Classes and objects (3)

Catalog

1. operators overloading

1.1  Assignment operator overload

1.2  == operators overloading

1.3  != operators overloading

1.4 < operators overloading

1.5  <=  operators overloading

1.6 > The operator

 1.7 >= The operator

1.8 In front of ++ And post ++

1.9 In front of -- And post --

2. Write a Date class

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1. operators overloading

C++ In order to enhance the readability of the code, operator overloading is introduced , Operator overloading is a function with a special function name , Also has its return value class type , Function name and parameter list , Its return value type and parameter list are similar to ordinary functions .

The function name is ： keyword operator Followed by operator symbols that need to be overloaded .

The function prototype ： return type  operator The operator ( parameter list )

Here we use Date Class to do the following

class Date
{
public:

  // Overload the operator 
  //...  
private:
	int _year;
	int _month;
	int _day;
};

Be careful ：

1. You can't create a new operator by connecting other symbols ： such as operator ^ 、[email protected] etc. , Only existing operators can be overloaded .

2. .* 、::( Scope accessor ) 、sizeof() 、?:( ternary operators ) 、.( spot ) above 5 Operators cannot be overloaded .

1.1  Assignment operator overload

Be careful ：

1. Assignment operator Can only As class

When the assignment operator is not explicitly implemented in a class , The compiler will generate a default , At this time, the user overloads the assignment operator outside the class , It conflicts with the default assignment operator generated by the compiler , Therefore, the assignment operator can only Overloaded into member functions .

2. Assignment operators are not equivalent to copy constructors

Is this a call to the assignment operator or a copy construct ？

  We found that we ended up in the copy constructor .

It shows that when the object is initialized, the operator called by the assignment operator is not the overloaded operator , Instead, call the copy constructor ,

The assignment operator is called again only after the object has been initialized , Will use the overloaded operator .

Then how can we achieve ？

Date& operator=(const Date& d) {
		if (this != &d) {
			_year = d._year;
			_month = d._month;
			_day = d._day;
			return *this;
		}
	}

  Here we also use the parameter with reference and return value with reference learned in the previous section .

Return value with reference Even if there is a function, it still exists , therefore Return alias Don't worry about illegal access .

Parameter with reference Is to avoid the infinite recursion problem .

1.2  == operators overloading

In built-in types == Is to compare the values in the two sides , The same is true in custom types , However, custom types cannot be compared directly like built-in types , Because there are built-in types and user-defined types in user-defined types , In the use of == when , It needs to be overloaded .

bool operator==(const Date& d) {
		return _year == d._year && _month == d._month && _day == d._day;
	}

1.3  != operators overloading

!= The operator , When the two sides are not equal, return true, When both sides are equal, return false, Isn't that what == The opposite value of the operator

bool operator != (const Date& d) {
		return !(*this == d);
	}

Here we use the function Principle of reusability , It greatly reduces the amount of our code and makes the code clearer .

1.4 < operators overloading

Similarly, when all member variables in a custom type are < return true, Otherwise return to false

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

1.5  <=  operators overloading

As long as it is less than or equal to any one, it returns true, Otherwise return to false

bool operator <= (const Date& d) {
		return (*this<d)||(*this==d)
	}

1.6 > The operator

> The antonym of is <=, Also take advantage of reusability

bool operator>(const Date& d) {
	 return !(*this<=d);
}

 1.7 >= The operator

We can use >= The antonym of is < To write , You can also use > and = Meet a condition to write

 bool operator >= (const Date& d) {
        //return (*this>d)||(*this==d);
		return !(*this < d);
	}

1.8 In front of ++ And post ++

In the face of ++ When the operator is overloaded , In the form of ：operator++(), How do we distinguish when to use prepositions ++, When to use post ++ Well ？

For pre ++ Do not write parameters , In the form of ：operator++()

For post ++ Write parameters , In the form of ：operator++(int i)

The type of parameter is well specified , We just need to do it , Don't think too much .

