std::basic_stringbuf::seekoff - cppreference.com

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# std::basic_stringbuf::seekoff

From cppreference.com
< cpp‎ | io‎ | basic stringbuf

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.

- If `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
- If `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
- If `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.
- 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 dir == ios_base::beg, then `newoff` is zero
b) if dir == ios_base::cur, then `newoff` is the current position of the pointer (gptr()-eback() or pptr()-pbase())
c) if 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)
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 overallocation is used), the function fails
4) Otherwise, the pointer is assigned as if by gptr() = eback() + newoff + off or pptr() = pbase() + newoff + off

### [edit] 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

### [edit] Return value

pos_type(newoff) on success, pos_type(off_type(-1)) on failure and if pos_type cannot represent the resulting stream position.

### [edit] 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'

### [edit] See also

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

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