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std::basic_stringbuf<CharT,Traits,Allocator>::seekpos

From cppreference.com
< cpp‎ | io‎ | basic stringbuf
 
C++
 
Input/output library
 
std::basic_stringbuf
 
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.

Effectively executes seekoff(off_type(sp), std::ios_base::beg, which).

Contents

[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

sp 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 <sstream>
#include <iostream>
 
struct mybuf : std::stringbuf
{
    mybuf(const std::string& str) : std::stringbuf(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::stringbuf::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] 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 375 C++98 static constant members of std::ios_base were
misspecified as members of std::basic_ios 		corrected
LWG 564 C++98 if was unclear how to reposition gptr and/or pptr they are repositioned by seekoff()

[edit] See also

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