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
whichincludesios_base::inand this buffer is open for reading, then repositions the read pointerstd::basic_streambuf::gptrinside the get area as described below - If
whichincludesios_base::outand this buffer is open for writing, 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, andwhenis 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
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 | ||||||||
| 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 <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'
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 next pointer in the input sequence, output sequence, or both, using relative addressing (virtual protected member function of std::basic_stringbuf) |
| [virtual] | repositions the file position, using relative addressing (virtual protected member function of std::basic_filebuf) |
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