override final

参考

虚函数

函数重载有以下要求：

• 基类中的函数必须是虚函数
• 基类和派生类中的函数名字必须完全相同(析构函数例外)、
• 基类和派生类中的函数形参型别必须完全相同
• 基类和派生类中的函数常量性必须完全相同
• 基类和派生类中的函数返回值和异常规格必须兼容

c++11又多了一条：

• 基类和派生类中的函数引用饰词必须完全相同

引用饰词

#include <iostream>
using namespace std;

class mydata
{
public:
	mydata() { cout << "constructor" << endl; }
	~mydata() { cout << "destructor" << endl; }
	mydata(mydata && param) { cout << "move constructor" << endl; }
	mydata(mydata & param) { cout << "copy constructor" << endl; }
	mydata operator =(mydata && param) { cout << "move operator = " << endl; }
};

class myclass
{
public:
	mydata _m;

	myclass():_m()
	{

	}

	mydata& data() &	//*this是左值才调用
	{
		cout << "--data() &" << endl;
		return this->_m;
	}

	mydata data() &&	//*this是右值才调用
	{
		cout << "--data() &&" << endl;
		return std::move(this->_m);
	}
};

myclass makemyclass()
{
	return myclass();
}

int main() {
	
	myclass my;
	auto a1 = my.data();
	auto a2 = makemyclass().data();
	
}

override

c++11允许在派生类的成员函数末尾添加override,表明派生类将要重写该函数，编译器会校验重写的规则，避免了一些隐晦的含义。

#include <iostream>
using namespace std;

class base {
public:
	virtual void func1() const{
		cout << "base: virtual void func1() const" << endl;
	}
	virtual void func2(int x){
		cout << "base: virtual void func2(int x)" << endl;
	}
	virtual void func3() & {
		cout << "base: virtual void func3() &" << endl;
	}
	void func4() const {
		cout << "base: void func4() const" << endl;
	}
};

class derived : public base {
public:
	virtual void func1() {
		cout << "derived: virtual void func1()" << endl;
	}
	virtual void func2(unsigned int x) {
		cout << "derived: virtual void func2(unsigned int x)" << endl;
	}
	virtual void func3() && {
		cout << "derived: virtual void func3() &&" << endl;
	}
	void func4() const {
		cout << "derived: void func4() const" << endl;
	}
};

#if 0
class derived2 : public base {
public:
	virtual void func1() override;					//error
	virtual void func2(unsigned int x) override;	//error
	virtual void func3() && override;				//error
	void func4() const override;					//error
};
#endif

class derived3 : public base {
public:
	virtual void func1() const override;	//ok	
	virtual void func2(int x) override;		//ok
	virtual void func3() & override;		//ok	
	//void func4() const override;	//需要修改基类				
};

derived makederived()
{
	return derived();
}

int main() {
	derived der;
	base *bas = &der;

	bas->func1();		//base: virtual void func1() const	

	unsigned int x = 0;
	bas->func2(x);		//base : virtual void func2(int x)

	bas->func3();		//base : virtual void func3() &

	bas->func4();		//base : void func4() const
}

final

• final跟在类定义后面，表示该类不能被继承：class test1 final
• final不能修饰非虚函数
• final在基类的虚函数后面，表示派生类不可以重载该函数
• final在派生类的虚函数后面，表示派生类的其他派生类不能重载该派生类的此函数

struct Base
{
    virtual void foo();
};
 
struct A : Base
{
    //#a
    void foo() final; // Base::foo is overridden and A::foo is the final override
    void bar() final; // Error: bar cannot be final as it is non-virtual
};
 
struct B final : A // struct B is final
{
    void foo() override; // Error: foo cannot be overridden as it is final in A
};
 
struct C : B // Error: B is final
{
};

对于上面#a的解释:

#include <iostream>
using namespace std;

struct Base
{
	virtual void foo()
	{
		cout << "base: foo()" << endl;
	}
};

struct A : Base
{
	void foo() final // Base::foo is overridden and A::foo is the final override
	{
		cout << "A: foo()" << endl;
	}
	//void bar() final; // Error: bar cannot be final as it is non-virtual
};

int main() {
	A a;
	Base *p = &a;
	p->foo();
}

A: foo()

总结

• override与final都不是语言关键字（keyword），只有在特定的位置才有特别含意，其他地方仍旧可以作为一般指示字（identifier）使用。override仅出现在成员函数的末尾才有意义。其他地方使用意义就变了：

void override(void)
{
	cout << "override" << endl;
}

• 为意在重写的函数添加override
• 成员函数引用饰词使得对于左值和右值对象(*this)的处理能够区分开来

ending