C++キーワードのoverride

非オリジナル、stackoverflowから転載
確かにoverrideはkeywordではありません
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."(人に見せる)

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.(コンパイラに確認させる)

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 { ... } 
};

In derived2 the compiler will issue an error for "changing the type". Without override , at most the compiler would give a warning for "you are hiding virtual method by same name".