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

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< cpp‎ | io‎ | 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 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

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

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