c++的virtual和override作用

一、概述

c++的virtual提供了運作時的多态，可以用基類的指針調用子類對象的函數，通過override可以強制要求基類有對應的虛函數。

二、測試代碼

#pragma once
#include <stdio.h>
class Base
{
public:
	void A_1() 
	{
		printf("Base::A_1\n");
	};
	virtual void A_2() 
	{
		printf("Base::A_2\n");
	}
	virtual void A_3() 
	{
		printf("Base::A_3\n");
	}

	virtual void A_4()
	{
		printf("Base::A_4\n");
	}

	void A_5() 
	{
		printf("Base::A_5\n");
	}
};

class Drived : public Base
{
public:
	void A_1() //覆寫了基類的函數，用基類指針調用時調用到的時基類的A_1，用子類指針調用時調用到的時子類的A_1
	{
		printf("Drived::A_1\n");
	};
	virtual void A_2() //用基類或子類指針都調用到的是子類的A_2,之類的virtual說明的是是子類的A_2還可以被virtual
	{
		printf("Drived::A_2\n");
	}
	void A_3() //用基類或子類指針都調用到的是子類的A_2
	{
		printf("Drived::A_3\n");
	}

	virtual void A_4() override //子類加上override，如果基類沒有對應virtual函數就會編譯錯誤。避免拼錯和記錯沒有重寫基類函數
	{
		printf("Drived::A_4\n");
	}
	/*
	void A_5() override //編譯錯誤 'Drived::A_5': method with override specifier 'override' did not override any base class methods	
	{ 
		printf("Drived::A_5\n");
	}
	*/
};

class Drived2 : public Drived
{
public:
	void A_2() override 
	{
		printf("Drived2::A_2\n");
	}
	void A_3()  override//用基類或子類指針都調用到的是子類的A_2
	{
		printf("Drived2::A_3\n");
	}
};

class VirtualTest
{
public:
	void DoTest()
	{
		Drived *drived = new Drived();
		Base *base = drived;
		base->A_1();
		drived->A_1();
		printf("\n");

		base->A_2();
		drived->A_2();
		printf("\n");

		base->A_3();
		drived->A_3();
		printf("\n");

		base->A_4();
		drived->A_4();
		printf("\n");

		Drived2 *drived2 = new Drived2();
		base = drived2;
		drived = drived2;
		
		base->A_2();
		drived->A_2();
		drived2->A_2();
		printf("\n");

		base->A_3();
		drived->A_3();
		drived2->A_3();

	}
};
           

三、輸出

Base::A_1
Drived::A_1

Drived::A_2
Drived::A_2

Drived::A_3
Drived::A_3

Drived::A_4
Drived::A_4

Drived2::A_2
Drived2::A_2
Drived2::A_2

Drived2::A_3
Drived2::A_3
Drived2::A_3
           

四、總結

1、基類函數沒加virtual，子類有相同函數，實作的是覆寫。用基類指針調用時，調用到的是基類的函數；用子類指針調用時，調用到的是子類的函數。

2、基類函數加了virtual時，實作的時重寫。用基類指針或子類指針調用時，調用到的都是子類的函數。

3、函數加上override，強制要求基本相同函數需要是虛函數，否則會編譯報錯。

4、子類的virtual可加可不加，建議加override不加virtual。