Rvalue Reference and Move Semantics

References

tmp obj/values are unnamed, created automatically on stack when

1. const reference initialization
   • const int& a = 1
   • const A& a = 1, only works if A has conversion constructor
2. argument passing
3. function return value
4. evaluation of expression

• 5/2

  • temp objects (not defined inside function, e.g. params) will be destructed after the line terminate, not when the function ends

• lvalue reference is an alias to an object

• rvalue reference is an alias to a temporary object

int main() {
    int a = 5;
    int& b = a;
    b++;
 
    int& c = 5; // ERROR
    const int& d = 5;
    d++; // ERROR
 
    int&& e = 5;
    e++;
    int&& x = a; // ERROR
}

• lvalue ref only bind to lvalue
• rvalue ref only bind to rvalue
• except const lvalue ref

Move

• Return by value will be done by move instead of copy

// Example p.23
 
// Move constructor
// Note that str(w.str) is copying the pointer
Word(Word&& w) : freq(w.freq), str(w.str) {
    w.freq = 0;
    w.str = nullptr;
    cout << "move: ";
    print();
}
 
// Move assignment
Word& operator=(Word&& w) {
    if (this != &w) {
        delete [] str;
        freq = w.freq; str = w.str;
        w.freq = 0; w.str = nullptr;
        cout << "move assignment: "; print();
    }
    return *this;
}

// Pass by rvalue reference &&, construct by moving
// We need std::move here because after we initalized the rvalue reference, it is now actually a lvalue (a and b)
Word_Pair(Word&& a, Word&& b) : w1(std::move(a)), w2(std::move(b)) {
    cout << "\n-- Another move with inputs --\n"; a.print(); b.print(); 
}

• when using std::move, no tmp is created, so no destructor