std::basic_stringbuf<CharT,Traits,Allocator>::seekoff
From cppreference.com
< cpp | io | basic stringbuf
protected: virtual pos_type seekoff( off_type off, |
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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 std::ios_base::in and this buffer is open for reading (that is, if (which & std::ios_base::in) == std::ios_base::in), then repositions the read pointer std::basic_streambuf::gptr inside the get area as described below
- If which includes std::ios_base::out and this buffer is open for writing (that is, (which & std::ios_base::out) == std::ios_base::out), then repositions the write pointer std::basic_streambuf::pptr inside the put area as described below
- If which includes both std::ios_base::in and std::ios_base::out and the buffer is open for both reading and writing (that is, (which & (std::ios_base::in | std::ios_base::out)) == (std::ios_base::in | std::ios_base::out)), and dir is either std::ios_base::beg or std::ios_base::end, then repositions both read and write pointers as described below.
- Otherwise, this function fails.
If gptr and/or pptr is repositioned, it is done as follows:
1) The new pointer offset newoff of type
off_type
is determinedb) if dir == std::ios_base::cur, then newoff is the current position of the pointer (gptr() - eback() or pptr() - pbase())
c) if dir == std::ios_base::end, then newoff is the length of the entire initialized part of the buffer (if over-allocation is used, the high watermark pointer minus the beginning pointer)
2) If the pointer to be repositioned is a null pointer and newoff would be non-zero, this function fails.
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 over-allocation is used), the function fails.
4) Otherwise, the pointer is assigned as if by gptr() = eback() + newoff + off or pptr() = pbase() + newoff + off.
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[edit] 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:
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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:
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[edit] Return value
pos_type(newoff) on success, pos_type(off_type(-1)) on failure or if pos_type
cannot represent the resulting stream position.
[edit] Example
Run this code
#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'
[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 |
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LWG 55 | C++98 | seekoff returned an undefinedinvalid stream position on failure |
pos_type(off_type(-1)) is returned on failure |
LWG 375 | C++98 | static constant members of std::ios_base were misspecified as members of std::basic_ios |
corrected |
LWG 432 | C++98 | seekoff might succeed even if newoff + offwould point past the last initialized character |
seekoff failsin this case |
LWG 453 | C++98 | repositioning null gptr() and/or null pptr() with a new offset of zero always failed |
it can succeed in this case |
LWG 563 | C++98 | the end pointer could not be used to calculate newoff because it could not be precisely controlled by the program after resolving LWG issue 432 |
use the high watermark pointer instead |
[edit] See also
invokes seekoff() (public member function of std::basic_streambuf<CharT,Traits> )
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[virtual] |
repositions the next pointer in the input sequence, output sequence, or both using absolute addressing (virtual protected member function) |
[virtual] |
repositions the file position, using relative addressing (virtual protected member function of std::basic_filebuf<CharT,Traits> )
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[virtual] |
repositions the next pointer in the input sequence, output sequence, or both, using relative addressing (virtual protected member function of std::strstreambuf )
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