#include<iostream> using namespace std; //基类People class People{ protected: char *m_name; int m_age; public: People(char*, int); }; People::People(char *name, int age): m_name(name), m_age(age){} //派生类Student class Student: public People{ private: float m_score; public: Student(char *name, int age, float score); void display(); }; //People(name, age)就是调用基类的构造函数 Student::Student(char *name, int age, float score): People(name, age), m_score(score){ } void Student::display(){ cout<<m_name<<"的年龄是"<<m_age<<",成绩是"<<m_score<<"。"<<endl; } int main(){ Student stu("小明", 16, 90.5); stu.display(); return 0; }运行结果为:
Student::Student(char *name, int age, float score): People(name, age), m_score(score){ }
People(name, age)
就是调用基类的构造函数,并将 name 和 age 作为实参传递给它,m_score(score)
是派生类的参数初始化表,它们之间以逗号,
隔开。Student::Student(char *name, int age, float score): m_score(score), People(name, age){ }但是不管它们的顺序如何,派生类构造函数总是先调用基类构造函数再执行其他代码(包括参数初始化表以及函数体中的代码),总体上看和下面的形式类似:
Student::Student(char *name, int age, float score){ People(name, age); m_score = score; }当然这段代码只是为了方便大家理解,实际上这样写是错误的,因为基类构造函数不会被继承,不能当做普通的成员函数来调用。换句话说,只能将基类构造函数的调用放在函数头部,不能放在函数体中。
Student::Student(char *name, int age, float score): People("小明", 16), m_score(score){ }
A --> B --> C
那么创建 C 类对象时构造函数的执行顺序为:A类构造函数 --> B类构造函数 --> C类构造函数
构造函数的调用顺序是按照继承的层次自顶向下、从基类再到派生类的。#include <iostream> using namespace std; //基类People class People{ public: People(); //基类默认构造函数 People(char *name, int age); protected: char *m_name; int m_age; }; People::People(): m_name("xxx"), m_age(0){ } People::People(char *name, int age): m_name(name), m_age(age){} //派生类Student class Student: public People{ public: Student(); Student(char*, int, float); public: void display(); private: float m_score; }; Student::Student(): m_score(0.0){ } //派生类默认构造函数 Student::Student(char *name, int age, float score): People(name, age), m_score(score){ } void Student::display(){ cout<<m_name<<"的年龄是"<<m_age<<",成绩是"<<m_score<<"。"<<endl; } int main(){ Student stu1; stu1.display(); Student stu2("小明", 16, 90.5); stu2.display(); return 0; }运行结果:
Student::Student()
,它并没有指明要调用基类的哪一个构造函数,从运行结果可以很明显地看出来,系统默认调用了不带参数的构造函数,也就是People::People()
。Student::Student(char *name, int age, float score)
,它指明了基类的构造函数。People(name, age)
去掉,也会调用默认构造函数,第 37 行的输出结果将变为:
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