C++/Class/Friend
Версия от 14:21, 25 мая 2010; (обсуждение)
Содержание
Define friend function for
#include <iostream>
#include <cstring>
#include <cstdlib>
using namespace std;
class StringClass {
char *p;
int len;
public:
StringClass(char *ptr);
~StringClass() {
delete [] p;
}
friend ostream &operator<<(ostream &stream, StringClass &ob);
};
StringClass::StringClass(char *ptr)
{
len = strlen(ptr)+1;
p = new char [len];
if(!p) {
cout << "Allocation error\n";
exit(1);
}
strcpy(p, ptr);
}
ostream &operator<<(ostream &stream, StringClass &ob)
{
stream << ob.p;
return stream;
}
int main()
{
StringClass stringObject1("www.java2s.com"), stringObject2("www.java2s.com");
cout << stringObject1;
cout << endl << stringObject2 << endl;
return 0;
}
friend class for each other
#include <iostream>
#include <string.h>
using namespace std;
class Curly
{
public:
Curly(char *msg) { strcpy(message, msg); };
void show_message(void) { cout << message << endl; };
friend class Moe;
void show_moe(class Moe moe);
private:
char message[256];
};
class Moe
{
public:
Moe(char *msg) { strcpy(message, msg); };
void show_message(void) { cout << message << endl; };
friend class Curly;
void show_curly(class Curly curly);
private:
char message[256];
};
void Curly::show_moe(class Moe moe) { cout << moe.message << endl; };
void Moe::show_curly(class Curly curly) { cout << curly.message << endl; };
int main(void)
{
class Moe moe("nuck...");
class Curly curly("whoop...");
moe.show_message();
moe.show_curly(curly);
curly.show_message();
curly.show_moe(moe);
}
Friend function Demo
#include <iostream>
using namespace std;
class myclass {
int num;
public:
myclass(int x) {
num = x;
}
friend int isneg(myclass ob);
};
int isneg(myclass ob)
{
return (ob.num < 0) ? 1 : 0;
}
int main()
{
myclass a(-1), b(2);
cout << isneg(a) << " " << isneg(b);
cout << endl;
return 0;
}
Friends can access private members of a class.
#include <iostream>
using std::cout;
using std::endl;
class Count {
friend void setX( Count &, int );
public:
Count() { x = 0; }
void print() const { cout << x << endl; }
private:
int x;
};
void setX( Count &c, int val )
{
c.x = val; // legal: setX is a friend of Count
}
int main()
{
Count counter;
cout << "counter.x after instantiation: ";
counter.print();
cout << "counter.x after call to setX friend function: ";
setX( counter, 8 ); // set x with a friend
counter.print();
return 0;
}
Make the inserter into a friend function
#include <iostream>
#include <cstring>
using namespace std;
class PhoneNumber {
char name[80];
int areaCode;
int prefix;
int num;
public:
PhoneNumber(char *n, int a, int p, int nm)
{
strcpy(name, n);
areaCode = a;
prefix = p;
num = nm;
}
friend ostream &operator<<(ostream &stream, PhoneNumber o);
};
// Display name and phone number.
ostream &operator<<(ostream &stream, PhoneNumber o)
{
stream << o.name << " ";
stream << "(" << o.areaCode << ") ";
stream << o.prefix << "-" << o.num << "\n";
return stream; // must return stream
}
int main()
{
PhoneNumber a("T", 111, 555, 1234);
PhoneNumber b("A", 312, 555, 5768);
PhoneNumber c("T", 212, 555, 9991);
cout << a << b << c;
return 0;
}
Use friend function to access the non-public member variable
#include <iostream>
using namespace std;
class MyClass {
int a, b;
public:
friend int sum(MyClass x);
void set_ab(int i, int j);
};
void MyClass::set_ab(int i, int j)
{
a = i;
b = j;
}
int sum(MyClass object)
{
return object.a + object.b;
}
int main(void)
{
MyClass integer;
cout << "Adding 3 and 4:" << endl;
integer.set_ab(3,4);
cout << sum(integer);
}