为什么“虚拟”的方法在C ++中隐式传播？ [英] Why is the "virtuality" of methods implicitly propagated in C++?

问题描述

删除停止方法virtuality传播的能力的原因是什么？

What is the reason for removing the ability to stop the propagation of methods virtuality?

让我更清楚：在C ++中，无论你写virtual void foo ）或void foo（）在派生类中，它将是虚拟的，只要在基类中，foo被声明为virtual。

Let me be clearer: In C++, whether you write "virtual void foo()" or "void foo()" in the derived class, it will be virtual as long as in the base class, foo is declared virtual.

通过派生*指针调用foo（）将导致一个虚拟表查找（如果derived2函数覆盖foo），即使这个行为是程序员不想要的。

This means that a call to foo() through a derived* pointer will result in a virtual table lookup (in case a derived2 function overrides foo), even if this behavior is not wanted by the programmer.

让我给你一个例子（看起来很漂亮我），如何停止虚拟传播是有用的：

Let me give you an example (that looks pretty blatant to me) of how it would be useful to stop virtuality propagation:

template <class T>
class Iterator // Here is an iterator interface useful for defining iterators
{              // when implementation details need to be hidden
public:
    virtual T& next() { ... }
    ...
};

template <class T>
class Vector
{
public:
    class VectIterator : public Iterator<T>
    {
    public:
        T& next() { ... }
        ...
    };
    ...
};

在上面的例子中，Iterator基类可以用来实现类型擦除以更清楚和面向对象的方式。 （请参阅 http://www.artima.com/cppsource/type_erasure.html 了解示例类型擦除。）

In the example above, the Iterator base class can be used to achieve a form of "type erasure" in a much more clearer and Object-Oriented way. (See http://www.artima.com/cppsource/type_erasure.html for an example of type erasure.)

但是，在我的示例中，可以直接使用Vector :: VectIterator对象（这将在大多数情况下完成），以访问

But still, in my example one can use a Vector::VectIterator object directly (which will be done in most cases) in order to access the real object without using the interface.

如果没有传播虚拟化，即使从指针或引用调用Vector :: VectIterator :: next（）也不会是虚拟的

If virtuality was not propagated, calls to Vector::VectIterator::next() even from a pointer or reference would not be virtual and would be able to be inlined and to run efficiently, just as if the Iterator interface didn't exist.

推荐答案

C + +11为此目的添加了上下文关键字 final 。

C++11 added the contextual keyword final for this purpose.

class VectIterator : public Iterator<T>
{
public:
    T& next() final { ... }
    ...
};

struct Nope : VecIterator {
    T& next() { ... } // ill-formed
};

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