C++中分别使用左值形参和右值形参的拷贝构造和移动构造

#include <iostream>

class Data {
public:
  Data() { std::cout << "EMPTY." << std::endl; }
  Data(const Data &d) { std::cout << "lvalue copy." << std::endl; }
  Data(const Data &&d) { std::cout << "rvalue copy." << std::endl; }
  Data &operator=(const Data &d) {
    std::cout << "lvalue assign." << std::endl;
    return *this;
  }
  Data &operator=(const Data &&d) {
    std::cout << "rvalue assign." << std::endl;
    return *this;
  }
};

int main(int argc, char *argv[]) {
  std::cout << "data0: ";
  Data data0;
  std::cout << "data1: ";
  Data data1 = data0;
  std::cout << "data2: ";
  Data data2(data0);
  std::cout << "data3: ";
  Data data3 = std::move(data0);
  std::cout << "data4: ";
  Data data4(std::move(data1));
  std::cout << "data5: ";
  Data data5;
  data5 = data2;
  std::cout << "data6: ";
  Data data6;
  data6 = std::move(data2);
}

　　运行结果：

data0: EMPTY.
data1: lvalue copy.
data2: lvalue copy.
data3: rvalue copy.
data4: rvalue copy.
data5: EMPTY.
lvalue assign.
data6: EMPTY.
rvalue assign.