What does it mean? std::istream response_stream(&*response_);

You did not add much code to clarify, but as it is written it looks like a function declaration - in that case it makes sense.

Defining a parameter as & in a c++ function declaration means that the parameter is passed by reference, as in the next example

void same(int &n);  // declaration of the prototype with a reference
int main(){
    int iA = 5;   // Define a number
    same(iA);     // pass the variable to ur function BY REFERENCE
    cout << iA; // Without the reference passing I should see 5; By reference I can 
                // change the value of the parameter, so now iA equals 6;
    return 0; // Exit from main
}
void same(int &n){
    n = 6;
}

So by passing a parameter as &*response_ I'm passing a reference to the object pointed by response_.

I hope I have been clear enough (English is not my primary language) and to have helped you.

EDIT: I found an article on CodeProject Pointer to Pointer and Reference to Pointer[^], maybe it helps you.

- Denis