空的用户定义的移动构造函数

Question

我为理解移动 CTOR 行为而编写的以下代码片段让我很难理解它的输出：

#include <iostream>

class Temp
{
  public:

    Temp(){
      std::cout << "Temp DEFAULT CTOR called" << std::endl;
      mp_Val = nullptr;
    }

    Temp(int inp) { 
      std::cout << "Temp CTOR called" << std::endl;
      mp_Val = new int(inp); 
    }

    Temp(const Temp& inp) {
      std::cout << "Temp COPY CTOR called" << std::endl;
      mp_Val = new int(*inp.mp_Val); 
    }

    Temp& operator= (const Temp& inp) {
      std::cout << "Temp ASSIGNMENT OPER called" << std::endl;
      mp_Val = new int(*inp.mp_Val);

      return *this;
    }

    int* mp_Val;
};

class B
{
  public:

    B(){
      std::cout << "Class B DEFAULT CTOR" << std::endl; 
      mp_Val = nullptr;
    }
    B(int inp) { 
      std::cout << "Class B CTOR" << std::endl; 
      mp_Val = new Temp(inp);
    }

    B(const B& in) { 
      std::cout << "Class B COPY CTOR" << std::endl; 
      mp_Val = in.mp_Val;
    }

     B(B&& in){ 
      std::cout << "Class B MOVE CTOR" << std::endl; //Doubt: 1
    }

    Temp *mp_Val;
};

int main() {
  B obj1(200);
  B obj2 = std::move(obj1);
  auto temp = obj1.mp_Val;
  std::cout << "Obj1 B::Temp address: " << obj1.mp_Val << std::endl;

  std::cout << "Obj2 B::Temp address: " << obj2.mp_Val << std::endl; //Doubt: 2

  return 0;
}

输出：

Class B CTOR
Temp CTOR called
Class B MOVE CTOR
Obj1 B::Temp address: 0xd48030
Obj2 B::Temp address: 0x400880

GCC 版本：4.6.3

我的问题是关于标记为Doubt 2的行。地址不应该打印为0吗？根据我的理解，由于我在 class B 中定义了一个空移动 CTOR（标记为 Doubt 1），它应该调用 class Temp 的默认 CTOR（它没有调用明显从日志中）初始化其成员变量 mp_Val Temp.

显然我缺少一些东西。

Answer 1

As per my understanding, as I have defined an empty move CTOR (marked as Doubt 1) in class B, it should call the default CTOR of class Temp (which it's not calling as evident from the logs) to initialise its member variable mp_Val of type Temp.

您的成员变量不是Temp类型，而是Temp *类型。你是对的，缺少初始化器意味着该成员将被默认构造，并且对于 Temp 类型，这将涉及调用默认构造函数。但是，对于指针类型，默认构造使对象未初始化。

空的用户定义的移动构造函数

Empty user-defined move constructor

c++

move

move-constructor

move-semantics

c++11