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c++的virtual和override作用及說明

更新時間：2022年11月05日 09:18:42 作者：游戲貓

這篇文章主要介紹了c++的virtual和override作用及說明，具有很好的參考價值，希望對大家有所幫助。如有錯誤或未考慮完全的地方，望不吝賜教

一、概述

c++的virtual提供了運行時的多態(tài)，可以用基類的指針調(diào)用子類對象的函數(shù)，通過override可以強制要求基類有對應(yīng)的虛函數(shù)。

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() //覆蓋了基類的函數(shù)，用基類指針調(diào)用時調(diào)用到的時基類的A_1，用子類指針調(diào)用時調(diào)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(diào)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(diào)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(yīng)virtual函數(shù)就會編譯錯誤。避免拼錯和記錯沒有重寫基類函數(shù)
?? ?{
?? ??? ?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(diào)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