从一个小程序明白new和delete的奇特现象

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

class emp
{
		protected:
		string name;
		public:
		emp(){}
		virtual ~emp(){}
		virtual void print() = 0;

};

class empch:public emp
{

		public:
		empch(const string& str)
		{
				name = str;
		}
		void print()
		{
					cout<<name<<endl;
		}

};

int main()
{
		list<emp*> link;
		emp* p=NULL;
		string str = "nihao";

		p=new empch(str);
		link.push_back(p);
		delete p;
		p=NULL;

		p=new empch(str);
		link.push_back(p);
		delete p;
		p=NULL;
		p=new empch(str);
		link.push_back(p);
		delete p;
		p=NULL;
		list<emp*>::iterator q = link.begin();
		while(q!=link.end())　　　　　　　　　　//执行到此会出现段错误
		{
				(*q)->print();
				q++;
		}		

		return 0;
}

删除最后一个p，运行：

 1 #include <iostream>
 2 #include <list>
 3 using namespace std;
 4
 5 class emp
 6 {
 7         protected:
 8         string name;
 9         public:
10         emp(){}
11         virtual ~emp(){}
12         virtual void print() = 0;
13
14 };
15
16 class empch:public emp
17 {
18
19         public:
20         empch(const string& str)
21         {
22                 name = str;
23         }
24         void print()
25         {
26                     cout<<name<<endl;
27         }
28
29 };
30
31 int main()
32 {
33         list<emp*> link;
34         emp* p=NULL;
35         string str = "nihao";
36
37         p=new empch(str);
38         link.push_back(p);
39         delete p;
40         p=NULL;
41
42         p=new empch(str);
43         link.push_back(p);
44         delete p;
45         p=NULL;
46
47         p=new empch(str);
48         link.push_back(p);
49     //    delete p;　　　　　　　　//在此删除delete动作
50         p=NULL;
51
52         list<emp*>::iterator q = link.begin();
53
54         while(q!=link.end())
55         {
56                 (*q)->print();58                 q++;
59         }
60
61         return 0;
62 }

此时输出：

nihao
nihao
nihao

------------------
(program exited with code: 0)
Press return to continue

修改程序，将容器中保存的各个地址输出：

 1 #include <iostream>
 2 #include <list>
 3 using namespace std;
 4
 5 class emp
 6 {
 7         protected:
 8         string name;
 9         public:
10         emp(){}
11         virtual ~emp(){}
12         virtual void print() = 0;
13
14 };
15
16 class empch:public emp
17 {
18
19         public:
20         empch(const string& str)
21         {
22                 name = str;
23         }
24         void print()
25         {
26                     cout<<name<<endl;
27         }
28
29 };
30
31 int main()
32 {
33         list<emp*> link;
34         emp* p=NULL;
35         string str = "nihao";
36
37         p=new empch(str);
38         link.push_back(p);
39         delete p;
40         p=NULL;
41
42         p=new empch(str);
43         link.push_back(p);
44         delete p;
45         p=NULL;
46
47         p=new empch(str);
48         link.push_back(p);
49         p=NULL;
50
51         list<emp*>::iterator q = link.begin();
52
53         while(q!=link.end())
54         {
55                 (*q)->print();
56                 cout<<*q<<endl;
57                 q++;
58         }
59
60         return 0;
61 }

输出如下：

nihao
0x9ff4020
nihao
0x9ff4020
nihao
0x9ff4020

------------------
(program exited with code: 0)
Press return to continue

此时发现每个地址都是0x9ff4020，至此，我们应该就明白为什么会发生这样奇怪的情况了。那么到这里，问题又来了，为什么会出现这么个诡异的现象呢？