c_cpp 复制和移动语义

Posted 2021-05-14

#include <iostream>

using namespace std;

class A {
public:
    A(int id) : id(id) {
        cout << "CONSTRUCTOR ID" << endl;
    }

    A() {
        cout << "CONSTRUCTOR" << endl;
    }

    ~A() {
        cout << "DESTRUCTOR" << endl;
    }

    A(const A &other) {
        cout << "COPY CONSTRUCTOR" << endl;
    }

    A(A &&other) : id(0) {
        cout << "MOVE CONSTRUCTOR" << endl;
    }

    A &operator=(const A &other) {
        cout << "COPY on Lvalue" << endl;
        return *this;
    }

    A &operator=(A &&other) {
        cout << "MOVE on Rvalue" << endl;
        return *this;
    }

    // Invokes move constructor
    A &&move_test() {
        A a(11);

        return move(a);
    }

    // DANGER! Invokes copy constructor, but after a has been destructed
    A &copy_test() {
        A a(12);

        return a;
    }

    // Passes by value
    A value_test() {
        A a(12);

        return a;
    }

    // parameter is created by COPY constructor
    // return value is created by MOVE constructor
    A move_arg_test1(A a) {

    // return move(a); // same
        return a;
    }

    // parameter is passed by address
    // return value is created by COPY constructor
    A &move_arg_test2(A &a) {
        return a;
    }

    // parameter is passed by address (takes only Rvalue)
    // return value is created by MOVE constructor
    A &&move_arg_test3(A &&a) {

        return move(a);
    }

private:
    int id;
};

int main() {

    A a1(6);
    A a2(7);

//    A a2 = A(4); // calls constructors ???
//    A c = a1; // calls COPY constructor
//    a1 = a2; // calls COPY operator= on Lvalue
//    a1 = a2.move_test(); // calls MOVE operator= on Rvalue
//    a1 = A(4); // calls MOVE operator= on Rvalue

    A a(8);
//    A d = a.move_arg_test1(a);
//    A d = a.move_arg_test2(a);
//    A d = a.move_arg_test3(A(4));
//    A d = a.copy_test();
//    A e = a.value_test();
//    A c = a.move_test();

    return 0;
}