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# std::strstreambuf::seekoff

From cppreference.com
< cpp‎ | io‎ | strstreambuf

protected: virtual pos_type seekoff(off_type off, ios_base::seekdir way, 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, 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, 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, and `when` 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 way == ios_base::beg, then `newoff` is zero
b) if way == ios_base::cur, then `newoff` is the current position of the pointer (gptr()-eback() or pptr()-pbase())
c) if way == 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 is negative or out of bounds of the initialized part of the buffer, 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 <strstream>

    int main()
    {
        char a[] = "123";
        std::strstream ss(a, sizeof a); // 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 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: '";
        std::cout.write(a, sizeof a);
        std::cout << "'\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 next pointer in the input sequence, output sequence, or both, using relative addressing (virtual protected member function of `std::basic_stringbuf`) [edit]
seekoff [virtual] | repositions the file position, using relative addressing (virtual protected member function of `std::basic_filebuf`) [edit]

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