发布时间：2011-12-6 17:06
分类名称：C++
The Semantics of Function
Member 的各种调用方式
Nonstatic Member Functions
obj.magnitude(); 转换为：magnitude_7Point3dFv( register Point3d * const this);
this用来函数内部使用。关于函数名字变形和处理问题，见本书内容。不做详细说明。
Virtual Member Functions
ptr->normalize(); 转化为：( *(ptr->_vptr[1]) )(ptr);
看看，这就是所谓的"运行时"绑定。 _vptr[1]，也就是虚函数表中的第一个函数地址，会随着程序代码的不同情况，有所变化。例如，ptr是一个Derived Class，那么这个位置，很有可能被Derived Class 替换掉。那么这个调用就会去调用子类的虚函数，而不是基类的。
一个class只会有一个virtual table，每一table内含有其对应的class object中所有的active virtual functions的实际地址，这些函数包括：
单一继承下的Virtual functions
class Point
{
public:
virtual ~Point();
virtual Point &mult(float) = 0;
// ... ...
float x() const { return _x;}
virtual float y() const {return 0;}
virtual float z() const {return 0;}
// ... ...
protected:
Point(float x = 0.0);
float _x;
};
class Point2d : public Point
{
public:
Point2d(float x = 0.0, float y = 0.0) : Point(x), _y(y) { }
~Point2d();
// override base class virtual functions
Point2d& mult(float);
float y() const {return _y};
protected:
float _y;
};
class Point3d : public Point2d
{
public:
Point3d(float x = 0.0, float y = 0.0, float z = 0.0)
: Point2d(x, y), _z(z) { }
~Point3d();
// override base class virtual functions
Point3d& mult(float);
float z() const {return _z};
protected:
float _z;
};
要说明的是，以上三张表，在运行的时候实际上都是存在的。子类只是拷贝了父类的表格，然后进行改写。而不是直接拿来父类的表格在上面写写画画。为什么，待会儿我会摘取《ATL Under the hood》中的一节来说明。
多继承下的Virtual Functions
多重继承的问题在于Dervied class和第二个和其后继的base classes的this指针调节上。
class Base1
{
public:
Base1();
virtual ~Base1();
virtual void speakClearly();
virtual Base1 *clone() const;
protected:
float data_Base1;
};
class Base2
{
public:
Base2();
virtual ~Base2();
virtual void mumble();
virtual Base2 *clone() const;
protected:
float data_Base2;
};
class Derived : public Base1, public Base2
{
public:
Derived();
virtual ~Derived();
virtual Derived *clone() const;
protected:
float data_Derived;
};
Base2 *pBase2 = new Derived; ==>>转换为：
Derived *pTemp = new Derived;
Base2* pBase2 = pTemp ? (Base2 *)((char *)pTemp + sizeof(Base1)) : 0;
当要执行: delete pBase2的时候，需要再次调整回，以便指到对象的起始点。然而，这个offset在编译时刻无法确定。所以调整需要在执行期间来进行。比较有效率的做法是使用Thunk技术（就是插入一小段汇编）。看看VS2010的Derived布局：
+---
| +--- (base class Base1)
0 | | {vfptr}
4 | | data_Base1
| +---
| +--- (base class Base2)
8 | | {vfptr}
12 | | data_Base2
| +---
16 | data_Derived
+---
Derived::$vftable@Base1@:
| &Derived_meta
| 0
0 | &Derived::{dtor}
1 | &Base1::speakClearly
2 | &Derived::clone
Derived::$vftable@Base2@:
| -8
0 | &thunk: this-=8; goto Derived::{dtor} ;看到没，这里有几个thunk字样，后面调整this，然后调用子类函数。
1 | &Base2::mumble
2 | &thunk: this-=8; goto Base2* Derived::clone
3 | &thunk: this-=8; goto Derived* Derived::clone
Derived::{dtor} this adjustor: 0
Derived::clone this adjustor: 0
Derived::__delDtor this adjustor: 0
Derived::__vecDelDtor this adjustor: 0
、
虚拟继承下的Virtual Functions
class Point2d
{
public:
Point2d(float x = 0.0, float y = 0.0){ }
virtual ~Point2d();
// override base class virtual functions
virtual void mumble();
virtual float z();
protected:
float _x, _y;
};
class Point3d : virtual public Point2d
{
public:
Point3d(float x = 0.0, float y = 0.0, float z = 0.0)
: Point2d(x, y), _z(z) { }
~Point3d();
// override base class virtual functions
Point3d& mult(float);
float z() const {return _z;}
protected:
float _z;
};
微软编译器布局如下：
class Point3d size(20):
+---
0 | {vbptr}
4 | _z
+---
+--- (virtual base Point2d)
8 | {vfptr}
12 | _x
16 | _y
+---
Point3d::$vbtable@:
0 | 0
1 | 8 (Point3dd(Point3d+0)Point2d)
Point3d::$vftable@:
| -8
0 | &Point3d::{dtor}
1 | &Point2d::mumble
2 | &Point2d::z
Point3d::{dtor} this adjustor: 8
Point3d::__delDtor this adjustor: 8
Point3d::__vecDelDtor this adjustor: 8
vbi: class offset o.vbptr o.vbte fVtorDisp
Point2d 8 0 4 0
不要在virtual base clas中声明nonstatic data member。为什么？你可以自己试试。
Static Member Functions
没有啥好说的，Static Member Functions就和一般的普通函数(非成员函数)一样，该怎么调用，就怎么调用。这种函数是憋屈了一段时间才被推出的，以前有种奇怪的调用方法：((Point3d *)0)->object_count(); 在引入static member functions之前，C++语言要求所有的member functions都必须由该class的object调用，所有就出现了上述调用方法。