How to make reusable buffer for C++ streaming operations

#include <string>
#include <vector>
#include <streambuf>
#include <sstream>
#include <type_traits>

struct mystreambuf : public std::streambuf 
{
    mystreambuf(char_type* buffer, std::streamsize bufferLength)
    {
        setg(buffer, buffer, buffer + bufferLength);
    }
protected:
    virtual pos_type seekoff(off_type offset, std::ios_base::seekdir direction, std::ios_base::openmode)
    {
        off_type curpos = gptr() - eback();
        std::streamsize bufferLength = egptr() - eback();
        off_type newpos = -1;

        if(direction == std::ios::cur) 
        {
            newpos = curpos + offset;
        }
        else if (direction == std::ios::end) 
        {
            newpos = bufferLength - offset;
        }
        else
        {
            newpos = offset;
        }

        setg(eback(), eback() + newpos, egptr());
        return newpos;
    }

    virtual pos_type seekpos(pos_type , std::ios_base::openmode )
    {
        return (std::streampos(std::_BADOFF));
    }
};

int main(int arcg, char* argv[])
{
    auto msg = "123.45";
    auto length = sizeof(msg);
    std::vector<char> buffer(msg, msg + length);

    mystreambuf streamBuffer(buffer.data(), length);

    std::istream localStream(&streamBuffer);

    double res = 0;
    localStream.seekg(1, localStream.beg);
    localStream >> res;

    localStream.clear();
    localStream.seekg(0, localStream.beg);

    res = 0;
    localStream >> res;

    auto msg2 = "33.5";
    length = sizeof(msg2);
    buffer.assign(msg2, msg2 + length);

    localStream.clear();
    localStream.seekg(0, localStream.beg);

    res = 0;
    localStream >> res;

    return 0;
}