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A friend function
#include <iostream>
using namespace std;
class MyClass {
int a, b;
public:
MyClass(int i, int j) { a=i; b=j; }
friend int friendFunction(MyClass x); // a friend function
};
// friendFunction() is a not a member function of any class.
int friendFunction(MyClass x)
{
/* Because friendFunction() is a friend of MyClass, it can
directly access a and b. */
int max = x.a < x.b ? x.a : x.b;
return max;
}
int main()
{
MyClass n(18, 111);
cout << "friendFunction(n) is " << friendFunction(n) << "\n";
return 0;
}friendFunction(n) is 18
A function can be a member of one class and a friend of another
#include <iostream>
using namespace std;
class Cylinder; // a forward declaration
enum colors { red, green, yellow };
class Cube {
colors color;
public:
Cube(colors c) { color= c; }
bool sameColor(Cylinder y);
};
class Cylinder {
colors color;
public:
Cylinder(colors c) { color = c; }
friend bool Cube::sameColor(Cylinder y);
};
bool Cube::sameColor(Cylinder y) {
if(color == y.color)
return true;
else
return false;
}
int main()
{
Cube cube1(red);
Cube cube2(green);
Cylinder cyl(green);
if(cube1.sameColor(cyl))
cout << "cube1 and cyl are the same color.\n";
else
cout << "cube1 and cyl are different colors.\n";
if(cube2.sameColor(cyl))
cout << "cube2 and cyl are the same color.\n";
else
cout << "cube2 and cyl are different colors.\n";
return 0;
}cube1 and cyl are different colors.
cube2 and cyl are the same color.
Friend functions and operator overloading
#include <iostream>
#include <string>
using namespace std;
class MyClass
{
friend void Peek(const MyClass& aMyClass);
friend ostream& operator<<(ostream& os, const MyClass& aMyClass);
public:
MyClass(const string& name = "");
private:
string m_Name;
};
MyClass::MyClass(const string& name):
m_Name(name)
{}
void Peek(const MyClass& aMyClass);
ostream& operator<<(ostream& os, const MyClass& aMyClass);
int main()
{
MyClass myObject("A");
Peek(myObject);
cout << myObject;
return 0;
}
void Peek(const MyClass& aMyClass)
{
cout << aMyClass.m_Name << endl;
}
ostream& operator<<(ostream& os, const MyClass& aMyClass)
{
os << "MyClass Object - ";
os << "m_Name: " << aMyClass.m_Name;
return os;
}Friend functions can access private members of a class
#include <iostream>
using std::cout;
using std::endl;
class MyClass
{
friend void setX( MyClass &, int ); // friend declaration
public:
MyClass() : x( 0 )
{
}
void print() const
{
cout << x << endl;
}
private:
int x;
};
void setX( MyClass &c, int val )
{
c.x = val;
}
int main()
{
MyClass counter;
counter.print();
setX( counter, 8 );
counter.print();
return 0;
}0
8
Friend functions can be shared by two or more classes
#include <iostream>
using namespace std;
class Cylinder; // a forward declaration
enum colors { red, green, yellow };
class Cube {
colors color;
public:
Cube(colors c) { color = c; }
friend bool sameColor(Cube x, Cylinder y);
};
class Cylinder {
colors color;
public:
Cylinder(colors c) { color= c; }
friend bool sameColor(Cube x, Cylinder y);
};
bool sameColor(Cube x, Cylinder y)
{
if(x.color == y.color)
return true;
else
return false;
}
int main()
{
Cube cube1(red);
Cube cube2(green);
Cylinder cyl(green);
if(sameColor(cube1, cyl))
cout << "cube1 and cyl are the same color.\n";
else
cout << "cube1 and cyl are different colors.\n";
if(sameColor(cube2, cyl))
cout << "cube2 and cyl are the same color.\n";
else
cout << "cube2 and cyl are different colors.\n";
return 0;
}cube1 and cyl are different colors.
cube2 and cyl are the same color.
friend square() function for Distance
#include <iostream>
using namespace std;
class Distance{
private:
int feet;
float inches;
public:
Distance() : feet(0), inches(0.0){ }
Distance(int ft, float in) : feet(ft), inches(in){ }
void showdist(){
cout << feet << "\"-" << inches << "\"";
}
friend float square(Distance);
};
float square(Distance d){
float fltfeet = d.feet + d.inches/12;
float feetsqrd = fltfeet * fltfeet;
return feetsqrd;
}
int main(){
Distance dist(3, 6.0);
float sqft;
sqft = square(dist);
dist.showdist();
cout << sqft;
return 0;
}Share friend function between classes
#include<iostream.h>
class MyClassB;
class MyClassA
{
char *name;
public:
MyClassA(char *s){name=s;}
friend void print(MyClassA &,MyClassB &);
};
class MyClassB
{
char *name;
public:
MyClassB(char *s){name=s;}
friend void print(MyClassA &,MyClassB &);
};
void print(MyClassA &a,MyClassB &b)
{
cout<<"the MyClassA is"<<a.name<<endl;
cout<<"the MyClassB is"<<b.name<<endl;
}
int main()
{
MyClassA s("Li Hu");
MyClassB t("Wan Ping");
print(s,t);
}the MyClassA isLi Hu
the MyClassB isWan Ping
