std::basic_...buf::seekoff(3) C++ Standard Libary std::basic_...buf::seekoff(3) NAME std::basic_stringbuf::seekoff - std::basic_stringbuf::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 std::basic_streambuf::gptr and/or std::basic_stream- buf::pptr, if possible, to the position that corresponds to exactly off characters from beginning, end, or current position of the get and/or put area of the buffer. * If which includes std::ios_base::in and this buffer is open for reading (that is, if (which & std::ios_base::in) == std::ios_base::in), then repo- sitions the read pointer std::basic_streambuf::gptr inside the get area as de- scribed below * If which includes std::ios_base::out and this buffer is open for writing (that is, (which & std::ios_base::out) == std::ios_base::out), then repo- sitions the write pointer std::basic_streambuf::pptr inside the put area as de- scribed below * If which includes both std::ios_base::in and std::ios_base::out and the buffer is open for both reading and writing (that is, (which & (std::ios_base::in | std::ios_base::out)) == (std::ios_base::in | std::ios_base::out)), and dir is either std::ios_base::beg or std::ios_base::end, then repositions both read and write pointers as described below. * Otherwise, this function fails. If gptr and/or pptr is repositioned, it is done as follows: 1) The new pointer offset newoff of type off_type is determined a) if dir == std::ios_base::beg, then newoff is zero b) if dir == std::ios_base::cur, then newoff is the current position of the pointer (gptr() - eback() or pptr() - pbase()) c) if dir == std::ios_base::end, then newoff is the length of the entire initialized part of the buffer (if over-allocation is used, the high watermark pointer minus the beginning pointer) 2) If the pointer to be repositioned is a null pointer and newoff would be non-zero, this function fails. 3) If newoff + off < 0 (the repositioning would move the pointer to be- fore the beginning of the buffer) or if newoff + off would point past the end of the buffer (or past the last initialized character in the buffer if over-allocation is used), the function fails. 4) Otherwise, the pointer is assigned as if by gptr() = eback() + newoff + off or pptr() = pbase() + newoff + off. Parameters off - relative position to set the next pointer(s) 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 whether the input sequences, the output sequence, or both are affected. It can be one or a combination of the following con- stants: which - Constant Explanation in affect the input sequence out affect the output sequence Return value pos_type(newoff) on success, pos_type(off_type(-1)) on failure or if pos_type cannot represent the resulting stream position. Example // Run this code #include <iostream> #include <sstream> int main() { std::stringstream ss("123"); // in/out std::cout << "put pos = " << ss.tellp() << " get pos = " << ss.tellg() << '\n'; // absolute positioning both pointers ss.rdbuf()->pubseekoff(1, std::ios_base::beg); // move both 1 forward std::cout << "put pos = " << ss.tellp() << " get pos = " << ss.tellg() << '\n'; // try to move both pointers 1 forward from current position if (-1 == ss.rdbuf()->pubseekoff(1, std::ios_base::cur)) std::cout << "moving both pointers from current position failed\n"; std::cout << "put pos = " << ss.tellp() << " get pos = " << ss.tellg() << '\n'; // move the write pointer 1 forward, but not the read pointer // can also be called as ss.seekp(1, std::ios_base::cur); ss.rdbuf()->pubseekoff(1, std::ios_base::cur, std::ios_base::out); std::cout << "put pos = " << ss.tellp() << " get pos = " << ss.tellg() << '\n'; ss << 'a'; // write at put position std::cout << "Wrote 'a' at put position, the buffer is now " << ss.str() << '\n'; char ch; ss >> ch; std::cout << "reading at get position gives '" << ch << "'\n"; } Output: put pos = 0 get pos = 0 put pos = 1 get pos = 1 moving both pointers from current position failed put pos = 1 get pos = 1 put pos = 2 get pos = 1 Wrote 'a' at put position, the buffer is now 12a reading at get position gives '2' 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 seekoff returned an undefined pos_type(off_type(-1)) invalid stream position on failure is returned on failure static constant members of LWG 375 C++98 std::ios_base were corrected misspecified as members of std::basic_ios seekoff might succeed even if newoff LWG 432 C++98 + off seekoff fails would point past the last initialized in this case character repositioning null gptr() and/or null LWG 453 C++98 pptr() it can succeed in this case with a new offset of zero always failed the end pointer could not be used to LWG 563 C++98 calculate newoff because it could use the high watermark not be precisely controlled by the pointer instead program after resolving LWG issue 432 See also pubseekoff invokes seekoff() (public member function of std::basic_stream- buf<CharT,Traits>) seekpos repositions the next pointer in the input sequence, output sequence, or [virtual] both using absolute addressing (virtual protected member function) seekoff repositions the file position, using relative addressing [virtual] (virtual protected member function of std::basic_file- buf<CharT,Traits>) seekoff repositions the next pointer in the input sequence, output sequence, or [virtual] both, using relative addressing (virtual protected member function of std::strstreambuf) http://cppreference.com 2024.06.10 std::basic_...buf::seekoff(3)
NAME | Synopsis | Parameters | Return value | Example | Output: | See also
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