Let's first look at ++

Date& operator++() {
		*this += 1;
		return *this;
	}

In front of ++ We just need to remember first ++ Reuse

After ++

Date operator++(int i) {
		Date ret(*this);
		*this += 1;
		return ret;
	}

First create an object to copy the current value , And then again ++, Return the created temporary object .

1.9 In front of -- And post --

And 1.8 Empathy , Attach code

//  In front of --

	Date& operator++() {
		*this -= 1;
		return *this;
	}



	//  After --

	Date operator--(int i) {
		Date ret(*this);
		*this -= 1;
		return ret;
	}

If the return value can have & best , Because there can be one less copy step , The rate of increase , Therefore, the belt can be used & Use on return &.

2. Write a Date class

I learned so much about operator overloading and previous related knowledge , We can write a completed class

class Date
{

public:

	// Judgement of leap year 
	bool judge(int year) {
		if (year % 400 == 0 || (year % 4 == 0 && year % 100 != 0)) {
			return true;
		}
		return false;
	}
	//  Gets the number of days in a month of a year 
	int GetMonthDay(int year, int month) {
		int MonthDayArrays[13] = { 0,31,28,31,30,31,30,31,31,30,31,30,31 };
		if (month == 2 && judge(year)) {
			return 29;
		}
		return MonthDayArrays[month];
	}

	//  All default constructors 

	Date(int year = 1900, int month = 1, int day = 1) {
		if (year >= 1 && month >= 1 && month <= 12 &&
			day >= 1 && day <= GetMonthDay(year, month)) {
			_year = year;
			_month = month;
			_day = day;
		}
	}

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

	//  copy constructor 

  // d2(d1)

	Date(const Date& d) {
		_year = d._year;
		_month = d._month;
		_day = d._day;
	}



	//  Assignment operator overload 

  // d2 = d3 -> d2.operator=(&d2, d3)

	Date& operator=(const Date& d) {
		if (this != &d) {
			_year = d._year;
			_month = d._month;
			_day = d._day;
			return *this;
		}
	}



	//  Destructor 

	//~Date();


	//  date += Days 

	Date& operator+=(int day) {
		if (day < 0) {
			return *this -= -day;
		}
		_day += day;
		while (_day > GetMonthDay(_year, _month)) {
			_day -= GetMonthDay(_year, _month);
			_month++;
			if (_month == 13) {
				_year++;
				_month = 1;
			}
		}
		return *this;
	}

	//  date + Days 

	Date operator+(int day) const { 
		Date ret(*this);
		ret += day;
		return ret;
	}

	//  date - Days 

	Date operator-(int day) const {
		Date ret(*this);
		ret -= day;
		return ret;
	}

	//  date -= Days 

	Date& operator-=(int day) {
		if (day < 0) {
			return *this += -day;
		}
		_day -= day;
		while (_day <= 0) {
			--_month;
			if (_month == 0) {
				_year--;
				_month = 12;
			}
			_day += GetMonthDay(_year, _month);
		}
		return *this;
	}

	// > Operator overloading 

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

	// == Operator overloading 

	bool operator==(const Date& d) {
		return _year == d._year && _month == d._month && _day == d._day;
	}

	// >= Operator overloading 

	inline bool operator >= (const Date& d) {
		return !(*this < d);
	}

	// < Operator overloading 

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

	// <= Operator overloading 

	bool operator <= (const Date& d) {
		return !(*this>d);
	}

	// != Operator overloading 

	bool operator != (const Date& d) {
		return !(*this == d);
	}
	//  date - date   Return days 

	int operator-(const Date& d) {
		int temp = 0;
		Date max = d;
		Date min = *this;
		int flag = -1;
		if (*this>d)
		{
			max = *this;
			min = d;
			flag = 1;
		}
		for (int i = min._year; i < max._year; i++) {
			temp += judge(i) ? 366 : 365;
		}
		for (int i = 1; i < max._month; i++) {
			temp += GetMonthDay(d._year, i);
		}
		for (int i = 1; i < min._month; i++) {
			temp -= GetMonthDay(_year, i);
		}
		temp += max._day - min._day;
		return temp*flag;
	}
private:
	int _year;
	int _month;
	int _day;

};

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Complete a small exercise ！！