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
whichincludesios_base::inand this buffer is open for reading (that is, if((which & ios_base::in) == ios_base::in), then repositions the read pointerstd::basic_streambuf::gptrinside the get area as described below - If
whichincludesios_base::outand this buffer is open for writing (that is,(which & ios_base::out) == ios_base::out), then repositions the write pointerstd::basic_streambuf::pptrinside the put area as described below - If
whichincludes bothios_base::inandios_base::outand the buffer is open for both reading and writing ((which & (ios_base::in|ios_base::out)) ==(ios_base::in|ios_base::out)), anddiris eitherios_base::begorios_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:
| ||||||||
| which | - | defines whether the input sequences, the output sequence, or both are affected. It can be one or a combination of the following constants:
|
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
#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) |
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