std::basic_stringbuf::seekoff

From cppreference.com
< cpp‎ | io‎ | basic stringbuf
 
C++
 
Input/output library
I/O manipulators
C-style I/O
Buffers
(deprecated in C++98)
(C++20)
Streams
Abstractions
File I/O
String I/O
Array I/O
(deprecated in C++98)
(deprecated in C++98)
(deprecated in C++98)
Synchronized Output
(C++20)
Types
Error category interface
(C++11)
(C++11)
 
std::basic_stringbuf
 
protected:

virtual pos_type seekoff(off_type off,
                         ios_base::seekdir dir,

                         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 (that is, if ((which & ios_base::in) == ios_base::in), 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 (that is, (which & ios_base::out) == ios_base::out), 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 ((which & (ios_base::in|ios_base::out)) ==(ios_base::in|ios_base::out)), and dir 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 dir == ios_base::beg, then newoff is zero
b) if dir == ios_base::cur, then newoff is the current position of the pointer (gptr()-eback() or pptr()-pbase())
c) if dir == 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 < 0 (the repositioning would move the pointer to before the beginning of the buffer) or if newoff + off would point past the end of the buffer (or past the last initialized character in the buffer if overallocation is used), 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
dir - 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 <sstream>
 
int main()
{
    std::stringstream ss("123"); // 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 1 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 " << ss.str() << '\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'

See also

[virtual]
 repositions the next pointer in the input sequence, output sequence, or both using absolute addressing
(virtual protected member function of std::basic_streambuf)
[virtual]
 repositions the next pointer in the input sequence, output sequence, or both, using relative addressing
(virtual protected member function of std::basic_streambuf)
[virtual]
 repositions the file position, using relative addressing
(virtual protected member function of std::basic_filebuf)
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/io/basic_stringbuf/seekoff&oldid=80458"