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< title > Item 12:使用override声明重载函数 - Effective Modern C++< / title >
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< ol class = "chapter" > < li class = "chapter-item expanded " > < a href = "../Introduction.html" > 简介< / a > < / li > < li class = "chapter-item expanded " > < div > 第一章 类型推导< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item1.html" > Item 1:理解模板类型推导< / a > < / li > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item2.html" > Item 2:理解auto类型推导< / a > < / li > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item3.html" > Item 3:理解decltype< / a > < / li > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item4.html" > Item 4:学会查看类型推导结果< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第二章 auto< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../2.Auto/item5.html" > Item 5:优先考虑auto而非显式类型声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../2.Auto/item6.html" > Item 6:auto推导若非己愿， < / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第三章 移步现代C++< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item7.html" > Item 7:区别使用()和{}创建对象< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item8.html" > Item 8:优先考虑nullptr而非0和NULL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item9.html" > Item 9:优先考虑别名声明而非typedefs< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item10.html" > Item 10:优先考虑限域枚举而非未限域枚举< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item11.html" > Item 11:优先考虑使用deleted函数而非使用未定义的私有声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item12.html" class = "active" > Item 12:使用override声明重载函数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item13.html" > Item 13:优先考虑const_iterator而非iterator< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item14.html" > Item 14:如果函数不抛出异常请使用noexcept< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item15.html" > Item 15:尽可能的使用constexpr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item16.html" > Item 16:让const成员函数线程安全< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item17.html" > Item 17:理解特殊成员函数函数的生成< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第四章 智能指针< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item18.html" > Item 18:对于独占资源使用std::unique_ptr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item19.html" > Item 19:对于共享资源使用std::shared_ptr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item20.html" > Item 20:当std::shared_ptr可能悬空时使用std::weak_ptr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item21.html" > Item 21:优先考虑使用std::make_unique和std::make_shared而非new< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item22.html" > Item 22:当使用Pimpl惯用法， < / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第五章 右值引用，移动语义，完美转发< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../5.RRefMovSemPerfForw/item23.html" > Item 23:理解std::move和std::forward< / a > < / li > < li class = "chapter-item expanded " > < a href = "../5.RRefMovSemPerfForw/item24.html" > Item 24:区别通用引用和右值引用< / a > < / li > < li class = "chapter-item expanded " > < a href = "../5.RRefMovSemPerfForw/item25.html" > Item 25:对于右值引用使用std::move， < / a > < / li > < li class = "chapter-item expanded " > < a href = "../5
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< h2 id = "条款十二使用override声明重写函数" > < a class = "header" href = "#条款十二使用override声明重写函数" > 条款十二：使用< code > override< / code > 声明重写函数< / a > < / h2 >
< p > < strong > Item 12: Declare overriding functions < code > override< / code > < / strong > < / p >
< p > 在C++面向对象的世界里，涉及的概念有类，继承，虚函数。这个世界最基本的概念是派生类的虚函数< strong > 重写< / strong > 基类同名函数。令人遗憾的是虚函数重写可能一不小心就错了。似乎这部分语言的设计理念是不仅仅要遵守墨菲定律，还应该尊重它。< / p >
< p > 虽然“重写（< em > overriding< / em > ）”听起来像“重载（< em > overloading< / em > ）”，然而两者完全不相关，所以让我澄清一下，正是虚函数重写机制的存在，才使我们可以通过基类的接口调用派生类的成员函数：< / p >
