c++中的(static) type和 (dynamic) type 概念是基于多態(tài)（polymorphism） ，例如：Vehicle*指針如果實際是指向一個Car對象，那么這個指針的靜態(tài)類型就是Vechicle，Car則是他的動態(tài)類型。靜態(tài)類型發(fā)生在編譯器編譯時，動態(tài)類型發(fā)生在動態(tài)綁定時。
我們常說的override（覆蓋）就是針對虛函數(shù)而言。
對虛函數(shù)的實現(xiàn)應(yīng)該說各個編譯器是不一樣的，大多數(shù)的編譯器是這樣的：
為每一個有虛函數(shù)的類增加一個虛表，這個虛表是靜態(tài)的，還有一個虛指針，為每個類對象。例如：
// Your original C++ source code
class Base {
public:
virtual arbitrary_return_type virt0(...arbitrary params...);
virtual arbitrary_return_type virt1(...arbitrary params...);
virtual arbitrary_return_type virt2(...arbitrary params...);
virtual arbitrary_return_type virt3(...arbitrary params...);
virtual arbitrary_return_type virt4(...arbitrary params...);
...
};
1 編譯器會為這個類的虛函數(shù)添加一個虛表，類似下面的：
// Pseudo-code (not C++, not C) for a static table defined within file Base.cpp
// Pretend FunctionPtr is a generic pointer to a generic member function
// (Remember: this is pseudo-code, not C++ code)
FunctionPtr Base::__vtable[5] = {
&Base::virt0, &Base::virt1, &Base::virt2, &Base::virt3, &Base::virt4
};
2 然后增加一個指向虛表的指針為每一個類對象，這個指針是隱藏的
// Your original C++ source code
class Base {
public:
...
FunctionPtr* __vptr; ← supplied by the compiler, hidden from the programmer
...
};
3 編譯器在構(gòu)造中初始化這個指針
Base::Base(...arbitrary params...)
: __vptr(&Base::__vtable[0]) ← supplied by the compiler, hidden from the programmer
...
{
...
}
在派生類中，它也會增加一個隱藏的虛表，但是它可以overrides基類的虛函數(shù)如：
// Pseudo-code (not C++, not C) for a static table defined within file Der.cpp
// Pretend FunctionPtr is a generic pointer to a generic member function
// (Remember: this is pseudo-code, not C++ code)
FunctionPtr Der::__vtable[5] = {
&Der::virt0, &Der::virt1, &Der::virt2, &Base::virt3, &Base::virt4
};
最后看看底層是如何調(diào)用的如：
void mycode(Base* p)
{
p->virt3();
}
主要三部分：
1.獲取隱藏的指向虛表的指針，并把它放在 register中如r1；
2.獲取指針r2=r1+3*4（假定一個指針有四個字節(jié)） ，并把它放到register中。
3 根據(jù)r2的地址調(diào)用函數(shù)。
所以說，調(diào)用一個虛函數(shù)至少和非虛函數(shù)差不多.
在這里我們可以看出一個虛指針的長度，至少是四個字節(jié)，但是要注意編譯器對它的具體實現(xiàn)。
純虛函數(shù)怎樣用，下面的例子可以說明這個問題。
象下面的代碼就可以用純虛函數(shù)來實現(xiàn)：
typedef std::vector<Vehicle*> VehicleList;
void myCode(VehicleList& v)
{
for (VehicleList::iterator p = v.begin(); p != v.end(); ++p) {
Vehicle& v = **p; // just for shorthand
// generic code that works for any vehicle...
...
// perform the "foo-bar" operation.
// note: the details of the "foo-bar" operation depend
// on whether we're working with a car or a truck.
if (v is a Car) {
// car-specific code that does "foo-bar" on car v
...
} else if (v is a Truck) {
// truck-specific code that does "foo-bar" on truck v
...
} else {
// semi-generic code that does "foo-bar" on something else
...
}
// generic code that works for any vehicle...
...
}
}
用純虛函數(shù)實現(xiàn)如下：
class Vehicle {
public:
// performs the "foo-bar" operation
virtual void fooBar() = 0;
};
typedef std::vector<Vehicle*> VehicleList;
void myCode(VehicleList& v)
{
for (VehicleList::iterator p = v.begin(); p != v.end(); ++p) {
Vehicle& v = **p; // just for shorthand
// generic code that works for any vehicle...
...
// perform the "foo-bar" operation.
v.fooBar();
// generic code that works for any vehicle...
...
}
}
也可以用繼承的方法來實現(xiàn)
class Car : public Vehicle {
public:
virtual void fooBar();
};
void Car::fooBar()
{
// car-specific code that does "foo-bar" on 'this'
... ← this is the code that was in {...} of if (v is a Car)
}
class Truck : public Vehicle {
public:
virtual void fooBar();
};
void Truck::fooBar()
{
// truck-specific code that does "foo-bar" on 'this'
... ← this is the code that was in {...} of if (v is a Truck)
}
有純虛函數(shù)的是抽象基類，強(qiáng)迫派生類接受基類的接口并實現(xiàn)，在COM（組件）中比較常見，。
virtual constructor（虛構(gòu)造）的一個實現(xiàn)方法之一：
class Shape {
public:
virtual ~Shape() { } // A virtual destructor
virtual void draw() = 0; // A pure virtual function
virtual void move() = 0;
...
virtual Shape* clone() const = 0; // Uses the copy constructor
virtual Shape* create() const = 0; // Uses the default constructor
};
class Circle : public Shape {
public:
Circle* clone() const; // Covariant Return Types; see below
Circle* create() const; // Covariant Return Types; see below
...
};
Circle* Circle::clone() const { return new Circle(*this); }
Circle* Circle::create() const { return new Circle(); }
void userCode(Shape& s)
{
Shape* s2 = s.clone();
Shape* s3 = s.create();
...
delete s2; // You need a virtual destructor here
delete s3;
}
注意在VC6中必須寫成Shape*，VC7就不用改，支持返回類型可以變。