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_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.

* 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 repositions the
read pointer std::basic_streambuf::gptr inside the get area as described 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 repositions the
write pointer std::basic_streambuf::pptr inside the put area as described 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 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 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 constants:
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 retroactively 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_streambuf<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_filebuf<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)