std::strstreambuf::seekpos - cppreference.com

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# std::strstreambuf::seekpos

From cppreference.com
< cpp‎ | io‎ | strstreambuf

protected: virtual pos_type seekpos(pos_type sp, 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 indicated by `sp`

If std::ios_base::in is set in `which`, attempts to reposition `gptr()` (the next pointer in the get area). If std::ios_base::out is set in `which`, attempts to reposition `pptr()` (the next pointer in the put area). If neither bit is set in `which`, the operation fails.

Each next pointer is repositioned as follows:

1) If the next pointer is null, the operation fails
2) Otherwise, the new offset `newoff` (of type `off_type`) is determined by calling sp.offset(). If `newoff` is negative, out of bounds of the buffer, or invalid, the operation fails.
3) Otherwise, the next pointer is assigned as if by gptr() = eback()+newoff or pptr() = pbase()+newoff.

### [edit] Parameters

sp | - | stream position, such as one obtained by seekoff() or `seekpos()`
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

The resultant offset converted to `pos_type` on success or pos_type(off_type(-1)) on failure.

### [edit] Notes

`seekpos()` is called by std::basic_streambuf::pubseekpos(), which is called by the single-argument versions of std::basic_istream::seekg() and std::basic_ostream::seekp()

### [edit] Example

Run this code

    #include <strstream>
    #include <cstring>
    #include <iostream>

    struct mybuf : std::strstreambuf
    {
        mybuf(const char* str) : std::strstreambuf(str, std::strlen(str)) {}
        pos_type seekpos(pos_type sp, std::ios_base::openmode which) {
             std::cout << "Before seekpos(" << sp << "), size of the get area is "
                       << egptr()-eback() << " with "
                       << egptr()-gptr() << " read positions available\n";
             pos_type rc = std::strstreambuf::seekpos(sp, which);
             std::cout << "seekpos() returns " << rc << ".\nAfter the call, "
                       << "size of the get area is "
                       << egptr()-eback() << " with "
                       << egptr()-gptr() << " read positions available\n";
            return rc;
        }
    };

    int main()
    {
        mybuf buf("12345");
        std::iostream stream(&buf);
        stream.seekg(2);
    }

Output:

    Before seekpos(2), size of the get area is 5 with 5 read positions available
    seekpos() returns 2.
    After the call, size of the get area is 5 with 3 read positions available

### [edit] See also

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

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