'override' keyword in C++

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本文详细解释了C++中override关键字的作用及用法。通过示例说明如何正确使用该关键字来实现基类虚拟方法的覆盖，并展示了不当使用时可能出现的问题。

转自：https://stackoverflow.com/questions/18198314/what-is-the-override-keyword-in-c-used-for

转自：https://msdn.microsoft.com/en-us/library/jj678987.aspx

The `override` keyword serves two purposes:

It shows the reader of the code that "this is a virtual method, that is overriding a virtual method of the base class."
The compiler also knows that it's an override, so it can "check" that you are not altering/adding new methods that you think are overrides.

To explain the latter:

class base
{
  public:
    virtual int foo(float x) = 0; 
};


class derived: public base
{
   public:
     int foo(float x) override { ... do stuff with x and such ... }
}

class derived2: public base
{
   public:
     int foo(int x) override { ... } //complie error!! 使用override 后的函数形式必须与base类相同
};

Another Example

Use `override` to help prevent inadvertent inheritance behavior in your code. The following example shows where, without using `override`, the member function behavior of the derived class may not have been intended. The compiler doesn't emit any errors for this code.

class BaseClass  
{  
    virtual void funcA();  
    virtual void funcB() const;  
    virtual void funcC(int = 0);  
    void funcD();  
};  
  
class DerivedClass: public BaseClass  
{  
    virtual void funcA(); // ok, works as intended  
  
    virtual void funcB(); // DerivedClass::funcB() is non-const, so it does not  
                          // override BaseClass::funcB() const and it is a new member function  
  
    virtual void funcC(double = 0.0); // DerivedClass::funcC(double) has a different  
                                      // parameter type than BaseClass::funcC(int), so  
                                      // DerivedClass::funcC(double) is a new member function  
  
};

When you use `override`, the compiler generates errors instead of silently creating new member functions.

class BaseClass  
{  
    virtual void funcA();  
    virtual void funcB() const;  
    virtual void funcC(int = 0);  
    void funcD();  
};  
  
class DerivedClass: public BaseClass  
{  
    virtual void funcA() override; // ok  
  
    virtual void funcB() override; // compiler error: DerivedClass::funcB() does not   
                                   // override BaseClass::funcB() const  
  
    virtual void funcC( double = 0.0 ) override; // compiler error:   
                                                 // DerivedClass::funcC(double) does not   
                                                 // override BaseClass::funcC(int)  
  
    void funcD() override; // compiler error: DerivedClass::funcD() does not   
                           // override the non-virtual BaseClass::funcD()  
};