std::basic_stringbuf<CharT,Traits,Allocator>::seekoff

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std::basic_stringbuf
 
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 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
dir - defines base position to apply the relative offset to. It can be one of the following constants:
Constant Explanation
beg the beginning of a stream
end the ending of a stream
cur 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
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
invalid stream position on failure 		pos_type(off_type(-1))
is returned on failure
LWG 375 C++98 static constant members of std::ios_base were
misspecified as members of std::basic_ios 		corrected
LWG 432 C++98 seekoff might succeed even if newoff + off
would point past the last initialized character 		seekoff fails
in this case
LWG 453 C++98 repositioning null gptr() and/or null pptr()
with a new offset of zero always failed 		it can succeed in this case
LWG 563 C++98 the end pointer could not be used to calculate newoff because it could
not be precisely controlled by the program after resolving LWG issue 432 		use the high watermark
pointer instead

See also

 invokes seekoff()
(public member function of std::basic_streambuf<CharT,Traits>)
[virtual]
 repositions the next pointer in the input sequence, output sequence, or both using absolute addressing
(virtual protected member function)
[virtual]
 repositions the file position, using relative addressing
(virtual protected member function of std::basic_filebuf<CharT,Traits>)
[virtual]
 repositions the next pointer in the input sequence, output sequence, or both, using relative addressing
(virtual protected member function of std::strstreambuf)
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