解構函式前加virtual作用

摘要： 最近在利用地鐵通勤時間學習c++相關知識，在csdn看到一篇關於為什麼解構函式前加virtual的文章，csdn原文：解構函式前加Virtual作用 大家知道，解構函式是為了在物件不被使用之後釋放它的資源，虛擬函式是為了...

#include<iostream>
using namespace std;
 
class Base
{
public:
Base() {}; //Base的建構函式
~Base() //Base的解構函式
{
cout << "Output from the destructor of class Base!" << endl;
};

virtual void DoSomething()
{
cout << "Do something in class Base!" << endl;
};
};
 
class Derived : public Base
{
public:
Derived() {}; //Derived的建構函式
~Derived() //Derived的解構函式
{
cout << "Output from the destructor of class Derived!" << endl;
};

void DoSomething()
{
cout << "Do something in class Derived!" << endl;
};
};
 
int main()
{
Derived *pTest1 = new Derived(); //Derived類的指標
pTest1->DoSomething();
delete pTest1;
 
cout << endl;
 
Base *pTest2 = new Derived(); //Base類的指標
pTest2->DoSomething();
delete pTest2;
 
return 0;
}

程式輸出:

1 Do something in class Derived!
2 Output from the destructor of class Derived!
3 Output from the destructor of class Base!
4 
5 Do something in class Derived!
6 Output from the destructor of class Base!

程式碼第36行可以正常釋放pTest1的資源，而程式碼第42行沒有正常釋放pTest2的資源，因為從結果看Derived類的解構函式並沒有被呼叫。通常情況下類的解構函式裡面都是釋放記憶體資源，而解構函式不被呼叫的話就會造成記憶體洩漏。原因是指標pTest2是Base型別的指標，釋放pTest2時只進行Base類的解構函式。在程式碼第8行前面加上virtual關鍵字後的執行結果如下：

1 Do something in class Derived!
2 Output from the destructor of class Derived!
3 Output from the destructor of class Base!
4 
5 Do something in class Derived!
6 Output from the destructor of class Derived!
7 Output from the destructor of class Base!