c++11 关键字 override 使用

写在最前。。。

请支持原创~~ 

1. 功能

用在类中成员函数声明的地方，用以标记一个virtual function 是重写另一个 virtual function；

 

2. 语法

• 只声明时，override 紧跟参数的右括号，如果是纯虚函数，override 会出现在 = 0 之前；
• 类中定义时，override 在 函数体之前；

3. 举例

 

#include <iostream>struct A
{virtual void foo();void bar();virtual ~A();
};// member functions definitions of struct A:
void A::foo() { std::cout << "A::foo();\n"; }
A::~A() { std::cout << "A::~A();\n"; }struct B : A
{
//  void foo() const override; // Error: B::foo does not override A::foo// (signature mismatch)void foo() override; // OK: B::foo overrides A::foo
//  void bar() override; // Error: A::bar is not virtual~B() override; // OK: `override` can also be applied to virtual// special member functions, e.g. destructorsvoid override(); // OK, member function name, not a reserved keyword
};// member functions definitions of struct B:
void B::foo() { std::cout << "B::foo();\n"; }
B::~B() { std::cout << "B::~B();\n"; }
void B::override() { std::cout << "B::override();\n"; }int main()
{B b;b.foo();b.override(); // OK, invokes the member function `override()`int override{42}; // OK, defines an integer variablestd::cout << "override: " << override << '\n';
}

重写的函数加上override，那么该成员函数需要满足：

• 成员函数为 虚函数；
• 成员函数从父类继承，在子类重写；

另外，如同 关键字 final， 只是一个标识，在使用成员函数时有特殊的意义。而在其他情况下可以作为一个对象名、函数名、类名使用。

 

结果：

B::foo();
B::override();
override: 42
B::~B();
A::~A();

 

4. 原文摘录 

Specifies that a virtual function overrides another virtual function.
The identifier override, if used, appears immediately after the declarator in the syntax of a member function declaration or a member function definition inside a class definition.

1) In a member function declaration, override may appear in virt-specifier-seq immediately after the declarator, and before the pure-specifier, if used.
2) In a member function definition inside a class definition, override may appear in virt-specifier-seq immediately after the declarator and just before function-body.
In both cases, virt-specifier-seq, if used, is either override or final, or final override or override final.

In a member function declaration or definition, override specifier ensures that the function is virtual and is overriding a virtual function from a base class. The program is ill-formed (a compile-time error is generated) if this is not true.

override is an identifier with a special meaning when used after member function declarators: it's not a reserved keyword otherwise.