“C++動態綁定”相關問題探討

一、相關問題：

1. 基類、派生類的構造和析構順序

2. 基類、派生類中virtual的取捨

二、測試代碼：

#include <iostream>
class A {
public:
    A() {
        std::cout  << "A()" << std::endl;
    }
    virtual void print() { std::cout << "A print()" << std::endl; }
    virtual ~A() {
        std::cout  << "~A()" << std::endl;
    }
};
class B : public A {
public:
    B() : A(){
        std::cout  << "B()" << std::endl;
    }
    virtual void print() { std::cout << "B print()" << std::endl; }
    virtual ~B() {
        std::cout  << "~B()" << std::endl;
    }
};
void print() {

}
int main(int argc, char *argv[])
{
    A* c = new B();
    c->print();
    delete c;
    return 0;
}

 三、探討與結論：

1. 基類、派生類的構造和析構順序為：基類構造-派生類構造-派生類析構-基類析構

上述代碼輸出結果為：

2. 基類、派生類中virtual的取捨：若要實現動態綁定，基類中virtual關鍵字不可捨棄，派生類中virtual關鍵字可有可無；若基類中有關鍵字virtual，則普通函數調用派生類函數，析構函數先調用派生類，再調用基類；若基類中無關鍵字virtual，則普通函數和析構函數均只調用基類函數。

測試代碼1：基類無關鍵字virtual

#include <iostream>
class A {
public:
    A() {
        std::cout  << "A()" << std::endl;
    }
    void print() { std::cout << "A print()" << std::endl; }
    ~A() {
        std::cout  << "~A()" << std::endl;
    }
};
class B : public A {
public:
    B() : A(){
        std::cout  << "B()" << std::endl;
    }
    virtual void print() { std::cout << "B print()" << std::endl; }
    virtual ~B() {
        std::cout  << "~B()" << std::endl;
    }
};
void print() {

}
int main(int argc, char *argv[])
{
    A* c = new B();
    c->print();
    delete c;
    return 0;
}

 輸出結果為：

測試代碼2：派生類無關鍵字virtual

#include <iostream>
class A {
public:
    A() {
        std::cout  << "A()" << std::endl;
    }
    virtual void print() { std::cout << "A print()" << std::endl; }
    virtual ~A() {
        std::cout  << "~A()" << std::endl;
    }
};
class B : public A {
public:
    B() : A(){
        std::cout  << "B()" << std::endl;
    }
    void print() { std::cout << "B print()" << std::endl; }
    ~B() {
        std::cout  << "~B()" << std::endl;
    }
};
void print() {

}
int main(int argc, char *argv[])
{
    A* c = new B();
    c->print();
    delete c;
    return 0;
}

 輸出結果為：