C++深入探究類與對(duì)象之友元與運(yùn)算符重載

更新時(shí)間：2022年04月24日 15:07:17 作者：緣友一世

友元就是讓一個(gè)函數(shù)或者類，訪問(wèn)另一個(gè)類中的私有成員；打個(gè)比方，這相當(dāng)于是說(shuō)：朋友是值得信任的，所以可以對(duì)他們公開(kāi)一些自己的隱私，運(yùn)算符重載的實(shí)質(zhì)就是函數(shù)重載或函數(shù)多態(tài)，運(yùn)算符重載是一種形式的C++多態(tài)，目的在于讓人能夠用同名的函數(shù)來(lái)完成不同的基本操作

友元

生活中你的家有客廳(Public)，有你的臥室(Private),客廳所有來(lái)的客人都可以進(jìn)去，但是你的臥室是私有的，也就是說(shuō)只有你能進(jìn)去,但是呢，你也可以允許你的好閨蜜好基友進(jìn)去。

在程序里，有些私有屬性也想讓類外特殊的一些函數(shù)或者類進(jìn)行訪問(wèn)，就需要用到友元的技術(shù)。

友元的目的就是讓一個(gè)函數(shù)或者類訪問(wèn)另一個(gè)類中私有成員。

友元的關(guān)鍵字為: friend

友元的三種實(shí)現(xiàn)：

全局函數(shù)做友元
類做友元
成員函數(shù)做友元

1 全局函數(shù)做友元

class Building
{
	//告訴編譯器 goodGay全局函數(shù) 是 Building類的好朋友，可以訪問(wèn)類中的私有內(nèi)容
	friend void goodGay(Building * building);
public:
	Building()
	{
		this->m_SittingRoom = "客廳";
		this->m_BedRoom = "臥室";
	}
public:
	string m_SittingRoom; //客廳
private:
	string m_BedRoom; //臥室
};
void goodGay(Building * building)
{
	cout << "好基友正在訪問(wèn)： " << building->m_SittingRoom << endl;
	cout << "好基友正在訪問(wèn)： " << building->m_BedRoom << endl;
}
void test01()
{
	Building b;
	goodGay(&b);
}
int main(){
	test01();
	system("pause");
	return 0;
}

2 類做友元

class Building;
class goodGay
{
public:
	goodGay();
	void visit();
private:
	Building *building;
};
class Building
{
	//告訴編譯器 goodGay類是Building類的好朋友，可以訪問(wèn)到Building類中私有內(nèi)容
	friend class goodGay;
public:
	Building();
public:
	string m_SittingRoom; //客廳
private:
	string m_BedRoom;//臥室
};
Building::Building()
{
	this->m_SittingRoom = "客廳";
	this->m_BedRoom = "臥室";
}
goodGay::goodGay()
{
	building = new Building;
}
void goodGay::visit()
{
	cout << "好基友正在訪問(wèn)" << building->m_SittingRoom << endl;
	cout << "好基友正在訪問(wèn)" << building->m_BedRoom << endl;
}
void test01()
{
	goodGay gg;
	gg.visit();
}
int main(){
	test01();
	system("pause");
	return 0;
}

3 成員函數(shù)做友元

class Building;
class goodGay
{
public:
	goodGay();
	void visit(); //只讓visit函數(shù)作為Building的好朋友，可以發(fā)訪問(wèn)Building中私有內(nèi)容
	void visit2(); 
private:
	Building *building;
};
class Building
{
	//告訴編譯器  goodGay類中的visit成員函數(shù) 是Building好朋友，可以訪問(wèn)私有內(nèi)容
	friend void goodGay::visit();
public:
	Building();
public:
	string m_SittingRoom; //客廳
private:
	string m_BedRoom;//臥室
};
Building::Building()
{
	this->m_SittingRoom = "客廳";
	this->m_BedRoom = "臥室";
}
goodGay::goodGay()
{
	building = new Building;
}
void goodGay::visit()
{
	cout << "好基友正在訪問(wèn)" << building->m_SittingRoom << endl;
	cout << "好基友正在訪問(wèn)" << building->m_BedRoom << endl;
}
void goodGay::visit2()
{
	cout << "好基友正在訪問(wèn)" << building->m_SittingRoom << endl;
	//cout << "好基友正在訪問(wèn)" << building->m_BedRoom << endl;
}
void test01()
{
	goodGay  gg;
	gg.visit();
}
int main(){
	test01();
	system("pause");
	return 0;
}

