C++复制分配和移动分配不被称为

Question

我想实现复制和移动任务，但我不明白我应该如何使用它们。我已阅读以下主题
When did copy assignment operator called?
但它没有为我工作。

类：

class Directory{ 

    string name; 
public: 
     Directory(string name):name(name) { 

     } 
     ~Directory() { 
      cout << "Deleting was called" <<endl; 

      } 

     Directory& operator=(Directory& other){ 
      cout << "cp assigment" <<endl; 
      return *this; 
     } 
     Directory& operator=(Directory&& other){ 
      cout << "move assigment" <<endl; 
      return *this; 
     } 
}; 

主要

int main() 
{ 

    Directory* dir = new Directory("alex"); 
    Directory* dir2; 
    dir = dir2; 

    cout<<"done"<<endl; 
}; 

我想知道拷贝赋值和分配移动时调用。提前致谢。

Answer 1

我的第一个评论，我建议删除所有* s和new。

因此，主要功能变成：

int main() 
{ 
    Directory dir = Directory("alex"); 
    Directory dir2; 
    dir2 = dir; // <-- fixed, original was: dir = dir2; 

    cout<<"done"<<endl; 
    return 0; // <-- fixed, return is strictly recommended for every non-void function 
} 

编译...

错误：东西是错误的Directory dir = Directory("alex");（删除拷贝构造函数的使用）。使用由Directory("alex")创建的临时实例初始化dir。

这是很容易改变：

int main() 
{ 
    Directory dir("alex"); // <-- fixed: direct construction 
    Directory dir2; 
    dir2 = dir; 

    cout<<"done"<<endl; 
    return 0; 
} 

编译...

错误：东西是错误的Directory dir2;。

A yepp。您定义了构造函数Directory(string name);。这将禁止在此处需要的默认构造函数的自动创建。

我们要么可以在默认的构造函数添加到class Directory：

Directory() = default; 

，或者我们可以改善现有的非默认的构造函数，以便它可以被使用的默认构造函数：

Directory(string name = string()): name(name) { } 

整个来源：

#include <iostream> 
#include <string> 
using namespace std; 

class Directory{ 

    string name; 
public: 
     Directory(string name = string()):name(name) { 

     } 
     ~Directory() { 
      cout << "Deleting was called" <<endl; 

      } 

     Directory& operator=(Directory& other){ 
      cout << "cp assigment" <<endl; 
      return *this; 
     } 
     Directory& operator=(Directory&& other){ 
      cout << "move assigment" <<endl; 
      return *this; 
     } 
}; 

int main() { 
    //Directory dir = Directory("alex"); 
    Directory dir("alex"); 
    Directory dir2; 
    dir2 = dir; 

    cout<<"done"<<endl; 
    // your code goes here 
    return 0; 
} 

现在，它编译es和作品。

输出：

cp assigment 
done 
Deleting was called 
Deleting was called 

你可以看到它住在ideone。

Answer 2

也许尝试这样的：

#include <iostream> 
#include <string> 

using namespace std; 

class Directory{ 
public: 
     string name; 

     Directory() { 
      cout << "Constructor 1 was called" <<endl;    
     } 

     Directory(string name):name(name) { 
      cout << "Constructor 2 was called" <<endl;    
     } 

     ~Directory() { 
      cout << "Deleting was called" <<endl; 
     } 

     Directory(const Directory& other){ 
      cout << "cp cons" <<endl; 
     } 

     Directory& operator=(const Directory& other){ 
      cout << "cp assigment" <<endl; 
      return *this; 
     } 

     Directory& operator=(Directory&& other){ 
      cout << "move assigment" <<endl; 
      return *this; 
     } 
}; 


int main() 
{ 

    Directory dir = Directory("alex"); 
    Directory dir2; 
    dir2 = dir; 

    cout << "done " << dir.name << dir2.name << endl; 
}; 

我改变了代码，以便它不使用指针，增加额外的构造函数（注意拷贝构造函数），并增加了一些额外的印刷。

我得到这样的输出：

Constructor 2 was called 
Constructor 1 was called 
cp assigment 
done alex 
Deleting was called 
Deleting was called 

从这里就可以看到你的拷贝赋值是不正确的，它仍然打印“亚历克斯”，但我猜你是只在功能感兴趣被调用，而不是他做。