std::strstreambuf::seekoff - cppreference.com (original) (raw)

| protected: virtual pos_type seekoff( off_type off, ios_base::seekdir way, ios_base::openmode which = ios_base::in | ios_base::out ); | | (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 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::out and 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 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)

1. If newoff + off is negative or out of bounds of the initialized part of the buffer, the function fails

2. Otherwise, the pointer is assigned as if by gptr() = eback() + newoff + off or pptr() = pbase() + newoff + off

[edit] Parameters

[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

#include #include   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.

[edit] See also

| | 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 std::basic_streambuf<CharT,Traits>) [edit] | | | 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>) [edit] | | | repositions the file position, using relative addressing (virtual protected member function of std::basic_filebuf<CharT,Traits>) [edit] |