&lt;三&gt;對象的淺拷貝和深拷貝問題

來源：本站原創(chuàng)|時間：2023-04-02|欄目：C++|點擊：次

先看示例代碼

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

class Student{

public:

    Student(int _age , const char * _name)
    {
         
         this->age=_age;

         int len=strlen(_name)+1;

         char *tep=new char[len];

         this->pName=tep;

         strcpy(this->pName,_name);

    }

    ~Student(){
        
        delete[]this->pName;
        this->pName=nullptr;
       
    }

    void showStudent(){
        cout<<this->pName<<" "<<this->age<<endl;
    }

 private:
    int  age;
    char *pName;  
};


int main(){

   Student s1(20,"zhangsan");
   s1.showStudent();

   Student s2=s1;
  
   s2.showStudent();

   return 1;
}

上面示例代碼中,對象的默認拷貝方式是內(nèi)存數(shù)據(jù)拷貝,如果對象占用了外部資源，那么就會出現(xiàn)問題了,這里的外部資源
就是在堆上申請的空間存放名字用,
s1,s2兩個對象中的名字指針都是指向了同一塊堆內(nèi)存區(qū)域,在結(jié)束main函數(shù)的是,s2先析構(gòu),s2會析構(gòu)調(diào)堆上的內(nèi)存空間,
s1再析構(gòu),由于s1對象的pName 指針和s2對象的pName指針指向的是同一塊堆內(nèi)存區(qū)域,但是該區(qū)域內(nèi)存已經(jīng)被釋放了,所以遇到了問題

鑒于上面的問題,我們引出了淺拷貝,深拷貝的問題和解決方法.

Student s2=s1; 會調(diào)用拷貝構(gòu)造函數(shù)(該拷貝構(gòu)造函數(shù)可以是系統(tǒng)自動生成的,或者你自己定義一個拷貝構(gòu)造函數(shù))
系統(tǒng)自動生成的拷貝構(gòu)造函數(shù)做的是內(nèi)存數(shù)據(jù)拷貝,所以就出現(xiàn)了上面的問題.因此我們需要定義屬于自己的拷貝構(gòu)造函數(shù)

改造代碼如下

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class Student2{

public:

    Student2(int _age , const char * _name)
    {        
         this->age=_age;
         int len=strlen(_name)+1;
         char *tep=new char[len];   
         this->pName=tep;
         strcpy(this->pName,_name);
         cout<<"執(zhí)行構(gòu)造函數(shù)"<<endl;
    }

    //自定義拷貝構(gòu)造函數(shù)
    Student2(const Student2 &_stu2){

        this->age=_stu2.age;    
        int len =strlen(_stu2.getPName())+1;
        char *newPName =new char[len];
        this->pName=newPName;
        strcpy(this->pName,_stu2.getPName());
        cout<<"執(zhí)行拷貝構(gòu)造函數(shù)"<<endl;

    }

    ~Student2(){
        if(this->pName!=nullptr){
            delete[]this->pName;
            this->pName=nullptr;
        }
    }

    void showStudent(){

        int *p=(int *)this->pName;
        cout<<this->pName<<" "<<this->age<<" pName Heap Address ="<<p<<endl;

    }

    const char * getPName() const{
         return this->pName;
    }

 private:
    int  age;
    char *pName;  

};




int main(){

   Student2 s1(20,"zhangsan");
   
   s1.showStudent();

   Student2 s2=s1;

   s2.showStudent(); 


   return 1;
}

上面的代碼還存在一個問題如下，如果在main函數(shù)中是下面的代碼

Student2 s1(20,"zhangsan");

Student2 s2(30,"lisi");

s2=s1;

在main函數(shù)結(jié)束的是同樣會遇到問題,這個時候就引出賦值函數(shù)，我們需要定義自己的賦值函數(shù)

在 s2=s1這一段代碼其實是這樣調(diào)用的
s2.operator=(s1)

在你不定義自己的賦值函數(shù)的時候,系統(tǒng)會幫我們生成一個賦值函數(shù),該賦值函數(shù)的賦值方式就是內(nèi)存的數(shù)值拷貝,這種方式
在Student對象,會有問題,所以我們需要向自定義拷貝構(gòu)造一樣,定義一個屬于自己的賦值函數(shù)

