C++第六天笔记2016年02月23日（周二）A.M

Posted 2020-06-16

1.    赋值运算符的重载：

什么情况下需要赋值运算符的重载？

当类中有指向对空间的指针变量时，需要重载赋值运算符以实现深拷贝。

浅拷贝问题：

当类中有指向对空间的指针变量时，前拷贝有可能会导致二次删除或内存泄露问题。

重载赋值操作符分5步：a=a;

1.1  判断是否为自赋值à自赋值则直接跳至第5步。

1.2  删除旧的堆空间（指针指向的旧的堆空间）

1.3  申请新的堆空间

1.4  内容拷贝

1.5  return* this;

浅拷贝：

 1 #include <iostream>
 2 #include"cstring"
 3 using namespace std;
 4 # define MAX_CHAR 10
 5 
 6 class Account
 7 {
 8 public:
 9     friend ostream& operator<<(ostream& out,Account& a);
10     Account(char* name="unknown",char* mr="Miss",float y=0.0):title(new char[strlen(mr)+1]){
11         strcpy(title, mr);
12         strcpy(owner, name);
13         balance=y;
14     }
15     void changeTitle(char* newTitle)
16     {
17         if (strlen(newTitle)>strlen(title)) {
18             char* tmp=title;
19             title=new char[strlen(newTitle)+1];
20             strcpy(title, newTitle);
21             delete [] tmp;
22         }
23         else
24             strcpy(title, newTitle);
25     }
26     void changeName(char* newName)
27     {
28         strcpy(owner, newName);
29     }
30     ~Account()
31     {
32         delete [] title;
33     }
34 private:
35     char* title;
36     char owner[MAX_CHAR];
37     float balance;
38 };
39 ostream& operator<<(ostream& out,Account& b)
40 {
41     out<<"who:"<<b.title<<" "<<b.owner<<" how much="<<b.balance<<"\n";
42     out<<endl;
43     return out;
44 }
45 int main(int argc, const char * argv[]) {
46     Account acc("Lou","Mr",200);
47     Account acc1;
48     cout<<acc;cout<<acc1;
49     acc1=acc;
50     cout<<acc1;
51     acc1.changeTitle("jean");
52     cout<<acc1;cout<<acc;
53     acc1.changeName("Dr.");
54     cout<<acc1;cout<<acc;
55     return 0;
56 }

赋值运算符重载：

 1 #include <iostream>
 2 #include"cstring"
 3 using namespace std;
 4 
 5 class Vector{
 6 public:
 7     Vector(){}
 8     Vector(int s,int an_array[]);
 9     ~Vector(){delete [] rep;}
10     int get_size(){return size;}
11     int *get_rep(){return rep;}
12     const Vector& operator=(const Vector& x);
13     int & operator[](int index)const {return rep[index];}
14 private:
15     int* rep;
16     int size;
17 };
18 Vector::Vector(int s,int an_array[]):size(s),rep(new int[s])
19 {
20     for (int i=0; i<size; i++) {
21         rep[i]=an_array[i];
22     }
23 }
24 const Vector& Vector::operator=(const Vector& x)
25 {
26     if (this!=NULL) {
27         size=x.size;
28         delete [] rep;
29         rep=new int[size];
30         for (int i=0; i<size; i++) {
31             rep[i]=x[i];
32         }
33 //        if (rep!=x.rep) {
34 //            delete [] rep;
35 //            rep=new int[size];
36 //            for (int i=0; i<size; i++) {
37 //                rep[i]=x.rep[i];
38 //            }
39 //        }
40     }
41     return *this;
42 }
43 ostream & operator<<(ostream& out,Vector& a)
44 {
45     for (int i=0; i<a.get_size(); i++) {
46         cout<<a[i]<<" ";
47     }
48     cout<<endl;
49     return out;
50 }
51 int main(int argc, const char * argv[]) {
52     
53     int a[5]={54,2,4,6,5};
54     int b[3]={4,6,5};
55     Vector v1(5,a);
56     Vector v2(3,b);
57     cout<<v2;
58     v2=v1;
59     cout<<v1;
60     cout<<v2;
61     if (v1[0]==v2[0]) {
62         cout<<"v1与v2占用同一块堆空间";
63     }
64     else
65         cout<<"v1与v2占用的堆空间彼此独立"<<endl;
66     return 0;
67 }