18-初始化列表

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写在前面

初始化列表,旨在初始化的时候为属性赋值

码上建功

//先看一个最基本的构造函数,带初始化属性列表的
struct Person {
    int m_age;
    int m_height;

    Person() {
        this->m_age = 10;     //初始化赋值,只能用this获得属性,this类似于OC中的self
        this->m_height = 20;
    }
    void display() {
        cout << "m_age is " << this->m_age << endl;
        cout << "m_height is " << this->m_height << endl;
    }
};
运行一下
Person person;
person.display();
看下打印结果:
m_age is 10
m_height is 20

//初始化的时候给属性赋值
struct Person {
    int m_age;
    int m_height;
    // 初始化列表 :m_age(age), m_height(height)
    //用一个冒号隔开,前面是需要传入的参数,后面是要赋值的属性
    Person(int age, int height) :m_height(height), m_age(age)   {
     //对属性进行一些加工操作
    }
    void display() {
        cout << "m_age is " << this->m_age << endl;
        cout << "m_height is " << this->m_height << endl;
    }
};
使用
Person person2(15, 25);
person2.display();
打印结果
m_age is 15
m_height is 25

当然在初始化的时候也可以通过函数调用返回初始化列表的值

int myAge() {
    cout << "myAge()" << endl;
    return 30;
}

int myHeight() {
    cout << "myHeight()" << endl;
    return 180;
}
struct Person {
    int m_age;
    int m_height;
    // 初始化列表 :m_age(age), m_height(height)
    //用一个冒号隔开,前面是需要传入的参数,后面是要赋值的属性
    Person():m_height(myHeight()), m_age(myAge()) {
    
    }
    void display() {
        cout << "m_age is " << this->m_age << endl;
        cout << "m_height is " << this->m_height << endl;
    }
};
调用
Person person;
person.display();
打印结果:
myAge()
myHeight()
m_age is 30
m_height is 180

当然你也可以这样来初始化
struct Person {
    int m_age;
    int m_height;

    Person(int age, int height) {
     cout << "Person(int age, int height) " << this << endl;
     this->m_age = age;
     this->m_height = height;
     }

    void display() {
        cout << "m_age is " << this->m_age << endl;
        cout << "m_height is " << this->m_height << endl;
    }
};
调用
Person person2(15, 25);
person2.display();
打印
m_age is 15
m_height is 25

多个初始化列表方法时

struct Person {
    int m_age;
    int m_height;

    Person() :Person(0, 0) { }
    Person(int age, int height) :m_age(age), m_height(height) { }

    void display() {
        cout << "m_age is " << this->m_age << endl;
        cout << "m_height is " << this->m_height << endl;
    }
};
调用:
Person person;
person.display();
Person person2(15, 25);
person2.display();
打印结果:
m_age is 0
m_height is 0
m_age is 15
m_height is 25

如果函数声明和实现是分离的

struct Person {
    int m_age;
    int m_height;
    // 默认参数只能写在函数的声明中
    Person(int age = 0, int height = 0);
};

// 构造函数的初始化列表只能写在实现中
Person::Person(int age, int height) :m_age(age), m_height(height) {
    
}
调用
Person person;
person.display();
Person person2(10);
person2.display();
Person person3(20, 180);
person3.display();
打印结果:
m_age is 0
m_height is 0
m_age is 10
m_height is 0
m_age is 20
m_height is 180

装逼一下

一种便捷的初始化成员变量的方式 
只能用在构造函数中 
初始化顺序只跟成员变量的声明顺序有关
◼ 如果函数声明和实现是分离的 
初始化列表只能写在函数的实现中 
默认参数只能写在函数的声明中
 

完整代码demo,请移步GitHub:DDGLearningCpp