運(yùn)算符重載

運(yùn)算符重載概念：對(duì)已有的運(yùn)算符重新進(jìn)行定義，賦予其另一種功能，以適應(yīng)不同的數(shù)據(jù)類型

1 加號(hào)運(yùn)算符重載

作用：實(shí)現(xiàn)兩個(gè)自定義數(shù)據(jù)類型相加的運(yùn)算

class Person {
public:
	Person() {};
	Person(int a, int b)
	{
		this->m_A = a;
		this->m_B = b;
	}
	//成員函數(shù)實(shí)現(xiàn) + 號(hào)運(yùn)算符重載
	Person operator+(const Person& p) {
		Person temp;
		temp.m_A = this->m_A + p.m_A;
		temp.m_B = this->m_B + p.m_B;
		return temp;
	}
public:
	int m_A;
	int m_B;
};
//全局函數(shù)實(shí)現(xiàn) + 號(hào)運(yùn)算符重載
//Person operator+(const Person& p1, const Person& p2) {
//	Person temp(0, 0);
//	temp.m_A = p1.m_A + p2.m_A;
//	temp.m_B = p1.m_B + p2.m_B;
//	return temp;
//}
//運(yùn)算符重載 可以發(fā)生函數(shù)重載 
Person operator+(const Person& p2, int val)  
{
	Person temp;
	temp.m_A = p2.m_A + val;
	temp.m_B = p2.m_B + val;
	return temp;
}
void test() {
	Person p1(10, 10);
	Person p2(20, 20);
	//成員函數(shù)方式
	Person p3 = p2 + p1;  //相當(dāng)于 p2.operaor+(p1)
	cout << "mA:" << p3.m_A << " mB:" << p3.m_B << endl;
	Person p4 = p3 + 10; //相當(dāng)于 operator+(p3,10)
	cout << "mA:" << p4.m_A << " mB:" << p4.m_B << endl;
}
int main() {
	test();
	system("pause");
	return 0;
}

總結(jié)1：對(duì)于內(nèi)置的數(shù)據(jù)類型的表達(dá)式的的運(yùn)算符是不可能改變的

總結(jié)2：不要濫用運(yùn)算符重載

2 左移運(yùn)算符重載

作用：可以輸出自定義數(shù)據(jù)類型

class Person {
	friend ostream& operator<<(ostream& out, Person& p);
public:
	Person(int a, int b)
	{
		this->m_A = a;
		this->m_B = b;
	}
	//成員函數(shù) 實(shí)現(xiàn)不了  p << cout 不是我們想要的效果
	//void operator<<(Person& p){
	//}
private:
	int m_A;
	int m_B;
};
//全局函數(shù)實(shí)現(xiàn)左移重載
//ostream對(duì)象只能有一個(gè)
ostream& operator<<(ostream& out, Person& p) {
	out << "a:" << p.m_A << " b:" << p.m_B;
	return out;
}
void test() {
	Person p1(10, 20);
	cout << p1 << "hello world" << endl; //鏈?zhǔn)骄幊?
}
int main() {
	test();
	system("pause");
	return 0;
}

總結(jié)：重載左移運(yùn)算符配合友元可以實(shí)現(xiàn)輸出自定義數(shù)據(jù)類型

3 遞增運(yùn)算符重載

作用：通過(guò)重載遞增運(yùn)算符，實(shí)現(xiàn)自己的整型數(shù)據(jù)

class MyInteger {
	friend ostream& operator<<(ostream& out, MyInteger myint);
public:
	MyInteger() {
		m_Num = 0;
	}
	//前置++
	MyInteger& operator++() {
		//先++
		m_Num++;
		//再返回
		return *this;
	}
	//后置++
	MyInteger operator++(int) {
		//先返回
		MyInteger temp = *this; //記錄當(dāng)前本身的值，然后讓本身的值加1，但是返回的是以前的值，達(dá)到先返回后++；
		m_Num++;
		return temp;
	}
private:
	int m_Num;
};
ostream& operator<<(ostream& out, MyInteger myint) {
	out << myint.m_Num;
	return out;
}
//前置++ 先++ 再返回
void test01() {
	MyInteger myInt;
	cout << ++myInt << endl;
	cout << myInt << endl;
}
//后置++ 先返回 再++
void test02() {
	MyInteger myInt;
	cout << myInt++ << endl;
	cout << myInt << endl;
}
int main() {
	test01();
	//test02();
	system("pause");
	return 0;
}