代碼如下

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class Student3{

public:

    Student3(int _age , const char * _name)
    {        
         this->age=_age;
         int len=strlen(_name)+1;
         char *tep=new char[len];   
         this->pName=tep;
         strcpy(this->pName,_name);
         cout<<"執(zhí)行構(gòu)造函數(shù)"<<endl;
    }

    //自定義拷貝構(gòu)造函數(shù)
    Student3(const Student3 & _stu){

        this->age=_stu.age;    
        int len =strlen(_stu.getPName())+1;
        char *newPName =new char[len];
        this->pName=newPName;
        strcpy(this->pName,_stu.getPName());
        cout<<"執(zhí)行拷貝構(gòu)造函數(shù)"<<endl;

    }

    //自定義賦值函數(shù)
    Student3 & operator= (const Student3 & _stu){

        //防止自賦值
        if(this==&_stu){return *this;}

        //注意: 需要先釋放當(dāng)前的堆內(nèi)存空間??！
        delete []this->pName;

        this->age=_stu.age;    
        int len =strlen(_stu.getPName())+1;
        char *newPName =new char[len];
        this->pName=newPName;
        strcpy(this->pName,_stu.getPName());
        cout<<"執(zhí)行賦值函數(shù)"<<endl;
        return *this;
    }

    ~Student3(){
        if(this->pName!=nullptr){
            delete[]this->pName;
            this->pName=nullptr;
        }
    }

    void showStudent(){

        int *p=(int *)this->pName;
        cout<<this->pName<<" "<<this->age<<" pName Heap Address ="<<p<<endl;

    }

    const char * getPName() const{
         return this->pName;
    }

 private:
    int  age;
    char *pName;  

};

int main(){

   Student3 s3(20,"zhangsan");
   
   s3.showStudent();

   Student3 s4(30,"lisi");

   s4=s3;
   
   s4.showStudent();

   return 1;
}

運用深拷貝淺拷貝實現(xiàn)String 代碼如下

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

class MyString{

public:

     //構(gòu)造函數(shù)
     MyString(const char * src){

        //創(chuàng)建空串
        if(src==nullptr){
            this->pchar=new char[1];
            this->pchar[0]='\0';
        }
        else{
            int len =strlen(src)+1;
            this->pchar=new char[len];
            strcpy(this->pchar,src);
        }
        cout<<"執(zhí)行構(gòu)造函數(shù), 堆內(nèi)存空間地址"<<(int *)this->pchar <<endl;

     }

     //拷貝構(gòu)造
     MyString(const MyString & myString){
      
        //重新分配內(nèi)存空間 
        int len =myString.stringLen()+1;
        this->pchar=new char[len];
        strcpy(this->pchar,myString.pchar);  
        cout<<"執(zhí)行拷貝構(gòu)造函數(shù), 新創(chuàng)建堆內(nèi)存空間地址"<<(int *)this->pchar <<endl;
 
     }

     //賦值函數(shù)
     MyString & operator=(const MyString & myString){

         //防止自賦值
         if(this==&myString){return *this;}
         //釋放現(xiàn)有的堆內(nèi)存空間
         delete[]this->pchar;
         this->pchar=nullptr;

        int len =myString.stringLen()+1;

        this->pchar=new char[len];
        strcpy(this->pchar,myString.pchar);
        
        cout<<"執(zhí)行賦值函數(shù), 新創(chuàng)建堆內(nèi)存空間地址"<<(int *)this->pchar <<endl;

        return *this;

     }

     int stringLen() const {     
         return strlen(this->pchar);
     }
    
     //析構(gòu)
     ~MyString(){
         delete [] this->pchar;
         this->pchar=nullptr;
     }

     void printChar() const{      
         cout<<this->pchar<<endl;
     }

private:
    char *pchar;
   

};


int main(){

    MyString s1("abcd");
    s1.printChar();

    MyString s2=s1;//執(zhí)行拷貝構(gòu)造
    s2.printChar();

    MyString s3("1234");
    s3=s1;//執(zhí)行賦值函數(shù)
    s3.printChar();

    
    return 1;
}