< pre > < code class = "language-cpp" > class Base {
public:
virtual void doWork(); //基类虚函数
…
};
class Derived: public Base {
public:
virtual void doWork(); //重写Base::doWork
… //（ ）
};
std::unique_ptr< Base> upb = //创建基类指针指向派生类对象
2023-01-31 11:01:36 +08:00
std::make_unique< Derived> (); //关于std::make_unique
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… //请参见Item21
upb-> doWork(); //通过基类指针调用doWork，
//实际上是派生类的doWork
//函数被调用
< / code > < / pre >
< p > 要想重写一个函数，必须满足下列要求：< / p >
< ul >
< li > 基类函数必须是< code > virtual< / code > < / li >
< li > 基类和派生类函数名必须完全一样（除非是析构函数)< / li >
< li > 基类和派生类函数形参类型必须完全一样< / li >
< li > 基类和派生类函数常量性< code > const< / code > ness必须完全一样< / li >
< li > 基类和派生类函数的返回值和异常说明（< em > exception specifications< / em > ）必须兼容< / li >
< / ul >
< p > 除了这些C++98就存在的约束外， ： < / p >
< ul >
< li > 函数的引用限定符（< em > reference qualifiers< / em > ） ， < code > virtual< / code > 也能使用它们：< / li >
< / ul >
< pre > < code class = "language-cpp" > class Widget {
public:
…
void doWork() & ; //只有*this为左值的时候才能被调用
void doWork() & & ; //只有*this为右值的时候才能被调用
};
…
Widget makeWidget(); //工厂函数（返回右值）
Widget w; //普通对象（左值）
…
w.doWork(); //调用被左值引用限定修饰的Widget::doWork版本
//（ & ）
makeWidget().doWork(); //调用被右值引用限定修饰的Widget::doWork版本
//（ & & ）
< / code > < / pre >
< p > 后面我还会提到引用限定符修饰成员函数，但是现在，只需要记住如果基类的虚函数有引用限定符，派生类的重写就必须具有相同的引用限定符。如果没有，那么新声明的函数还是属于派生类，但是不会重写父类的任何函数。< / p >
< p > 这么多的重写需求意味着哪怕一个小小的错误也会造成巨大的不同。代码中包含重写错误通常是有效的，但它的意图不是你想要的。因此你不能指望当你犯错时编译器能通知你。比如，下面的代码是完全合法的，咋一看，还很有道理，但是它没有任何虚函数重写——没有一个派生类函数联系到基类函数。你能识别每种情况的错误吗，换句话说，为什么派生类函数没有重写同名基类函数？< / p >
< pre > < code class = "language-cpp" > class Base {
public:
virtual void mf1() const;
virtual void mf2(int x);
virtual void mf3() & ;
void mf4() const;
};
class Derived: public Base {
public:
virtual void mf1();
virtual void mf2(unsigned int x);
virtual void mf3() & & ;
void mf4() const;
};
< / code > < / pre >
< p > 需要一点帮助吗？< / p >
< ul >
< li >
< p > < code > mf1< / code > 在< code > Base< / code > 基类声明为< code > const< / code > ，但是< code > Derived< / code > 派生类没有这个常量限定符< / p >
< / li >
< li >
< p > < code > mf2< / code > 在< code > Base< / code > 基类声明为接受一个< code > int< / code > 参数，但是在< code > Derived< / code > 派生类声明为接受< code > unsigned int< / code > 参数< / p >
< / li >
< li >
< p > < code > mf3< / code > 在< code > Base< / code > 基类声明为左值引用限定，但是在< code > Derived< / code > 派生类声明为右值引用限定< / p >
< / li >
< li >
< p > < code > mf4< / code > 在< code > Base< / code > 基类没有声明为< code > virtual< / code > 虚函数< / p >
< / li >
< / ul >
< p > 你可能会想， ， ， ， ， ， ， （ ， ） < / p >
< p > 由于正确声明派生类的重写函数很重要， ， ： < code > override< / code > 。还是上面那个例子，我们可以这样做：< / p >
< pre > < code class = "language-cpp" > class Derived: public Base {
public:
virtual void mf1() override;
virtual void mf2(unsigned int x) override;
virtual void mf3() & & override;
virtual void mf4() const override;
};
< / code > < / pre >
< p > 代码不能编译，当然了，因为这样写的时候，编译器会抱怨所有与重写有关的问题。这也是你想要的，以及为什么要在所有重写函数后面加上< code > override< / code > 。< / p >
< p > 使用< code > override< / code > 的代码编译时看起来就像这样（假设我们的目的是< code > Derived< / code > 派生类中的所有函数重写< code > Base< / code > 基类的相应虚函数）:< / p >
< pre > < code class = "language-cpp" > class Base {
public:
virtual void mf1() const;
virtual void mf2(int x);
virtual void mf3() & ;
virtual void mf4() const;
};
class Derived: public Base {
public:
virtual void mf1() const override;
virtual void mf2(int x) override;
virtual void mf3() & override;
void mf4() const override; //可以添加virtual，
};
< / code > < / pre >
< p > 注意在这个例子中< code > mf4< / code > 有别于之前，它在< code > Base< / code > 中的声明有< code > virtual< / code > 修饰，所以能正常工作。大多数和重写有关的错误都是在派生类引发的，但也可能是基类的不正确导致。< / p >
< p > 比起让编译器（ ： ） ， < code > override< / code > 。如果你考虑修改修改基类虚函数的函数签名，< code > override< / code > 还可以帮你评估后果。如果派生类全都用上< code > override< / code > ，你可以只改变基类函数签名，重编译系统，再看看你造成了多大的问题（即，多少派生类不能通过编译），然后决定是否值得如此麻烦更改函数签名。没有< code > override< / code > ，你只能寄希望于完善的单元测试，因为，正如我们所见，派生类虚函数本想重写基类，但是没有，编译器也没有探测并发出诊断信息。< / p >
