“C++動態(tài)綁定”相關(guān)問題探討

路人甲Java 2021-08-20

展開全文

一、相關(guān)問題：

1. 基類、派生類的構(gòu)造和析構(gòu)順序

2. 基類、派生類中virtual的取舍

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(jié)論：

1. 基類、派生類的構(gòu)造和析構(gòu)順序為：基類構(gòu)造-派生類構(gòu)造-派生類析構(gòu)-基類析構(gòu)

上述代碼輸出結(jié)果為：

2. 基類、派生類中virtual的取舍：若要實現(xiàn)動態(tài)綁定，基類中virtual關(guān)鍵字不可舍棄，派生類中virtual關(guān)鍵字可有可無；若基類中有關(guān)鍵字virtual，則普通函數(shù)調(diào)用派生類函數(shù)，析構(gòu)函數(shù)先調(diào)用派生類，再調(diào)用基類；若基類中無關(guān)鍵字virtual，則普通函數(shù)和析構(gòu)函數(shù)均只調(diào)用基類函數(shù)。

測試代碼1：基類無關(guān)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(jié)果為：

測試代碼2：派生類無關(guān)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(jié)果為：