how copy from one stringstream object to another in C++?

Indeed, streams are non-copyable (though they are movable).

Depending on your usage, the following works quite well:

#include <iostream>
#include <sstream>

int main()
{
    std::stringstream ss1;
    ss1 << "some " << 123 << " stuff" << std::flush;

    std::stringstream ss2;
    ss2 << ss1.rdbuf(); // copy everything inside ss1's buffer to ss2's buffer

    std::cout << ss1.str() << std::endl;
    std::cout << ss2.str() << std::endl;
}

Output:

some 123 stuff
some 123 stuff