From cppreference.com
protected:virtualpos_typeseekoff(off_typeoff,ios_base::seekdirway,ios_base::openmodewhich=ios_base::in|ios_base::out);(deprecated in C++98)
(removed in C++26)Repositions
and/or
, 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 ios_base::in and this buffer is open for reading, then repositions the read pointer
inside the get area as described below.
If which includes ios_base::out and this buffer is open for writing, then repositions the write pointer
inside the put area as described below.
If which includes both ios_base::in and ios_base::out and the buffer is open for both reading and writing, and way is either ios_base::beg or ios_base::end, then repositions both read and write pointers as described below.
Otherwise, this function fails.
If the pointer (either gptr or pptr or both) is repositioned, it is done as follows:
1) If the pointer to be repositioned is a null pointer and the new offset newoff would be non-zero, this function fails.
2) The new pointer offset newoff of type off_type is determined
a) if way==ios_base::beg, then newoff is zero
b) if way==ios_base::cur, then newoff is the current position of the pointer (gptr()-eback() or pptr()-pbase())
c) if way==ios_base::end, then newoff is the length of the entire initialized part of the buffer (if overallocation is used, the high watermark pointer minus the beginning pointer)
3) If newoff+off is negative or out of bounds of the initialized part of the buffer, 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 way - defines base position to apply the relative offset to. It can be one of the following constants: Constant Explanation
the beginning of a stream
the ending of a stream
the current position of stream position indicator which - defines whether the input sequences, the output sequence, or both are affected. It can be one or a combination of the following constants: Constant Explanation
affect the input sequence
affect the output sequence Return value
pos_type(newoff) on success, pos_type(off_type(-1)) on failure and if pos_type cannot represent the resulting stream position.
Example
Run this code
#include<iostream>#include<strstream>intmain(){chara[]="123";std::strstreamss(a,sizeofa);// in/outstd::cout<<"put pos = "<<ss.tellp()<<" get pos = "<<ss.tellg()<<'\n';// absolute positioning both pointersss.rdbuf()->pubseekoff(1,std::ios_base::beg);// move both forwardstd::cout<<"put pos = "<<ss.tellp()<<" get pos = "<<ss.tellg()<<'\n';// try to move both pointers 1 forward from current positionif(-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 positionstd::cout<<"Wrote 'a' at put position, the buffer is now: '";std::cout.write(a,sizeofa);std::cout<<"'\n";charch;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 retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
C++98 seekoff returned an undefined
invalid stream position on failure pos_type(off_type(-1))
is returned on failure See also
[virtual]
repositions the next pointer in the input sequence, output sequence, or both using absolute addressing
(virtual protected member function)
[virtual]
repositions the next pointer in the input sequence, output sequence, or both, using relative addressing
(virtual protected member function of std::basic_streambuf<CharT,Traits>)
[virtual]
repositions the next pointer in the input sequence, output sequence, or both, using relative addressing
(virtual protected member function of std::basic_stringbuf<CharT,Traits,Allocator>)
[virtual]
repositions the file position, using relative addressing
(virtual protected member function of std::basic_filebuf<CharT,Traits>)