Why does string stream operator<< erases the original value

Question

Can someone explain why the following code produce " you!" instead of "hello you!".

int main() {
    std::stringstream ss("hello");
    ss << " you!";
    cout << ss.str() << endl;
}

Answer 1

openmode must have ate set.

This is what you're looking for:

int main() {
    std::stringstream ss("hello", std::ios_base::out | std::ios_base::ate);
    ss << " you!";
    cout << ss.str() << endl;
}