Constructor of member function

Catalog

The concept of constructors ：

The properties of constructors ：

There can only be one constructor ：

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The concept of constructors ：

A constructor is a special member function , Same name as class name , It is automatically called by the compiler when creating class type objects , Ensure that each data member has an appropriate initial value , And only called once in the object's life cycle .

The properties of constructors ：

（1） The function name is the same as the class name .

（2） No return value .

（3） The compiler automatically calls the corresponding constructor .

（4） Constructor can overload .

Let's take an example directly here

#include<iostream>
using namespace std;
class Data
{
public:
	Data()
	{
		cout << "Date()" << this << endl;
	}
	void InitData(int year = 1, int month = 1, int day = 1)
	{
		_year = year;
		_month = month;
		_day = day;
	}
	void PrintfData()
	{
		cout << _year << "/" << _month << "/" << _day << endl;
	}
private:
	int _year;
	int _month;
	int _day;
};
int main()
{
	Data d1,d2;
	d1.InitData(2022,5,21);
	d1.PrintfData();
	return 0;
}

So the result is ：

There can only be one constructor ：

Parameterless constructors and fully default constructors are called default constructors , And the default constructor can only have one .

Let's take a wrong case ：

#include<iostream>
using namespace std;
class Data
{
public:
	Data()
	{
		cout << "Date()" << this << endl;
	}
Data()
	{
		_year = year;
		_month = month;
		_day = day;
	}
	void InitData(int year = 1, int month = 1, int day = 1)
	{
		_year = year;
		_month = month;
		_day = day;
	}
	void PrintfData()
	{
		cout << _year << "/" << _month << "/" << _day << endl;
	}
private:
	int _year;
	int _month;
	int _day;
};
int main()
{
	Data d1
	return 0;
}

In the above code , There are two default constructors , Because constructors without parameters and full default constructors are considered as default constructors , So , Now there are two constructors , The compiler doesn't know which constructor to call , So , You're going to report a mistake , So we can delete one .

About compiler generated default member functions , A lot of people have doubts ： In the case that we don't implement constructors , The compiler generates the default constructor . But it seems that the default constructor is useless ？ Object calls the compiler generated default constructor , But the object year/month/_day, Still random . In other words, the default constructor generated by the compiler is useless here ？
answer ：C++ Divide types into built-in types ( Basic types ) And custom types . A built-in type is a type that has been defined by the syntax ： Such as int/char..., Custom type is what we use class/struct/union Self defined type , Look at the following program , I found that the compiler will generate a default constructor for custom type members _t Call its default member function
 

class Time
{
public:
	Time()
	{
		cout << "Time()" << endl;
		_hour = 0;
		_minute = 0;
		_second = 0;
	}
private:
	int _hour;
	int _minute;
	int _second;
};
class Date
{
private:
	//  Basic types ( Built in type )
	int _year;
	int _month;
	int _day;
	//  Custom type 
	Time _t;
};
int main()
{
	Date d;
	return 0;
}

What does that mean , The compiler doesn't care int,char This basic type , Instead, I will ignore custom types

This is the result of the output