摘要：
Override关键字：在成员函数声明或定义中，Override确保函数是虚拟函数，并从基类重写虚拟函数。位置：在函数调用运算符之后，在函数体或纯虚拟函数ID“=0”之前。
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++#