std::strstreambuf::seekpos

 

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strstreambuf::seekoff

strstreambuf::seekpos

 

protected: virtual pos_type seekpos( pos_type sp,                           std::ios_base::openmode which =                               std::ios_base::in | std::ios_base::out );		(deprecated in C++98) (removed in C++26)

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:

• If the next pointer is null, the operation fails.
• 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.
• Otherwise, the next pointer is assigned as if by gptr() = eback() + newoff or pptr() = pbase() + newoff.

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

Return value

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

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().

Example

Run this code

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

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	seekpos returned an undefined invalid stream position on failure	pos_type(off_type(-1)) is returned on failure

See also

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