std::strstreambuf::seekoff

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< cpp‎ | io‎ | strstreambuf
 
C++
 
Input/output library
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(C++11)
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std::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 way 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

Parameters

off - relative position to set the next pointer(s) to
way - 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

Return value

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

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'

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

See also

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