std::strstreambuf::seekoff
From cppreference.com
< cpp | io | strstreambuf
|
protected
:
virtual pos_type seekoff( off_type off, |
(deprecated in C++98) (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
- If which includes ios_base::in and this buffer is open for reading, then repositions the read pointer std::basic_streambuf::gptr
- If which includes ios_base::out and this buffer is open for writing, then repositions the write pointer std::basic_streambuf::pptr
- 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 - 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
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
3) If newoff + off
4) Otherwise, the pointer is assigned as if by
gptr(
)
= eback(
)
+ newoff +
or
pptr(
)
= pbase(
)
+ newoff +
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:
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||||||||
| 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))
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'
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))
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See also
|
[virtual]
|
repositions the next pointer in the input sequence, output sequence, or both using absolute addressing (virtual protected member function) |
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[virtual]
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repositions the next pointer in the input sequence, output sequence, or both, using relative addressing (virtual protected member function of std::basic_streambuf<CharT,Traits>) |
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[virtual]
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repositions the next pointer in the input sequence, output sequence, or both, using relative addressing (virtual protected member function of std::basic_stringbuf<CharT,Traits,Allocator>) |
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[virtual]
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repositions the file position, using relative addressing (virtual protected member function of std::basic_filebuf<CharT,Traits>) |