18-初始化列表

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](https://github.com/dudongge/DDGLearningCpp)