Effective C++ Item 12 复制对象时勿忘其每一个成分

本文为senlie原创，转载请保留此地址：http://blog.csdn.net/zhengsenlie

经验1：copying函数应该确保复制“对象内的所有成员变量”及”所有base class成分“。

示例：没对base class成分赋值

<pre name="code" class="cpp">#include <iostream>
#include <string>
using namespace std;

class Customer{
public:
Customer(){
}
Customer(string n):name(n){}
Customer(const Customer &rhs) : name(rhs.name){
}
Customer &operator=(const Customer &rhs){
name = rhs.name;
return *this;
}
string getCustomer() const{return name;}
private:
std::string name;
};

class PriorityCustomer: public Customer{
public:
PriorityCustomer(int p, string n):Customer(n), priority(p){}
PriorityCustomer(const PriorityCustomer &rhs): priority(rhs.priority){//derived class为调用base class的默认构造函数初始化
}
PriorityCustomer &operator=(const PriorityCustomer &rhs){
//没对base class成分进行赋值
priority = rhs.priority;
return *this;
}
int getPriority() const	{return priority;}
string getCustermerName() const{return getCustermerName();}
private:
int priority;
};

int main(){
PriorityCustomer pc1(1, "PriorityCustomer 1");
PriorityCustomer pc2(pc1);
PriorityCustomer pc3;
pc3 = pc1;
cout << pc2.getPriority() << endl
<< pc3.getPriority() << endl;
cout << pc2.getCustomerName() << endl
<< pc3.getCustomerName() << endl;
}

输出：

1

1

(空)

(空)

解析：

PriorityCustomer的copy构造函数和copyassignment函数没有指定参数传给它的base class构造函数，因此PriorityCustomer对象的Customer成分会被Customer的默认构造函数初始化，所以它的name成员变量为被初始化为空字符串

纠正：对base class成分赋值

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

class Customer{
public:
Customer(){
}
Customer(string n):name(n){}
Customer(const Customer &rhs) : name(rhs.name){
}
Customer &operator=(const Customer &rhs){
name = rhs.name;
return *this;
}
string getName() const{return name;}
private:
std::string name;
};

class PriorityCustomer: public Customer{
public:
PriorityCustomer(){}
PriorityCustomer(int p, string n):Customer(n), priority(p){}
PriorityCustomer(const PriorityCustomer &rhs): Customer(rhs),priority(rhs.priority){//调用base class的copy构造函数
}
PriorityCustomer &operator=(const PriorityCustomer &rhs){
Customer::operator=(rhs); //对base class成分进行赋值动作
priority = rhs.priority;
return *this;
}
int getPriority() const	{return priority;}
string getCustomerName() const{return getName();}
private:
int priority;
};

int main(){
PriorityCustomer pc1(1, "PriorityCustomer 1");
PriorityCustomer pc2(pc1);
PriorityCustomer pc3;
pc3 = pc1;
cout << pc2.getPriority() << endl
<< pc3.getPriority() << endl;
cout << pc2.getCustomerName() << endl
<< pc3.getCustomerName() << endl;

system("pause");
}

输出：

1

1

PriorityCustomer 1

PriorityCustomer 1
解析：

当你编写一个copying函数（包括copyconstructor和copy assignment）时，请确保（1）复制所有local成员变量，（2）调用所有base classes内的适当的copying函数

经验2：不要尝试以某个copying函数实现另一个copying函数。应该将共同机能放进第三个函数中，并由两个copying函数共同调用。