總結(jié)：前置遞增返回引用，后置遞增返回值

4 賦值運(yùn)算符重載

c++編譯器至少給一個(gè)類添加4個(gè)函數(shù)

默認(rèn)構(gòu)造函數(shù)(無(wú)參，函數(shù)體為空)
默認(rèn)析構(gòu)函數(shù)(無(wú)參，函數(shù)體為空)
默認(rèn)拷貝構(gòu)造函數(shù)，對(duì)屬性進(jìn)行值拷貝
賦值運(yùn)算符 operator=, 對(duì)屬性進(jìn)行值拷貝

如果類中有屬性指向堆區(qū)，做賦值操作時(shí)也會(huì)出現(xiàn)深淺拷貝問(wèn)題

示例：

class Person
{
public:
	Person(int age)
	{
		//將年齡數(shù)據(jù)開(kāi)辟到堆區(qū)
		m_Age = new int(age);
	}
	//重載賦值運(yùn)算符 
	Person& operator=(Person &p)
	{
		if (m_Age != NULL)
		{
			delete m_Age;
			m_Age = NULL;
		}
		//編譯器提供的代碼是淺拷貝
		//m_Age = p.m_Age;
		//提供深拷貝 解決淺拷貝的問(wèn)題
		m_Age = new int(*p.m_Age);
		//返回自身
		return *this;
	}
	~Person()
	{
		if (m_Age != NULL)
		{
			delete m_Age;
			m_Age = NULL;
		}
	}
	//年齡的指針
	int *m_Age;
};
void test01()
{
	Person p1(18);
	Person p2(20);
	Person p3(30);
	p3 = p2 = p1; //賦值操作
	cout << "p1的年齡為：" << *p1.m_Age << endl;
	cout << "p2的年齡為：" << *p2.m_Age << endl;
	cout << "p3的年齡為：" << *p3.m_Age << endl;
}
int main() {
	test01();
	//int a = 10;
	//int b = 20;
	//int c = 30;
	//c = b = a;
	//cout << "a = " << a << endl;
	//cout << "b = " << b << endl;
	//cout << "c = " << c << endl;
	system("pause");
	return 0;
}

5 關(guān)系運(yùn)算符重載

**作用：**重載關(guān)系運(yùn)算符，可以讓兩個(gè)自定義類型對(duì)象進(jìn)行對(duì)比操作

示例：

class Person
{
public:
	Person(string name, int age)
	{
		this->m_Name = name;
		this->m_Age = age;
	};
	bool operator==(Person & p)
	{
		if (this->m_Name == p.m_Name && this->m_Age == p.m_Age)
		{
			return true;
		}
		else
		{
			return false;
		}
	}
	bool operator!=(Person & p)
	{
		if (this->m_Name == p.m_Name && this->m_Age == p.m_Age)
		{
			return false;
		}
		else
		{
			return true;
		}
	}
	string m_Name;
	int m_Age;
};
void test01()
{
	//int a = 0;
	//int b = 0;
	Person a("孫悟空", 18);
	Person b("孫悟空", 18);
	if (a == b)
	{
		cout << "a和b相等" << endl;
	}
	else
	{
		cout << "a和b不相等" << endl;
	}
	if (a != b)
	{
		cout << "a和b不相等" << endl;
	}
	else
	{
		cout << "a和b相等" << endl;
	}
}
int main() {
	test01();
	system("pause");
	return 0;
}

6 函數(shù)調(diào)用運(yùn)算符重載

函數(shù)調(diào)用運(yùn)算符 () 也可以重載
由于重載后使用的方式非常像函數(shù)的調(diào)用，因此稱為仿函數(shù)
仿函數(shù)沒(méi)有固定寫法，非常靈活

示例：

class MyPrint
{
public:
	void operator()(string text)
	{
		cout << text << endl;
	}
};
void test01()
{
	//重載的（）操作符 也稱為仿函數(shù)
	MyPrint myFunc;
	myFunc("hello world");
}
class MyAdd
{
public:
	int operator()(int v1, int v2)
	{
		return v1 + v2;
	}
};
void test02()
{
	MyAdd add;
	int ret = add(10, 10);
	cout << "ret = " << ret << endl;
	//匿名對(duì)象調(diào)用  
	cout << "MyAdd()(100,100) = " << MyAdd()(100, 100) << endl;
}
int main() {
	test01();
	test02();
	system("pause");
	return 0;
}

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