std::strstreambuf::seekpos - cppreference.com (original) (raw)
Repositions std::basic_streambuf::gptr and/or std::basic_streambuf::pptr, if possible, to the position indicated by sp.
If std::ios_base::in is set in which, attempts to reposition gptr() (the next pointer in the get area). If std::ios_base::out is set in which, attempts to reposition pptr() (the next pointer in the put area). If neither bit is set in which, the operation fails.
Each next pointer is repositioned as follows:
- If the next pointer is null, the operation fails.
- Otherwise, the new offset newoff (of type
off_type) is determined by calling sp.offset(). If newoff is negative, out of bounds of the buffer, or invalid, the operation fails. - Otherwise, the next pointer is assigned as if by gptr() = eback() + newoff or pptr() = pbase() + newoff.
[edit] Parameters
| sp | - | stream position, such as one obtained by seekoff() or seekpos() |
|---|---|---|
| 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 |
[edit] Return value
The resultant offset converted to pos_type on success or pos_type(off_type(-1)) on failure.
[edit] Notes
seekpos() is called by std::basic_streambuf::pubseekpos(), which is called by the single-argument versions of std::basic_istream::seekg() and std::basic_ostream::seekp().
[edit] Example
#include #include #include struct mybuf : std::strstreambuf { mybuf(const char* str) : std::strstreambuf(str, std::strlen(str)) {} pos_type seekpos(pos_type sp, std::ios_base::openmode which) { std::cout << "Before seekpos(" << sp << "), size of the get area is " << egptr() - eback() << " with " << egptr() - gptr() << " read positions available.\n"; pos_type rc = std::strstreambuf::seekpos(sp, which); std::cout << "seekpos() returns " << rc << ".\nAfter the call, " << "size of the get area is " << egptr() - eback() << " with " << egptr() - gptr() << " read positions available.\n"; return rc; } }; int main() { mybuf buf("12345"); std::iostream stream(&buf); stream.seekg(2); }
Output:
Before seekpos(2), size of the get area is 5 with 5 read positions available. seekpos() returns 2. After the call, size of the get area is 5 with 3 read positions available.
[edit] 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 | seekpos returned an undefinedinvalid stream position on failure | pos_type(off_type(-1))is returned on failure |
[edit] See also
| | repositions the next pointer in the input sequence, output sequence, or both, using relative addressing (virtual protected member function) [edit] | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | | repositions the next pointer in the input sequence, output sequence, or both using absolute addressing (virtual protected member function of std::basic_streambuf<CharT,Traits>) [edit] | | | repositions the next pointer in the input sequence, output sequence, or both using absolute addressing (virtual protected member function of std::basic_stringbuf<CharT,Traits,Allocator>) [edit] | | | repositions the file position, using absolute addressing (virtual protected member function of std::basic_filebuf<CharT,Traits>) [edit] |