Function & Method的区别 为什么类中成员函数的指针需要显式地声明？

前些日子，在stack overflow上看到一个关于类中函数指针的问题：

There is a class

class A {
public:
    A() {};

private:
    void func1( int) {};
    void func2( int) {};
};

I want to add a function pointer which will be set in constructor and points to func1 or func2.

So I can call this pointer (as class member) from every class procedure and set this pointer in constructor.

How can I do it?

One Answer:

class A {
public:
    A(bool b) : func_ptr_(b ? &A::func1 : &A::func2) {};

    void func(int i) {(this->*func_ptr_)(i);}

private:
    typedef void (A::*func_ptr_t_)();
    func_ptr_t_ func_ptr_;

    void func1(int) {};
    void func2(int) {};
};

That said, polymorphism might be a better way to do whatever you want to do with this.

关于类中的函数指针，当时一直不能理解为什么成员函数的指针一定要有显示地（A：：func1）标识出来，成员函数与C里常用的函数有什么区别呢？

前些天，看了C++ Effective中关于成员函数里有个隐藏的this指针作为参数，我还是没把C的函数与类中成员函数的区别联系到函数指针用法上来。

直到今天，看了stanford的公开课programming paradigms的第五集，当jerry cain谈到lsearch这个function中的void* (*cmp)(void*, void*)必须得是真正的function，关于他如何说明这个真正，我贴上他的讲义：

When I say function, I’m talking about this object-oriented-less unit, which is just some 
block of code that gets called as a function that has no object or class declaration around 
it.  
When I’m talking about the type of number functions or functions that are inside classes, 
I don’t refer to them as functions, I refer to them as methods. The difference between a 
function and a method, they look very similar, except that methods actually have the 
address of the relevant object lying around as this invisible parameter via this invisible 
parameter called this.  
The type of function that gets passed right here has to be either a global function that has 
nothing to do with the class or it has to be a method inside a class that’s declared as static. 
Which means that it does not have any this pointer passed around on your behalf behind 
the scenes.