Something about virtual function and pure virtual function

最新推荐文章于 2022-02-15 17:58:59 发布

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#function #construction #class #c

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本文探讨了C++中虚拟函数的必要性及其实现方式，特别强调了纯虚函数的特点和在子类中的应用。通过示例展示了如何正确使用虚拟函数和纯虚函数，确保类的正确继承性和多态性。

Firstly, let's demonstrate a fact that virtual function must be implemented.

class A {
public:
	A() { a = 0; }
    virtual void a1();
    int a2();
private:
    int a;
};

int main() {
	A a;
	return 0;
}

A strange error about construction occurs. Now, let's see the correct version.

class A {
public:
	A() { a = 0; }
    virtual void a1() { a = 1;}
    int a2();
private:
    int a;
};

int main() {
	A a;
	return 0;
}

It is right. By the way, you can see the common member function could avoid implementation.

In contrast, the pure virtual function mustn't be implemented.

class A {
public:
	A() { a = 0; }
    virtual void a3() =0 { a = 1;}
private:
    int a;
};

int main() {
	return 0;
}

Another example:

class A {
public:
	A() { a = 0; }
    virtual void a3() =0;
private:
    int a;
};

class B: public A {
public:
	B(): A() { }
};

class C: public B {
public:
	C(): B() {}
	void a3() {}
};

int main() {
	B b;
	C c;
	return 0;
}

However, the correct should be

class A {
public:
	A() { a = 0; }
    virtual void a3() =0;
private:
    int a;
};

class B: public A {
public:
	B(): A() { }
};

class C: public B {
public:
	C(): B() {}
	void a3() {}
};

int main() {
//	B b;
	C c;
	return 0;
}

From this experiment, we can see that pure virtual function doesn't necessarily be implemented in its subclass and hence its subclass is still a virtual class.