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：

#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;　　　　　　　　//在此删除delete动作
p=NULL;

list<emp*>::iterator q = link.begin();

while(q!=link.end())
{
(*q)->print();                 q++;
}

return 0;
}

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：

#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);
p=NULL;

list<emp*>::iterator q = link.begin();

while(q!=link.end())
{
(*q)->print();
cout<<*q<<endl;
q++;
}

return 0;
}

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？