基類的解構函式不為虛擬函式

class Base {
public:
    Base(int a = 0) {
        cout << "Base default constructor" << endl;
        p = new int(a);
    }
    ~Base() {
        cout << "Base destructor" << endl;
        if (nullptr != p) {
            delete p;
            p = nullptr;
        }
    }
private
 
:
    int *p;
};
class Inherit : public Base {
public:
    Inherit(int b = 0) {
        cout << "Inherit default constructor" << endl;
        q = new int(b);
    }
    ~Inherit() {
        cout << "Inherit destructor" << endl;
        if (nullptr != q) {
            delete q;
            q = nullptr
 
;
        }
    }
private:
    int *q;
};

基類對派生類及物件不需要進行操作時。

int main(void)
{
    Base *b = new Base(2);
    delete b;
    b = nullptr;
    return 0;
}

　　這時不能定義虛擬函式，因為這樣會增加記憶體開銷。當類裡面有定義虛擬函式時，編譯器會給類增加一個虛擬函式表，裡面來存放虛擬函式指標，這樣就增加了類的儲存空間。只有當一個類被用來作為基類的時候，才把解構函式寫成虛擬函式。

基類的解構函式為虛擬函式

class Base {
public
 
:
    Base(int a = 0) {
        cout << "Base default constructor" << endl;
        p = new int(a);
    }
    virtual ~Base() {
        cout << "Base destructor" << endl;
        if (nullptr != p) {
            delete p;
            p = nullptr;
        }
    }
private:
    int *p;
};
class Inherit : public Base {
public:
    Inherit(int b = 0) {
        cout << "Inherit default constructor" << endl;
        q = new int(b);
    }
    ~Inherit() {
        cout << "Inherit destructor" << endl;
        if (nullptr != q) {
            delete q;
            q = nullptr;
        }
    }
private:
    int *q;
};

示例

int main(void)
{
    Base *b = new Inherit(3);
    delete b;
    b = nullptr;
    return 0;
}