protected: virtual pos_type seekoff(off_type off, ios_base::seekdir dir, ios_base::openmode which = ios_base::in | ios_base::out); |
Repositions std::basic_streambuf::gptr
and/or std::basic_streambuf::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.
which
includes ios_base::in
and this buffer is open for reading (that is, if ((which & ios_base::in) == ios_base::in
), then repositions the read pointer std::basic_streambuf::gptr
inside the get area as described below which
includes ios_base::out
and this buffer is open for writing (that is, (which & ios_base::out) == ios_base::out
), then repositions the write pointer std::basic_streambuf::pptr
inside the put area as described below which
includes both ios_base::in
and ios_base::out
and the buffer is open for both reading and writing ((which & (ios_base::in|ios_base::out)) ==(ios_base::in|ios_base::out)
), and dir
is either ios_base::beg
or ios_base::end
, then repositions both read and write pointers as described below. If the pointer (either gptr
or pptr
or both) is repositioned, it is done as follows:
newoff
would be non-zero, this function fails.newoff
of type off_type
is determineddir == ios_base::beg
, then newoff
is zerodir == ios_base::cur
, then newoff
is the current position of the pointer (gptr()-eback()
or pptr()-pbase()
)dir == 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)newoff + off < 0
(the repositioning would move the pointer to before 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 overallocation is used), the function failsgptr() = eback() + newoff + off
or pptr() = pbase() + newoff + off
off | - | relative position to set the next pointer(s) to | ||||||||
dir | - | defines base position to apply the relative offset to. It can be one of the following constants:
|
||||||||
which | - | defines whether the input sequences, the output sequence, or both are affected. It can be one or a combination of the following constants:
|
pos_type(newoff)
on success, pos_type(off_type(-1))
on failure and if pos_type cannot represent the resulting stream position.
#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'
[virtual] | repositions the next pointer in the input sequence, output sequence, or both using absolute 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_streambuf<CharT,Traits> ) |
[virtual] | repositions the file position, using relative addressing (virtual protected member function of std::basic_filebuf<CharT,Traits> ) |
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