std::strstreambuf::seekoff (3) - Linux Manuals

std::strstreambuf::seekoff: std::strstreambuf::seekoff

NAME

std::strstreambuf::seekoff - std::strstreambuf::seekoff

Synopsis

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

Parameters

off - relative position to set the next pointer(s) to
        defines base position to apply the relative offset to. It can be one of the following constants:

        Constant Explanation
way - beg the beginning of a stream
        end the ending of a stream
        cur the current position of stream position indicator

        defines whether the input sequences, the output sequence, or both are affected. It can be one or a combination of the following constants:

which - 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'