returns " << f1.rdbuf()->pub- seekoff(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::mb- state_t>>(f2.getloc()).encoding() << '\n' << "pubseekoff(3, beg) returns " << f2.rdbuf()->pub- seekoff(3, std::ios_base::beg) << '\n' << "pubseekoff(0, end) returns " << f2.rdbuf()->pub- seekoff(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 See also pubseekoff invokes seekoff() (public member function of std::basic_stream- buf<CharT,Traits>) seekpos repositions the file position, using absolute addressing [virtual] (virtual protected member function) fseek moves the file position indicator to a specific location in a file (function) http://cppreference.com 2022.07.31 std::basic_...buf::seekoff(3)
NAME | Synopsis | Parameters | Return value | Notes | Example | Output: | See also
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