C++11 之 override

override 关键字

作用：在成员函数声明或定义中， override 确保该函数为虚函数并覆写来自基类的虚函数。
位置：函数调用运算符之后，函数体或纯虚函数标识 “= 0” 之前。

使用以后有以下好处:
1．可以当注释用,方便阅读．
2．告诉阅读你代码的人，这是方法的复写．
3．编译器可以给你验证 override 对应的方法名是否是你父类中所有的，如果没有则报错．

override 使用举例

如果你想重写父类的方法，比如toString()方法：

正确的是：

public String toString() override {
	...
}

假如不小心把方法名写错了而没写 override ,这时编译器是可以编译通过的，因为编译器以为这个方法是你的子类中自己增加的方法。如：

// 注意这里的小写方法，实际上是错误的。
public String tostring() {...}

相反，如果你很机智，在知道自己要重写父类的方法，加上了 override 标签后，编译器会检查出重写方法错误，会保证你重写父类方法的正确性。

#include <memory>
#include <iostream>
using namespace std; 
class Base
{
public:
            virtual void foo();
};
void Base::foo()
{
        cout << "Base::foo" <<endl;
}
 
class Derived : public  Base
{
            void foo() override; // OK: Derived::foo overrides Base::foo
};
void Derived::foo()
{
        cout << "Derived::foo" <<endl;
}
int main()
{
        Base * b = new Derived();
        b->foo();
        return 0;
} 

root@ubuntu:~/c++# g++ -std=c++11  over.cpp -o over
root@ubuntu:~/c++# ./over
Derived::foo

非虚函数

#include <memory>
#include <iostream>
using namespace std;
class Base
{
public:
            //virtual void foo();
            void foo();
};
void Base::foo()
{
        cout << "Base::foo" <<endl;
}

class Derived : public  Base
{
            void foo() override; // OK: Derived::foo overrides Base::foo
};
void Derived::foo()
{
        cout << "Derived::foo" <<endl;
}
int main()
{
        Base * b = new Derived();
        b->foo();
        return 0;
}

over.cpp:17:11: error: ‘void Derived::foo()’ marked ‘override’, but does not override
      void foo() override; // OK: Derived::foo overrides Base::foo
           ^

#include <memory>
#include <iostream>
using namespace std; 
class Base
{
public:
            virtual void foo();
};
void Base::foo()
{
        cout << "Base::foo" <<endl;
}
 
class Derived : public  Base
{
            void foo(int a) override; // OK: Derived::foo overrides Base::foo
};
void Derived::foo(int a)
{
        cout << "Derived::foo" <<endl;
}
int main()
{
        Base * b = new Derived();
        b->foo(3);
        return 0;
} 

root@ubuntu:~/c++# g++ -std=c++11  over.cpp -o over
over.cpp:16:11: error: ‘void Derived::foo(int)’ marked ‘override’, but does not override
      void foo(int a) override; // OK: Derived::foo overrides Base::foo
           ^
over.cpp: In function ‘int main()’:
over.cpp:25:10: error: no matching function for call to ‘Base::foo(int)’
  b->foo(3);
          ^
over.cpp:9:6: note: candidate: virtual void Base::foo()
 void Base::foo()
      ^
over.cpp:9:6: note:   candidate expects 0 arguments, 1 provided

#include <memory>
#include <iostream>
using namespace std;
class Base
{
public:
            virtual void foo();
};
void Base::foo()
{
        cout << "Base::foo" <<endl;
}

class Derived : public  Base
{
            void foo() ; // OK: Derived::foo overrides Base::foo
            //void foo() override; // OK: Derived::foo overrides Base::foo
};
//void Derived::foo(int a)
//{
//      cout << "Derived::foo" <<endl;
//}
int main()
{
        Base * b = new Derived();
        b->foo();
        return 0;
}

root@ubuntu:~/c++# g++ -std=c++11  over.cpp -o over
/tmp/cc9ksedx.o: In function `Derived::Derived()':
over.cpp:(.text._ZN7DerivedC2Ev[_ZN7DerivedC5Ev]+0x14): undefined reference to `vtable for Derived'
over.cpp:(.text._ZN7DerivedC2Ev[_ZN7DerivedC5Ev]+0x18): undefined reference to `vtable for Derived'
collect2: error: ld returned 1 exit status

#include <memory>
#include <iostream>
using namespace std;
class Base
{
public:
            virtual void foo();
};
void Base::foo()
{
        cout << "Base::foo" <<endl;
}

class Derived : public  Base
{
           void Foo() override; // OK: Derived::foo overrides Base::foo
};
//void Derived::foo(int a)
//{
//      cout << "Derived::foo" <<endl;
//}
int main()
{
        Base * b = new Derived();
        //b->foo();
        return 0;
}

root@ubuntu:~/c++# g++ -std=c++11  over.cpp -o over
over.cpp:16:10: error: ‘void Derived::Foo()’ marked ‘override’, but does not override
     void Foo() override; // OK: Derived::foo overrides Base::foo
          ^
root@ubuntu:~/c++#