从抽象基类实现纯虚函数:覆盖说明符是否有任何意义? [英] Implementing pure virtual function from abstract base class: does override specifier have any meaning?

问题描述

我偶然发现了 override 的用例说明符，据我所知，似乎是多余的，并且也没有任何特殊的语义含义，但是也许我遗漏了一些东西，因此提出了这个问题.在继续之前，我应该指出，我已经尝试在SO上找到它的答案，但是我得到的最接近的是以下线程，并没有真正回答我的查询(也许有人可以指出实际上已经存在的一个问答集).回答我的问题).

I just stumbled over a use case of the override specifier that, as far as I can tell, seems redundant and also without any particular semantics meaning, but maybe I'm missing something, hence this question. Before proceeding, I should point out that I've tried to find an answer to it here on SO, but the nearest I got were the following threads, not really answering my query (maybe someone can point out a Q&A that actually already answers my question).

• C ++虚拟/纯虚拟解释
• C ++用纯虚拟方法覆盖纯虚拟方法

考虑以下抽象类:

struct Abstract {
  virtual ~Abstract() {};
  virtual void foo() = 0;
};

在直接从 Abstract 派生的非抽象类中实现 foo()时，是否有任何理由使用 override 说明符在下面的 DerivedB 中)?即，当已经需要实现 foo()来使派生类成为非抽象类(并且不会真正覆盖任何内容)时?

Is there any reason to use the override specifier when implementing foo() in a non-abstract class derived directly from Abstract (as in DerivedB below)? I.e., when the implementation of foo() is already required for the derived class to be non-abstract (and not really overriding anything)?

/* "common" derived class implementation, in my personal experience
   (include virtual keyword for semantics) */
struct DerivedA : public Abstract {
  virtual void foo() { std::cout << "A foo" << std::endl; }
};

/* is there any reason for having the override specifier here? */
struct DerivedB : public Abstract {
  virtual void foo() override { std::cout << "B foo" << std::endl; }
};

推荐答案

我不是 override 的忠实拥护者，但是，假设您普遍认为这很有用，那么，是的，将 override 放在覆盖纯虚函数的虚函数上很有用.考虑下面这个人为的例子:

I'm not a big fan of override, but, assuming it's something that you find useful in general, then, yes, putting override on a virtual function that overrides a pure virtual functions is useful. Consider this rather contrived example:

struct Base {
    virtual void f() = 0;
};

struct Derived : Base {
    virtual void f();
    virtual void f(int);
};

现在假设 Base 的维护者(也许甚至是您将来的自己)将 Base 更改为如下形式:

Now suppose that the maintainer of Base (perhaps even your future self) changes Base to look like this:

struct Base {
    virtual void f(int) = 0;
};

现在， Derived 的行为已悄然改变.使用 override 时，编译器将报告错误.

Now the behavior of Derived has quietly changed. With override the compiler would report an error.

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