纯虚函数与实现 [英] pure virtual function with implementation

问题描述

我基本的理解是，没有一个纯虚拟函数的实现，但是，我被告知可能有纯虚拟函数的实现。

  class A {
 public：
 virtual void f（）= 0; 
}; 
 
 void A :: f（）{
 cout<<Test<< endl; 
} 
  

以上代码是否正确？

有一个纯粹的虚拟函数与一个实现的目的是什么？

解决方案

> virtual 函数必须在直接实例化的派生类型中实现，但基类型仍然可以定义实现。派生类可以通过使用完全范围的名称（通过在您的示例中调用 A :: f（））显式调用基类实现（如果访问权限允许）如果 A :: f（）有 public 或 protected ）。类似的东西：

  class B：public A {
 
 virtual void f（）{
 // class B没有为f（）
 //做任何特殊的事情，所以我们将调用A的
 
 //注意A的声明f be public 
 //或保护以避免编译时问题
 
 A :: f（）; 
} 
 
}; 
  

我可以想到的我的头顶的用例是当有一个更多或者 - 不太合理的默认行为，但是类设计者希望只显式调用默认排序行为。它也可以是你想要派生类总是执行自己的工作，但也能够调用一组通用的功能的情况。

请注意，即使它是允许的通过语言，它不是我看到常用的东西（事实上，它可以做似乎让大多数C ++程序员，甚至经验丰富的人）。

My basic understanding is that there is no implementation for a pure virtual function, however, I was told there might be implementation for pure virtual function.

class A {
public:
    virtual void f() = 0;
};

void A::f() {
    cout<<"Test"<<endl;
}

Is code above OK?

What's the purpose to make it a pure virtual function with an implementation?

解决方案

A pure virtual function must be implemented in a derived type that will be directly instantiated, however the base type can still define an implementation. A derived class can explicitly call the base class implementation (if access permissions allow it) by using a fully-scoped name (by calling A::f() in your example - if A::f() were public or protected). Something like:

class B : public A {

    virtual void f() {
        // class B doesn't have anything special to do for f()
        //  so we'll call A's

        // note that A's declaration of f() would have to be public 
        //  or protected to avoid a compile time problem

        A::f();
    }

};

The use case I can think of off the top of my head is when there's a more-or-less reasonable default behavior, but the class designer wants that sort-of-default behavior be invoked only explicitly. It can also be the case what you want derived classes to always perform their own work but also be able to call a common set of functionality.

Note that even though it's permitted by the language, it's not something that I see commonly used (and the fact that it can be done seems to surprise most C++ programmers, even experienced ones).

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