Youyuan, youyuan

Youyuan, youyuan
Zookeeper

1. youyuan: C ++ controls the access to the private part of the class object, but sometimes it needs to be a private member of the class's external member class. In this case, C ++ provides a youyuan mechanism.

 To create a friend function:

A: place the function declaration in the class declaration, and add the keyword friend before the prototype.

Function Name (parameter list) returned by friend );

Friend classclassname;

B: Write a friend function definition. You do not need to use the keyword "friend" in the definition.

Friend int youyuan function name (const classname & );

# Include <iostream>

 

Usingnamespacestd;

 

Classdemo

{

Private:

Inti; // can only be accessed within the class

Public:

Demo () // Constructor

{

Cout <"demo" <this <endl;

}

Demo (constdemo & it) // copy Structure

{

// Once you have implemented the copy constructor, You need to assign values between class members. The Compiler does not care.

This-> I = it. I;

Cout <"copydemo" <this <endl;

}

~ Demo ()

{

Cout <"~ Demo "<this <endl;

}

Voidset_ I (inti)

{

This-> I = I;

}

Intget_ I ()

{

Returni;

}

// Declares a friend function of this class. This function can call the information of this class.

Friendvoidtest ();

// Declares a friend of this class. The information of this class can be called by the following class.

Friendclassdemo1;

// Demo and demo0 are mutual friends

Friendclassdemo0;

};

 

Voidtest ()

{

// Because the above

Demod;

// Here, d. I is the private variable in the demo. Because youyuan is used, test can be used.

D. I = 100;

Printf ("% d \ n", d. I );

}

 

Classdemo0

{

Private:

Inti;

Public:

Demo0 ()

{

Demod;

D. I = 123;

// 123 is printed, indicating that the class in youyuan can contain the variable in the category class.

Printf ("% d \ n", d. I );

}

};

 

Classdemo1

{

Public:

Demo1 ()

{

Demod;

D. I = 144;

Printf ("% d \ n", d. I );

}

Friendclassdemo2;

};

 

// The following example shows that a friend of mine is not a friend of mine.

Classdemo2

{

Public:

Demo2 ()

{

Demod;

// This sentence cannot be opened. If it is released, an error is returned.

// D. I = 155;

}

};

 

Intmain (void)

{

Test ();

 

Demo0d0;

 

Demo1d1;

 

// An error is reported during the following operation, indicating a feature of youyuan: My friends are not my friends.

// Demo2d2;

 

Return0;

}

2. Run:

3. youyuan relationship description

 

How do I use the friend functions?

You can use the private member variable and member function of an object of the sort class. The Code is as follows:
Class TestClass
{
Private:
Int length;
Int width;

Public:
TestClass (int len, int wide) {length = len; width = wide ;};
Friend int CaculateArea (TestClass test); // Add the keyword friend.
Friend int CaculateCircle (TestClass test );
};
Int CaculateArea (TestClass test) // you can access the private member variable length and width of the test object.
{
Return test. length * test. width;
}
Int CaculateCircle (TestClass test)
{
Return 2 * (test. length + test. width );
}

Int main ()
{
TestClass testClass (2, 3 );
Cout <CaculateArea (testClass) <endl;
Cout <CaculateCircle (testClass) <endl;
Return 0;

}
 
How do I use the friend functions?

You can use the private member variable and member function of an object of the sort class. The Code is as follows:
Class TestClass
{
Private:
Int length;
Int width;

Public:
TestClass (int len, int wide) {length = len; width = wide ;};
Friend int CaculateArea (TestClass test); // Add the keyword friend.
Friend int CaculateCircle (TestClass test );
};
Int CaculateArea (TestClass test) // you can access the private member variable length and width of the test object.
{
Return test. length * test. width;
}
Int CaculateCircle (TestClass test)
{
Return 2 * (test. length + test. width );
}

Int main ()
{
TestClass testClass (2, 3 );
Cout <CaculateArea (testClass) <endl;
Cout <CaculateCircle (testClass) <endl;
Return 0;

}