When is an rvalue evaluated?

Question

So I understand that s2 binds to the expression s1 + s1, but is this evaluated at the time s2 is assigned or is it lazy and evaluated when s2 += "Test"; is called? And also would s2 hold memory for a temporary string?

#include <iostream>
#include <string>

int main()
{
    std::string s1 = "Test";
    std::string&& s2 = s1 + s1;
    s2 += "Test";
    std::cout << s2 << '\n';
}

Answer 1

s2 binds to the expression s1 + s1, but is this evaluated at the time s2 is assigned

Yes.

And also would s2 hold memory for a temporary string?

Precisely, s2 is bound to a temporary std::string.

s1 + s1 will produce a temporary std::string, which will be bound to the reference s2 (and its lifetime is extended to the lifetime of the reference). Then s2 += "Test";, performs operator+=() on s2, i.e. the temporary std::string.