std::move
class Node {public: Node() {} Node(const Node& nd) { cout << "const Node& nd" << endl; } Node(Node&& nd) { cout << "Node&& nd" << endl; }};int main() { const Node nd; // unfortunitely, print "const Node& nd" Node cy_nd(std::move(nd)); return 0;} Because the move assignment operator takes an rvalue reference to a non-const std::string.
The rvalue can, however, be passed to the copy assignment operator, because an lvalue reference-to-const is permitted to bind to a const rvalue. std::forward
// const lvalue referencevoid inner(const int& i) { cout << "int&" << endl; }// non-const rvalue referencevoid inner(int&& i) { cout << "int&&" << endl; } // universal referencetemplate void outer(T&& i) { /*i is lvalue reference * print "int&" */ inner(i); /* using std::forward * if : i is rvalue reference * then : print "int&&" * else : still print "int&" */ inner(std::forward (i));}int main() { int i = 2; outer(2); return 0;} std::move unconditionally casts its argument to an rvalue,
std::forward does it under certain conditions. std::forward is a conditional cast : It casts to an rvalue only if its argument was initialized with an rvalue.