virtual

　　尽管在派生列表中同一个基类只能出现一次，但实际上派生类可以多次继承同一个类。派生类可以通过它的两个直接基类分别继承同一个间接基类，也可以直接继承某个基类，然后通过另一个基类再一次间接继承该类。 　　举个例子，IO标准库的istream和ostream分别继承了一个共同的名为base_ios的抽象基类。该抽象基类负责保存流的缓冲内容并管理流的条件状态。iostream是另外一个类，它从istream和ostream直接继承而来，可以同时读写流的内容。因为istream和ostream都继承自base_ios，所以iostream继承了base_is两次，一次是通过istream，另一次是通过ostream。 　　在默认情况下，派生类中含有继承链上每个类对应的子部分。如果某个类在派生过程中出现了多次，则派生类中将包含该类的多个子对象。 　　这种默认的情况对某些形如iostream的类显然是行不通的。一个iostream对象肯定是希望在同一个缓冲区进行读写操作，也会要求条件状态能同时反映输入和输出操作的情况。假如在iostram对象中真的包含了base_ios的两份拷贝，则上述的共享行为就无法实现而了。 　　 在C++语言中我们通过虚继承（virtual inheritance）的机制解决上述问题。虚继承的目的是令某个类做出声明，承诺愿意共享它的基类。其中，共享的基类子对象成为虚基类

Can we make a class copy constructor virtual in C++? How to use? No you can't, constructors can't be virtual. C++03 - 12.1 Constructors 4) A constructor shall not be virtual (10.3) or static (9.4). [...] If you need something like this, you can look up the virtual constructor idiom here . No you cannot. Furthermore, the whole concept does not make sense. Virtual functions are functions that are dispatched based on the value of an object (the dynamic type of the object). When a constructor is called, the object does not yet have a value (because it has not yet been constructed). Therefore, no

为什么析构函数要声明成virtual呢？ 因为，如果delete一个基类的指针时， 如果它指向的是一个子类的对象，那么析构函数不为虚就会导致无法调用子类析构函数，从而导致资源泄露。 如果一个类要被使用成 多态 的，那么这个 virtual是必须 的。比如： #include <iostream> using namespace std; class Animal { char* ap; public: Animal() { ap = new char; std::cout << "Animal ctor" << std::endl; } virtual void foo() { std::cout << "Animal::foo" << std::endl; } virtual ~Animal() { std::cout << "Animal dtor" << std::endl; delete ap; } }; class Dog : public Animal { char* dp; public: Dog() { dp = new char; std::cout << "Dog ctor" << std::endl; } virtual void foo() { std::cout << "Dog::foo" << std::endl; } virtual ~Dog() {

# include <iostream> using namespace std; class A { public: virtual void f() { cout << "A::f()" << endl; } }; class B:public A { private: virtual void f() { cout << "B::f()" << endl; } }; int main() { A *ptr = new B; ptr->f(); return 0; } This code works correctly and prints B::f(). I know how it works, but why is this code allowed? Access control is performed at compile time, not runtime. There's no way in general for the call to f() to know the runtime type of the object pointed to by ptr , so there's no check on the derived class's access specifiers. That's why the call is permitted. As for

When using some framework/api, sometimes it's pretty unclear if you must call base.Method if you override it, for example you can be pretty sure that you should call base.Maethod() when you are overriding event invocater, to propagate the event, in other situations it can be not so clear especially when there is no source code available, and no comments. I wounder how other programmers decide should they call base method or not in this situation, and if you are about to write some framework how to inform other programmers that you expect base method to be called or not in virtual members. Alex

can you please help me with the following questions... If I need a virtual printer that will convert a PostScript stream to a different format, do I have to implement a virtual printer from scratch or implement a rendering plug-in? The rendering plug-in seems to support only certain customizations. Also the data invariably goes to the spooler which is not needed in this case. If I implement a virtual printer driver does it completely replace the Microsoft PostScript Driver or the Microsoft Universal Driver? Since my driver is virtual, does it matter if I write a PostScript compliant or a