关于空指针类型的类变量的成员函数访问的问题 - Linux安全网 - Linux操作系统_Linux 命令_Linux教程

下面代码会有什么输出?
#i<a href='http://www.linuxso.com/command/nc.html' target='_blank'><u>nc</u></a>lude <ios<a href='http://www.linuxso.com/command/tr.html' target='_blank'><u>tr</u></a>eam>
using namespace std;

class A
{
public:
	vo<a href='http://www.linuxso.com/command/id.html' target='_blank'><u>id</u></a> fun()
	{
		cout<<"fun"<<endl;
	}
	virtual void vfun()
	{
		cout<<"vfun"<<endl;
	}
private:
	int a;
};

int main()
{
	A *a = NULL;
	a->fun();
	a->vfun();
	system("pause");
	return 0;
}

</pre></blockquote>
<p><br>
fun输出fun,</p>
<p>vfunc爆异常错误</p>
<p>---------------------------------------------</p>
<p>为什么有这样的结果？</p>
<ol>
<li> 从c++ 对象模型中知道，类的非虚成员函数和普通c的成员函数没有什么区别，而数据成员是每个类的实例化对象独享，虚函数会放在一个vptr指针指向的表，那个表存有函数的入口地址.</li></ol>
<p>            比如 上述的func() 可能会有这个形式：</p>
<p>                          void func(A* this)</p>
<p>                           {}</p>
<p>    2.   在编译的时候,func这个函数直接会被汇编到t<a href='http://www.linuxso.com/command/ex.html' target='_blank'><u>ex</u></a>t段里面，加载到内存后，其地址及入场等等信息都是完整存在的.</p>
<p>           我们在main你们调用这个函数的这句代码会直接引用到text段里面函数对于的地址，这个值可以说是死的。所以对象即便为空也是没关系的.</p>
<p>    3.虚函数的地址虽然也和一般函数一样在内存中加载了，但是虚函数不是在编译时刻求值的，而是动态时刻。这个动态时刻的概念就是，作为入参或者类的继承类型的实例化地址对应的具体函数的地址.通过vi<a href='http://www.linuxso.com/command/su.html' target='_blank'><u>su</u></a>al studio或gcc反汇编代码，会发现往往都是在类对象实例化的时候，在构造函数里面插入的。这个地方 a这个指针显然没有new从而没有调用构造函数，所以看其vptr指针(vs貌似叫做vftable)可以肯定地址是空的.所以会爆错！</p>
<p>  4.如果a作为栈变量或者堆类型new出肯定都是不会爆错!</p>
<p> </p>
<p> </p>
                
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