Classes and Objects: Medium

类的6个默认成员函数

• 如果一个类中什么成员都没有,简称为空类.
• 空类中真的什么都没有吗？并不是,任何类在什么都不写时,编译器会自动生成以下6个默认成员函数.

默认成员函数：用户没有显式实现,编译器Member functions that are automatically generatedCalled the default member function.

构造函数

• 构造函数是特殊的成员函数,需要注意的是,构造函数虽然名称叫构造,但是构造函数的主要任务并不是开空间创建对象,而是初始化对象.

构造函数的特性 

1. 函数名与类名相同.
2. No concept of return value
3. 对象实例化时编译器自动调用对应的构造函数.
4. 构造函数可以重载.
5. It can also be called without passing parameters

#include<iostream>
using namespace std;
typedef int DataType;
class Date
{
public:
	void Init(int year, int month, int day)
	{
		_year = year;
		_month = month;
		_day = day;
	}

	void Print()
	{
		cout << _year << "-" << _month << "-" << _day << endl;
	}

private:
	int _year;
	int _month;
	int _day;
};

int main()
{
	Date A;
	A.Init(2022,7,27);

	A.Print();
	return 0;
}

• 在使用类的过程中,我们常常需要对类进行初始化,Initializing the class yourself is a bit cumbersome,And we risk forgetting to initialize,C++The default constructor appears in

---

#include<iostream>
using namespace std;
typedef int DataType;
class Date
{
public:
	Date()
	{
		_year = 2022;
		_month = 7;
		_day = 27;
	}

	Date(int year, int month, int day)
	{
		_year = year;
		_month = month;
		_day = day;
	}

	void Print()
	{
		cout << _year << "-" << _month << "-" << _day << endl;
	}

private:
	int _year;
	int _month;
	int _day;
};

int main()
{
	Date A;

	A.Print();
	return 0;
}

• 创建Aobject is initialized,You can also pass a value to initialize it yourself,

---

#include<iostream>
using namespace std;
typedef int DataType;
class Date
{
public:
	Date()
	{
		_year = 1900;
		_month = 1;
		_day = 1;
	}
	Date(int year = 1900, int month = 1, int day = 1)
	{
		_year = year;
		_month = month;
		_day = day;
	}
private:
	int _year;
	int _month;
	int _day;
};

int main()
{
	Date d1;
	return 0;
}

•  Compilation will not pass here,无参的构造函数,半缺省/全缺省的构造函数,Default constructors are called constructors,它们Only one of them can appear,

---

#include<iostream>
using namespace std;
typedef int DataType;
class Date
{
public:
	Date(int year, int month, int day)
	{
		_year = year;
		_month = month;
		_day = day;
	}
private:
	int _year;
	int _month;
	int _day;
};

int main()
{
	Date d1;
	return 0;
}

•  This is the wrong use of constructors,没有合适的默认构造函数可用

关于编译器生成的默认成员函数 

#include<iostream>
using namespace std;
typedef int DataType;
class Time
{
public:
	/*Time()
	{
		cout << "Time()" << endl;
		_hour = 0;
		_minute = 0;
		_second = 0;
	}*/
private:
	int _hour;
	int _minute;
	int _second;
};

class Date
{
private:
	// 基本类型(内置类型)
	int _year;
	int _month;
	int _day;
	// 自定义类型
	Time _t;
};

int main()
{
	Date d;
	return 0;
}

• 在C++The default generated constructor is specified in
  • a：内置类型成员不做处理
  • b：The custom type member goes back and calls his default constructor
• C++This design in can cause a problem,
  • For example in this code,生成d1will call its default constructor,而_year和_month和_day都是内置类型,只用_t是自定义类型,
  • 对于_tThis object will then call its default constructor,_hour和 _minute和 _second都是内置类型,But because of内置类型成员不做处理.As a result, it is still a random value after the default constructor is finally called

---

 

• C++11 中针对内置类型成员不初始化的缺陷,又打了补丁,即：内置类型成员变量在类中声明时可以给默认值.
• 注意：The default value given is not initialization,is the default value given