36 C++ - 赋值(=)运算符重载

class Person{
  friend ostream& operator<<(ostream& os,const Person& person){
    os << "ID:" << person.mID << " Age:" << person.mAge << endl;
    return os;
  }
public:
  Person(int id,int age){
    this->mID = id;
    this->mAge = age;
  }
  //重载赋值运算符
  Person& operator=(const Person& person){
    this->mID = person.mID;
    this->mAge = person.mAge;
    return *this;
  }
private:
  int mID;
  int mAge;
};
//1. =号混淆的地方
void test01(){
  Person person1(10, 20);
  Person person2 = person1; //调用拷贝构造
  //如果一个对象还没有被创建，则必须初始化，也就是调用构造函数
  //上述例子由于person2还没有初始化，所以会调用构造函数
  //由于person2是从已有的person1来创建的，所以只有一个选择
  //就是调用拷贝构造函数
  person2 = person1; //调用operator=函数
  //由于person2已经创建，不需要再调用构造函数，这时候调用的是重载的赋值运算符
}
//2. 赋值重载案例
void test02(){
  Person person1(20, 20);
  Person person2(30, 30);
  cout << "person1:" << person1;
  cout << "person2:" << person2;
  person2 = person1;
  cout << "person2:" << person2;
}
//常见错误，当准备给两个相同对象赋值时，应该首先检查一下这个对象是否对自身赋值了
//对于本例来讲，无论如何执行这些赋值运算都是无害的，但如果对类的实现进行修改，那么将会出现差异；
//3. 类中指针
class Person2{
  friend ostream& operator<<(ostream& os, const Person2& person){
    os << "Name:" << person.pName << " ID:" << person.mID << " Age:" << person.mAge << endl;
    return os;
  }
public:
  Person2(char* name,int id, int age){
    this->pName = new char[strlen(name) + 1];
    strcpy(this->pName, name);
    this->mID = id;
    this->mAge = age;
  }
#if 1
  //重载赋值运算符
  Person2& operator=(const Person2& person){
    //注意:由于当前对象已经创建完毕，那么就有可能pName指向堆内存
    //这个时候如果直接赋值，会导致内存没有及时释放
    if (this->pName != NULL){
      delete[] this->pName;
    }
    this->pName = new char[strlen(person.pName) + 1];
    strcpy(this->pName,person.pName);
    this->mID = person.mID;
    this->mAge = person.mAge;
    return *this;
  }
#endif
  //析构函数
  ~Person2(){
    if (this->pName != NULL){
      delete[] this->pName;
    }
  }
private:
  char* pName;
  int mID;
  int mAge;
};
void test03(){
  Person2 person1("John",20, 20);
  Person2 person2("Edward",30, 30);
  cout << "person1:" << person1;
  cout << "person2:" << person2;
  person2 = person1;
  cout << "person2:" << person2;
}