< p > C++既有很多关键字， （ < em > contextual keywords< / em > ） ， < code > override< / code > 和< code > final< / code > （向虚函数添加< code > final< / code > 可以防止派生类重写。< code > final< / code > 也能用于类，这时这个类不能用作基类）。这两个关键字的特点是它们是保留的，它们只是位于特定上下文才被视为关键字。对于< code > override< / code > ，它只在成员函数声明结尾处才被视为关键字。这意味着如果你以前写的代码里面已经用过< strong > override< / strong > 这个名字， ： < / p >
< pre > < code class = "language-cpp" > class Warning { //C++98潜在的传统类代码
public:
…
void override(); //C++98和C++11都合法（ ）
…
};
< / code > < / pre >
< p > 关于< code > override< / code > 想说的就这么多，但对于成员函数引用限定（< em > reference qualifiers< / em > ）还有一些内容。我之前承诺我会在后面提供更多的关于它们的资料，现在就是" 后面" 了。< / p >
< p > 如果我们想写一个函数只接受左值实参， < code > const< / code > 左值引用形参：< / p >
< pre > < code class = "language-cpp" > void doSomething(Widget& w); //只接受左值Widget对象
< / code > < / pre >
< p > 如果我们想写一个函数只接受右值实参，我们声明一个右值引用形参：< / p >
< pre > < code class = "language-cpp" > void doSomething(Widget& & w); //只接受右值Widget对象
< / code > < / pre >
< p > 成员函数的引用限定可以很容易的区分一个成员函数被哪个对象（即< code > *this< / code > ）调用。它和在成员函数声明尾部添加一个< code > const< / code > 很相似，暗示了调用这个成员函数的对象（即< code > *this< / code > ）是< code > const< / code > 的。< / p >
< p > 对成员函数添加引用限定不常见，但是可以见。举个例子，假设我们的< code > Widget< / code > 类有一个< code > std::vector< / code > 数据成员，我们提供一个访问函数让客户端可以直接访问它：< / p >
< pre > < code class = "language-cpp" > class Widget {
public:
using DataType = std::vector< double> ; //“using”的信息参见Item9
…
DataType& data() { return values; }
…
private:
DataType values;
};
< / code > < / pre >
< p > 这是最具封装性的设计，只给外界保留一线光。但先把这个放一边，思考一下下面的客户端代码：< / p >
< pre > < code class = "language-cpp" > Widget w;
…
auto vals1 = w.data(); //拷贝w.values到vals1
< / code > < / pre >
< p > < code > Widget::data< / code > 函数的返回值是一个左值引用（准确的说是< code > std::vector< double> & < / code > ）
因为左值引用是左值，所以< code > vals1< / code > 是从左值初始化的。因此< code > vals1< / code > 由< code > w.values< / code > 拷贝构造而得，就像注释说的那样。< / p >
< p > 现在假设我们有一个创建< code > Widget< / code > s的工厂函数， < / p >
< pre > < code class = "language-cpp" > Widget makeWidget();
< / code > < / pre >
< p > 我们想用< code > makeWidget< / code > 返回的< code > Widget< / code > 里的< code > std::vector< / code > 初始化一个变量：< / p >
< pre > < code class = "language-cpp" > auto vals2 = makeWidget().data(); //拷贝Widget里面的值到vals2
< / code > < / pre >
< p > 再说一次，< code > Widgets::data< / code > 返回的是左值引用，还有，左值引用是左值。所以，我们的对象（< code > vals2< / code > ）得从< code > Widget< / code > 里的< code > values< / code > 拷贝构造。这一次，< code > Widget< / code > 是< code > makeWidget< / code > 返回的临时对象（即右值），所以将其中的< code > std::vector< / code > 进行拷贝纯属浪费。最好是移动，但是因为< code > data< / code > 返回左值引用， （ ， ， ） （ ： ） < / p >
< p > 我们需要的是指明当< code > data< / code > 被右值< code > Widget< / code > 对象调用的时候结果也应该是一个右值。现在就可以使用引用限定，为左值< code > Widget< / code > 和右值< code > Widget< / code > 写一个< code > data< / code > 的重载函数来达成这一目的：< / p >
< pre > < code class = "language-cpp" > class Widget {
public:
using DataType = std::vector< double> ;
…
DataType& data() & //对于左值Widgets,
{ return values; } //返回左值
DataType data() & & //对于右值Widgets,
{ return std::move(values); } //返回右值
…
private:
DataType values;
};
< / code > < / pre >
< p > 注意< code > data< / code > 重载的返回类型是不同的，左值引用重载版本返回一个左值引用（即一个左值），右值引用重载返回一个临时对象（即一个右值）。这意味着现在客户端的行为和我们的期望相符了：< / p >
< pre > < code class = "language-cpp" > auto vals1 = w.data(); //调用左值重载版本的Widget::data，
//拷贝构造vals1
auto vals2 = makeWidget().data(); //调用右值重载版本的Widget::data,
//移动构造vals2
< / code > < / pre >
< p > 这真的很棒，但别被这结尾的暖光照耀分心以致忘记了该条款的中心。这个条款的中心是只要你在派生类声明想要重写基类虚函数的函数，就加上< code > override< / code > 。< / p >
< p > < strong > 请记住：< / strong > < / p >
< ul >
< li > 为重写函数加上< code > override< / code > < / li >
< li > 成员函数引用限定让我们可以区别对待左值对象和右值对象（即< code > *this< / code > )< / li >
< / ul >
< / main >
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