std::basic_stringbuf<CharT,Traits,Allocator>::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.

# Declarations

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

# 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: Constant Explanation beg the beginning of a stream end the ending of a stream cur the current position of stream position indicator
which: defines whether the input sequences, the output sequence, or both are affected. It can be one or a combination of the following constants: Constant Explanation in affect the input sequence out affect the output sequence

# Return value

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

# Example

#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";
}

# Defect reports

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
LWG 375	C++98	static constant members of std::ios_base weremisspecified 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 couldnot be precisely controlled by the program after resolving LWG issue 432	use the high watermarkpointer instead