生活中你的家有客厅(Public)，有你的卧室(Private),客厅所有来的客人都可以进去，但是你的卧室是私有的，也就是说只有你能进去,但是呢，你也可以允许你的好闺蜜好基友进去。
在程序里，有些私有属性也想让类外特殊的一些函数或者类进行访问，就需要用到友元的技术。
友元的目的就是让一个函数或者类访问另一个类中私有成员。
友元的关键字为: friend
友元的三种实现：
class Building
{
//告诉编译器 goodGay全局函数 是 Building类的好朋友，可以访问类中的私有内容
friend void goodGay(Building * building);
public:
Building()
{
this->m_SittingRoom = "客厅";
this->m_BedRoom = "卧室";
}
public:
string m_SittingRoom; //客厅
private:
string m_BedRoom; //卧室
};
void goodGay(Building * building)
{
cout << "好基友正在访问： " << building->m_SittingRoom << endl;
cout << "好基友正在访问： " << building->m_BedRoom << endl;
}
void test01()
{
Building b;
goodGay(&b);
}
int main(){
test01();
system("pause");
return 0;
}class Building;
class goodGay
{
public:
goodGay();
void visit();
private:
Building *building;
};
class Building
{
//告诉编译器 goodGay类是Building类的好朋友，可以访问到Building类中私有内容
friend class goodGay;
public:
Building();
public:
string m_SittingRoom; //客厅
private:
string m_BedRoom;//卧室
};
Building::Building()
{
this->m_SittingRoom = "客厅";
this->m_BedRoom = "卧室";
}
goodGay::goodGay()
{
building = new Building;
}
void goodGay::visit()
{
cout << "好基友正在访问" << building->m_SittingRoom << endl;
cout << "好基友正在访问" << building->m_BedRoom << endl;
}
void test01()
{
goodGay gg;
gg.visit();
}
int main(){
test01();
system("pause");
return 0;
}class Building;
class goodGay
{
public:
goodGay();
void visit(); //只让visit函数作为Building的好朋友，可以发访问Building中私有内容
void visit2();
private:
Building *building;
};
class Building
{
//告诉编译器 goodGay类中的visit成员函数 是Building好朋友，可以访问私有内容
friend void goodGay::visit();
public:
Building();
public:
string m_SittingRoom; //客厅
private:
string m_BedRoom;//卧室
};
Building::Building()
{
this->m_SittingRoom = "客厅";
this->m_BedRoom = "卧室";
}
goodGay::goodGay()
{
building = new Building;
}
void goodGay::visit()
{
cout << "好基友正在访问" << building->m_SittingRoom << endl;
cout << "好基友正在访问" << building->m_BedRoom << endl;
}
void goodGay::visit2()
{
cout << "好基友正在访问" << building->m_SittingRoom << endl;
//cout << "好基友正在访问" << building->m_BedRoom << endl;
}
void test01()
{
goodGay gg;
gg.visit();
}
int main(){
test01();
system("pause");
return 0;
}运算符重载概念：对已有的运算符重新进行定义，赋予其另一种功能，以适应不同的数据类型
作用：实现两个自定义数据类型相加的运算
class Person {
public:
Person() {};
Person(int a, int b)
{
this->m_A = a;
this->m_B = b;
}
//成员函数实现 + 号运算符重载
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;
};
//全局函数实现 + 号运算符重载
//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;
//}
//运算符重载 可以发生函数重载
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);
//成员函数方式
Person p3 = p2 + p1; //相当于 p2.operaor+(p1)
cout << "mA:" << p3.m_A << " mB:" << p3.m_B << endl;
Person p4 = p3 + 10; //相当于 operator+(p3,10)
cout << "mA:" << p4.m_A << " mB:" << p4.m_B << endl;
}
int main() {
test();
system("pause");
return 0;
}总结1：对于内置的数据类型的表达式的的运算符是不可能改变的
总结2：不要滥用运算符重载
作用：可以输出自定义数据类型
class Person {
friend ostream& operator<<(ostream& out, Person& p);
public:
Person(int a, int b)
{
this->m_A = a;
this->m_B = b;
}
//成员函数 实现不了 p << cout 不是我们想要的效果
//void operator<<(Person& p){
//}
private:
int m_A;
int m_B;
};
//全局函数实现左移重载
//ostream对象只能有一个
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; //链式编程
}
int main() {
test();
system("pause");
return 0;
}总结：重载左移运算符配合友元可以实现输出自定义数据类型
作用： 通过重载递增运算符，实现自己的整型数据
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; //记录当前本身的值，然后让本身的值加1，但是返回的是以前的值，达到先返回后++；
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;
}总结： 前置递增返回引用，后置递增返回值
c++编译器至少给一个类添加4个函数
如果类中有属性指向堆区，做赋值操作时也会出现深浅拷贝问题
示例：
class Person
{
public:
Person(int age)
{
//将年龄数据开辟到堆区
m_Age = new int(age);
}
//重载赋值运算符
Person& operator=(Person &p)
{
if (m_Age != NULL)
{
delete m_Age;
m_Age = NULL;
}
//编译器提供的代码是浅拷贝
//m_Age = p.m_Age;
//提供深拷贝 解决浅拷贝的问题
m_Age = new int(*p.m_Age);
//返回自身
return *this;
}
~Person()
{
if (m_Age != NULL)
{
delete m_Age;
m_Age = NULL;
}
}
//年龄的指针
int *m_Age;
};
void test01()
{
Person p1(18);
Person p2(20);
Person p3(30);
p3 = p2 = p1; //赋值操作
cout << "p1的年龄为：" << *p1.m_Age << endl;
cout << "p2的年龄为：" << *p2.m_Age << endl;
cout << "p3的年龄为：" << *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;
}**作用：**重载关系运算符，可以让两个自定义类型对象进行对比操作
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("孙悟空", 18);
Person b("孙悟空", 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;
}示例：
class MyPrint
{
public:
void operator()(string text)
{
cout << text << endl;
}
};
void test01()
{
//重载的（）操作符 也称为仿函数
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;
//匿名对象调用
cout << "MyAdd()(100,100) = " << MyAdd()(100, 100) << endl;
}
int main() {
test01();
test02();
system("pause");
return 0;
}
