Question C++: why pure virtual function has definition?Please look into sample code here 

ksrameshkanth

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Here the class Base has pure virutual function, But it is allowing to have definition,
even it can be called from drived class. There is no compilation error and Run time error. 
why?
Anyone Can please give details about it?

Is there any special purpose to allow this?

#include <iostream.h>

class Base

{
　　public:
　　virtual void Fun( )=; //纯虚函数接口

};

//纯虚函数实现
void Base::Fun(void)
{　　
　　cout << "\n I am in Pure virtual function\n";

}

class Derived:Public Base

{
　　public:

　　void Fun()

　　{

　　　　cout<<"\n I am in the Derived class";

　　　　Base::Fun( );//显式调用了基类中的纯虚函数

　　}

};

void main()

{

　　Derived d;

　　Base *b = &d;

　　b->Fun();

} 

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panayotisk

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If I remember well, somewhere in "Effective C++" Meyers mentions a reason for a pure virtual function to have a body:

Derived classes that implement this pure virtual function may call this implementation smwhere in their code.

If part of the code of two different derived classes is similar then it makes sense to move it up in the hierarchy, 
even if the function should be pure virtual. 
在<<Effective C++>>(没有拜读过)讲明，派生类可以显式地调用基类中的纯虚函数，这样就可以将将不同子类中公共的事务放在父类中完成

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ksrameshkanth

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Thanks for your quick response. I agree with your point that the pure virtual function purpose is to force the derived class to 
override it.But my doubt if we are able to call that function from the derived classes, 
why are we not allowed to create an object for that class and call that function with that object.

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panayotisk

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Using implementation code from a base class has nothing to do with allowing 
creation of objects of the base class. These are totally unrelated.

When designing ask yourself:

- Do I need to be able to instantiate objects of the base class? 
　　If not the base class should be abstract (contains at least one pure virtual function).

- Is there common code that derived classes may want to use? 
　　Then consider providing this in the body of a pure virtual function.
在派生类中显式地调用基类中纯虚函数（基类做了定义）与基类能否实例化没有任何关系

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humptydumpty

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Purpose of a Pure Virtual Function is to make a Class to Abstract Class.

Often in a design, you want the base class to present only an interface for

its derived classes. That is, you don’t want anyone to actually create an

object of the base class, only to upcast to it so that its interface can be

used. This is accomplished by making that class abstract, which happens if

you give it at least one pure virtual function. You can recognize a pure

virtual function because it uses the virtual keyword and is followed by = 0.

If anyone tries to make an object of an abstract class, the compiler

prevents them. This is a tool that allows you to enforce a particular design.
纯虚函数时为了定义一个抽象类，在该类中必有一些虚函数为纯虚函数，从而只声明了一个接口，而在某个派生类中必然实现该接口

When an abstract class is inherited, all pure virtual functions must be

implemented, or the inherited class becomes abstract as well. Creating a

pure virtual function allows you to put a member function in an interface

without being forced to provide a possibly meaningless body of code for

that member function. At the same time, a pure virtual function forces

inherited classes to provide a definition for it.
纯虚函数的意义在于不可以实例化该类，且不用写"{}"这样无意义的东西，并且要求其某个派生类必然要实现该接口

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 Graham

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If you have a "normal" virtual function, its implementation (body) becomes 
a sort of "default" implementation. That is, derived classes will inherit 
that implementation and use it if they don't override the function. 
This may or may not be a problem if the author of a derived class forgets to 
override a particular virtual function.

By making the original function pure (but still with a body), you are saying to 
the author of a derived class 
"there's a default implementation of this function; 
if you want to use, you will have to be explicit about it".
对于一般的虚函数，其实现是作为派生类的默认实现，即若派生类没有override基类中实现，则默认调用
基类中函数体；
但是如果在父类中实现了纯虚函数（接口）的函数体，则此时对于派生类意味着：
　　　　在父类中实现了接口的函数体，如果需要使用，请显式地调用。

code:

class base

{

　　public:

    　　virtual void f() = ;

};

void base::f()

{

    // implementation

}

class derived : public base

{

　　public:

    　　virtual void f()

    　　{

        　　base::f();  // explicit use of default implementation

    　　}

};

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 exterminator

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There is a case where it becomes a necessity to provide the implementation for a pure virtual functions 
and that would be a pure virtual destructor.//纯虚析构函数

ISO C++ 12.4 (7):

A destructor can be declared virtual (10.3) or pure virtual (10.4); 
if any objects of that class or any derived class are created in the program, 
the destructor shall be defined. If a class has a base class with a virtual destructor, 
its destructor (whether user or implicitly declared) is virtual. 
析构函数可以为虚函数或纯虚函数，如果基类或其派生类被实例化，则析构函数必须定义（当然，对于有纯虚函数的类不能被实例化），
如果基类的析构函数为虚函数，则派生类的析构函数无论是否显示的声明为virtual，均为需函数

Also, its not that since it has a body you need to call it explicitly. 
Its the other way round. If it needs to be called, then its body need to be implemented. 
The standard states it clearly:

ISO C++ 10.4 (2):

A pure virtual function need be defined only if explicitly called with the qualified id syntax (5.1). 
如果在派生类中显式的调用基类中的纯虚函数，则基类必须实现纯虚函数接口的函数体

And hence when the call from the derived destructor is done while polymorphic destruction 
you need to have a body of the base pure virtual destructor implemented for it to be executed while Base's destruction.
由于在派生类实例被析构时会调用基类的析构函数（编译器完成），因此当我们把基类的析构函数声明为纯虚函数时，必须implement纯虚析构函数的函数体--------------------------------------------------------------------------------------------------------------------------------

Graham

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I would be happy with the statement if it's specified that he never puts data in an abstract class. Some people stick to this rule - I don't see the point of being that restrictive.

It comes down to whether you only ever use abstract classes to mimic "interfaces", or whether you see them as part of a broader picture, one that sees a use for abstract classes with state information.

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SuperKoko

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I already had abstract classes with a few fields, used by non-virtual methods who called other virtual methods.

I think that it is a restriction of the language to never put data in abstract classes, and it may need some duplicate code and data member... what is not good.

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请教了下师弟关于Java中知识，顺便几下（不会Java，太弱了）：
Java中的Interface类相当于C++中的：1、抽象类，函数全为纯虚函数；2、没有数据成员
Java中的抽象类与C++的抽象类相当；
Java中的派生类只能继承一个（抽象）类、可以继承多个接口类