1 #include <iostream>
 2 
 3 class My1{
 4     public:
 5         My1();
 6         ~My1();
 7 };
 8 
 9 class My2{
10     public:
11         My2();
12         ~My2();
13     My1 mymm;
14 };
15 
16 class My3{
17     public:
18         My3();
19         ~My3();
20     My1 mymm;
21 };
22 
23 My1::My1(){
24     std::cout<<"my1gouzao"<<std::endl;
25 }
26 My1::~My1(){
27     std::cout<<"my1xigou"<<std::endl;
28 }
29 My2::My2(){
30     std::cout<<"my2gouzao"<<std::endl;
31 }
32 My2::~My2(){
33     std::cout<<"my2xigou"<<std::endl;
34 }
35 My3::My3(){
36     std::cout<<"my3gouzao"<<std::endl;
37 }
38 My3::~My3(){
39     std::cout<<"my3xigou"<<std::endl;
40 }
41 
42 int main(){
43     My2 mym2;
44     My3 mym3;
45 }

例1：嵌套类的构造与析构

例2：在一个类中创建另一个类的实例（直接访问）

 1 #include <iostream>
 2 using namespace std;
 3 class My1{
 4     public:
 5         My1();
 6         ~My1();
 7     int i;
 8 };
 9 
10 class My2{
11     public:
12         My2();
13         ~My2();
14     void say();
15     My1 my1;    
16 };
17 
18 My1::My1(){
19     cout<<"my1gouzao"<<endl;
20 }
21 void My2::say(){
22     my1.i = 200;
23     cout<<my1.i<<endl;
24 }
25 My1::~My1(){
26     cout<<"my1xigou"<<endl;
27 }
28 My2::My2(){
29     cout<<"my2gouzao"<<endl;
30 }
31 My2::~My2(){
32     cout<<"my2xigou"<<endl;
33 }
34 
35 int main(){
36     My2 my2;
37     my2.say();
38 }

 例3：在一个类中创建另一个类的实例（接口访问）

 1 #include <iostream>
 2 using namespace std;
 3 class My1{
 4     public:
 5     My1();
 6     ~My1();
 7     void setage(int age);
 8     void show();
 9     private:
10     int m_age;
11 };
12 
13 class My2{
14     public:
15     My2();
16     ~My2();
17     My1 my1;
18     void say();
19 };
20 
21 My1::My1(){
22     cout<<"my1gouzao"<<endl;
23 }
24 My1::~My1(){
25     cout<<"my1xigou"<<endl;
26 }
27 void My1::setage(int age){
28     m_age=age;
29 }
30 void My1::show(){
31     cout<<"我今年"<<m_age<<"岁"<<endl;
32 }
33 My2::My2(){
34     cout<<"my2gouzao"<<endl;
35 }
36 My2::~My2(){
37     cout<<"my2xigou"<<endl;
38 }
39 void My2::say(){
40     my1.setage(2);
41     my1.show();
42 }
43 
44 int main(){
45     My1 my1;
46     My2 my2;
47     my1.setage(20);
48     my1.show();
49     my2.say();
50     my1.show();
51 }

这里用到了私有属性和封装。在My2中定义My1的实例，My1 my1和My2 my2中的my1是不同的，通过my2不能直接修改My1 my1属性的值

参考：

https://www.cnblogs.com/prettyshuang/p/5427689.html

https://www.cnblogs.com/haoyul/p/7287681.html

https://www.cnblogs.com/ljygoodgoodstudydaydayup/p/3854914.html