What are the knowledge points of Youyuan in C++?
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First, the concept of friends:
1. What is a friend?
Friend is a kind of relationship in C++.
Friend relations occur between functions and classes or between classes.
Friendship is a single item and cannot be passed on.
2. The usage of friends:
Declare friends in a class with the friend keyword
The friend of a class can be another class or a concrete function
Friends are not part of the class.
Friends are not restricted by the access level in the class.
Friends can directly access all members of a specific class
3. The grammar of Youyuan:
Declare a function or class in a class using the friend keyword:
Class Test
{
Double x
Double y
Friend void func (Test& t)
}
Void func (Test& t)
{
}
Note: the func () global function is a friend of the Test class, and func () can access all members of the Test class, but func () is not a member function of Test.
Sample code:
# include
# include
Class Test
{
Double x
Double y
Public:
Test (double xjinghe double y)
{
This- > xerox
This- > yellowy
}
Double getX ()
{
Return x
}
Double getY ()
{
Return y
}
Friend double func (Test& T1 Test& T2)
}
Double func (Test& T1 Test& T2)
{
Double ret = 0
Ret = (t2.y-t1.y) * (t2.y-t1.y) + (t2.x-t1.x) * (t2.x-t1.x)
Ret = sqrt (ret)
Return ret
}
Int main ()
{
Test T1 (1Pol 2)
Test T2 (100.20)
Printf ("T1 (% fjore% f)\ n", t1.getX (), t1.getY ())
Printf ("T2 (% fjore% f)\ n", t2.getX (), t2.getY ())
Printf ("(T1 ~ T2) =% f\ n", func (T1 ~ T2))
Return 0
}
Output result:
Root@txp-virtual-machine:/home/txp/add# gathers + test.cpp
Root@txp-virtual-machine:/home/txp/add#. / a.out
T1 (1.0000000.000000)
T2 (10.000000520.000000)
(T1, T2) = 20.124612
4. The embarrassment of Youyuan:
Friends are born to take into account the efficiency of the c language.
Friends directly destroy the encapsulation of object-oriented.
The efficiency of Youyuan in the actual development of products outweighs the gain.
Youyuan has been gradually abandoned in software engineering.
5. Points for attention:
Friendly relations are not transitive.
The friend of a class can be a member function of another class.
The friend of a class can be a complete class
All member functions are friends
Code testing:
# include
Class ClassC
{
Const char* n
Public:
ClassC (const char* n)
{
This- > n = n
}
Friend class ClassB
}
Class ClassB
{
Const char* n
Public:
ClassB (const char* n)
{
This- > n = n
}
Void getClassCName (ClassC& c)
{
Printf ("c.n =% s\ n", c.n)
}
Friend class ClassA
}
Class ClassA
{
Const char* n
Public:
ClassA (const char* n)
{
This- > n = n
}
Void getClassBName (ClassB& b)
{
Printf ("b.n =% s\ n", b.n)
}
/ *
Void getClassCName (ClassC& c)
{
Printf ("c.n =% s\ n", c.n)
}
, /
}
Int main ()
{
ClassA A ("A")
ClassB B ("B")
ClassC C ("C")
A.getClassBName (B)
B.getClassCName (C)
Return 0
}
Output result:
Root@txp-virtual-machine:/home/txp/add#. / a.out
B. n = B
C. N = C
If you open the masked part of the code above, the compilation will report an error (because the friend is not transitive):
Root@txp-virtual-machine:/home/txp/add# gathers + test.cpp
Test.cpp: In member function 'void ClassA::getClassCName (ClassC&)':
Test.cpp:5:17: error: 'const char* ClassC::n' is private
Const char* n
^
Test.cpp:48:32: error: within this context
Printf ("c.n =% s\ n", c.n)
^
6. Summary:
Friends are born to take into account the efficiency of the c language.
Friends directly destroy the encapsulation of object-oriented.
Friendly relations are not transitive.
The friend of a class can be a member function of another class.
The friend of a class can be a completed class
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