[C++ Elementary] analysis of ostream&, operator<<, operator<<(ostream& _cout, const Date& d), bool-guided structure embedded comparison function

 Sort out the knowledge of C++ overloading and judgment 

Table of contents

1. bool operator>(const Date& d) const [bool-guided structure embedded comparison function]

1. The overall meaning of the code

2. The meaning of the two const in the code

二、friend ostream& operator<<(ostream& _cout, const Date& d)

1. Related basic concepts 

2. Three references &

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The problem comes from the following date class

class Date
{
public:
	Date(int year = 1900, int month = 1, int day = 1)
		: _year(year)
		, _month(month)
		, _day(day)
	{}
	bool operator<(const Date& d)const
	{
		return (_year < d._year) ||
			(_year == d._year && _month < d._month) ||
			(_year == d._year && _month == d._month && _day < d._day);
	}
	bool operator>(const Date& d)const
	{
		return (_year > d._year) ||
			(_year == d._year && _month > d._month) ||
			(_year == d._year && _month == d._month && _day > d._day);
	}
	friend ostream& operator<<(ostream& _cout, const Date& d)
	{
		_cout << d._year << "-" << d._month << "-" << d._day;
		return _cout;
	}
private:
	int _year;
	int _month;
	int _day;
};



int main()
{
    Date date;
    cout<<date<<endl;
}

1. bool operator>(const Date& d) const [bool-guided structure embedded comparison function]

1. The overall meaning of the code

We can intercept a piece of code for analysis

bool operator>(const Date& d) const

The > overload of our common date class has the following form. It can be seen from observation that the > and < overload codes mentioned at the beginning are only derivatives of the following codes, and the essence has not changed

bool operator> (const Date& d) const
{
	if (_year > d._year ||
		_year == d._year && _month > d._month ||
		_year == d._year && _month == d._month && _day > d._day) 
		return true;
	else
		return false;
}

2. The meaning of the two const in the code

• The first const is to prevent the referenced Date parameter from being modified;
• The second const is a const member function. As for why it is needed, please refer to my other article.

const member function https://blog.csdn.net/Captain_ldx/article/details/127334021

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二、friend ostream& operator<<(ostream& _cout, const Date& d)

Next we explain this code 

friend ostream& operator<<(ostream& _cout, const Date& d)

1. Related basic concepts 

First of all, we need to know the relevant basic concepts

• ostream is the abbreviation of outputstream, that is, output stream. Used for output in C++, a typical output stream object is our common cout. In C++, you rarely customize the ostream object, and more often use cout directly.
• The type of ostream usually appears in operator overloading such as <<, and appears as a friend function of a certain class. Taking the above code as an example, if you call an object of class Date (assuming I named it val), cout<<val is equivalent to calling this function, which usually outputs some member variables of Date.
• friend Modified friend function
• operator overloaded operator, the above code overloaded <<

2. Three references &

The above 3 references of ostream& operator<<(ostream& _cout, const Date& date) &

1. The first ostream& returns a reference to an ostream class object. In order to perform continuous operations , such as cout<<a<<b; first perform cout<<a, and after the operation is complete, return the reference to cout, that is, return cout In itself, the original formula becomes cout<<b; so that it can be written continuously, cout<<a<<b<<c<<.....

2. The second ostream& _cout, _cout is a random name, it is the object of the ostream class, and the usage is the same as cout, its purpose is: you can pass the data to _cout through <<, and finally use _cout as a parameter outgoing. So use cout<<date (date is an object of the date class) when outputting, and then call operator<<(_cout,d) to get the value of d (here d is the date data in the example), and finally Pass cout as a parameter.

For the code _cout<<date<<endl; in the beginning example , it is actually executed:

		_cout << d._year << "-" << d._month << "-" << d._day;
		return _cout;

 The above code is a derivative of the code shown below, and the difference is only due to the different personal code styles. The specific meaning is the same.

    cout<<date.year<<'-'<<date.month<<'-' <<date.day; 
    return cout；

3. The third const Date& is mainly for quick entry and exit of parameters . If & is not used, a parameter will be copied instead of passing in the original object directly, and the copy constructor will be called (if not written, the compiler will automatically provide it) , wasting time and sometimes producing errors.

For example, when there is a pointer in the data member, the copy constructor provided by the compiler will automatically copy a pointer, pointing to the same address as the original pointer, so that changing the address of one will change the other, which is what we don’t want .

When we need it, we need to copy a new pointer to point to a different address. The two objects do not interfere with each other, as long as the values ​​of the variables pointed to by the pointer are equal, we need to write our own copy constructor, new and so on. So generally use &, add const to prevent accidental modification.

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