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std::basic_...buf::seekpos(3) C++ Standard Libarystd::basic_...buf::seekpos(3) NAME std::basic_filebuf::seekpos - std::basic_filebuf::seekpos Synopsis protected: virtual pos_type seekpos( pos_type sp, std::ios_base::openmode which = std::ios_base::in | std::ios_base::out ); Repositions the file pointer, if possible, to the position indicated by sp. If the associated file is not open (is_open() == false), fails immediately. Reposition performs as follows: 1) If the file is open for writing, writes the put area and any un- shift sequences required by the currently imbued locale, using overflow(). 2) Repositions the file pointer, as if by calling std::fsetpos(). 3) If the file is open for reading, updates the get area if neces- sary. If sp was not obtained by calling seekoff() or seekpos() on the same file, the behavior is undefined. Parameters sp - file position obtained by seekoff() or seekpos() called ear- lier on the same file 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 sp on success or pos_type(off_type(-1)) on failure. Notes seekpos() is called by std::basic_streambuf::pubseekpos(), which is called by the single-argument versions of std::basic_istream::seekg() and std::basic_ostream::seekp(). Many implementations do not update the get area in seekpos(), dele- gating to underflow() that is called by the next sgetc(). Example On some implementations, the get area is emptied by seekpos() and the second underflow() is necessary to observe the effects. // Run this code #include <fstream> #include <iostream> struct mybuf : std::filebuf { pos_type seekpos(pos_type sp, std::ios_base::openmode which) { std::cout << "Before seekpos(" << sp << "), size of the get area is " << egptr() - eback() << " with " << egptr() - gptr() << " read positions avail- able.\n"; pos_type rc = std::filebuf::seekpos(sp, which); std::cout << "seekpos() returns " << rc << ".\nAfter the call, " << "size of the get area is " << egptr() - eback() << " with " << egptr() - gptr() << " read positions avail- able.\n"; // uncomment if get area is emptied by seekpos() // std::filebuf::underflow(); // std::cout << "after forced underflow(), size of the get area is " // << egptr() - eback() << " with " // << egptr() - gptr() << " read positions avail- able.\n"; return rc; } }; int main() { mybuf buf; buf.open("test.txt", std::ios_base::in); std::istream stream(&buf); stream.get(); // read one char to force underflow() stream.seekg(2); } Possible output: Before seekpos(2), size of the get area is 110 with 109 read positions available. seekpos() returns 2. After the call, size of the get area is 110 with 108 read positions available. Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct behavior LWG 55 C++98 seekpos returned an undefined pos_type(off_type(-1)) invalid stream position on failure is re- turned on failure LWG 171 C++98 the sequence of the operations of made clear reposition was not clear See also pubseekpos invokes seekpos() (public member function of std::basic_stream- buf<CharT,Traits>) seekoff repositions the file position, using relative addressing [virtual] (virtual protected member function) fseek moves the file position indicator to a specific location in a file (function) http://cppreference.com 2024.06.10 std::basic_...buf::seekpos(3)
NAME | Synopsis | Parameters | Return value | Notes | Example | Possible output: | See also
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