问题描述

纯虚函数和虚函数有什么区别?

What is the difference between a pure virtual function and a virtual function?

我知道纯虚函数是一个没有实体的虚函数"，但这意味着什么以及下面这行实际做了什么:

I know "Pure Virtual Function is a Virtual function with no body", but what does this mean and what is actually done by the line below:

virtual void virtualfunctioname() = 0

推荐答案

虚函数使其类成为多态基类.派生类可以覆盖虚函数.通过基类指针/引用调用的虚拟函数将在运行时解析.也就是说，使用对象的动态类型而不是它的静态类型:

A virtual function makes its class a polymorphic base class. Derived classes can override virtual functions. Virtual functions called through base class pointers/references will be resolved at run-time. That is, the dynamic type of the object is used instead of its static type:

 Derived d;
 Base& rb = d;
 // if Base::f() is virtual and Derived overrides it, Derived::f() will be called
 rb.f();  

纯虚函数是声明以=0结尾的虚函数:

A pure virtual function is a virtual function whose declaration ends in =0:

class Base {
  // ...
  virtual void f() = 0;
  // ...

纯虚函数隐式地将它定义为抽象的类(与在Java中使用关键字显式声明抽象类不同).抽象类不能被实例化.派生类需要覆盖/实现所有继承的纯虚函数.如果不这样做，它们也会变得抽象.

A pure virtual function implicitly makes the class it is defined for abstract (unlike in Java where you have a keyword to explicitly declare the class abstract). Abstract classes cannot be instantiated. Derived classes need to override/implement all inherited pure virtual functions. If they do not, they too will become abstract.

C++ 的一个有趣的特性"是一个类可以定义一个具有实现的纯虚函数.(有什么好处值得商榷.)

An interesting 'feature' of C++ is that a class can define a pure virtual function that has an implementation. (What that's good for is debatable.)

请注意，C++11 为 delete 和 default 关键字带来了新用法，它们看起来类似于纯虚函数的语法:

Note that C++11 brought a new use for the delete and default keywords which looks similar to the syntax of pure virtual functions:

my_class(my_class const &) = delete;
my_class& operator=(const my_class&) = default;

参见这个问题和这个问题 有关此使用 delete 和 default 的更多信息.

See this question and this one for more info on this use of delete and default.

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