std::strstreambuf::seekoff
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< cpp | io | strstreambuf
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std::strstreambuf
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Protected member functions
strstreambuf::seekoff
protected:
(deprecated in C++98) virtual pos_type seekoff( off_type off,
ios_base::seekdir way,
(removed in C++26)
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::outand the buffer is open for both reading and writing, and way 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 determineda) 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
[edit] Parameters
off
-
relative position to set the next pointer(s) to
way
-
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:
repositions the next pointer in the input sequence, output sequence, or both using absolute addressing
(virtual protected member function) [edit]
repositions the next pointer in the input sequence, output sequence, or both, using relative addressing
(virtual protected member function of
repositions the next pointer in the input sequence, output sequence, or both, using relative addressing
(virtual protected member function of
Constant
Explanation
beg
the beginning of a stream
end
the ending of a stream
cur
the current position of stream position indicator
Constant
Explanation
in
affect the input sequence
out
affect the output sequence
[edit] Return value
pos_type(newoff) on success, pos_type(off_type(-1)) on failure and if pos_type cannot represent the resulting stream position.
[edit] 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'
[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 | seekoff returned an undefinedinvalid stream position on failure |
pos_type(off_type(-1)) is returned on failure |
[edit] See also
[virtual]
(virtual protected member function) [edit]
[virtual]
(virtual protected member function of
std::basic_streambuf<CharT,Traits>) [edit]
[virtual]
(virtual protected member function of
std::basic_stringbuf<CharT,Traits,Allocator>) [edit]