std::strstreambuf:: seekoff

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:

Parameters

Return value

pos_type ( newoff ) on success, pos_type ( off_type ( - 1 ) ) on failure and if pos_type cannot represent the resulting stream position.

Example

#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";
}

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.

See also