Friends, static keywords, static methods, and relationships between objects

1 Friends

Friends friend The mechanism allows a class to authorize other functions to access its non-public members .
The friend declaration takes the keyword friend start , It can only appear in class declarations , They are not affected by their public private and protected Impact of zone .
Friends are divided into external function friends , Member function friend , Like friends .
Characteristics of friends :

1. Without symmetry : A yes B Friends , It doesn't mean that B yes A Friends .
2. It's not transitive : A yes B Friends ,B yes C Friends , but A No C Friends .
3. Not inherited : Base Type inheritance Object type , If Object The type is A Friends , but Base The type is not A Friends .

1.1 External friend function ：

If an external function wants to access private members of the an internal function , Must be defined as Friends form .

If you want to access in the main function value Value ,

The treatment is as follows ：
5. You can add... To a class value Property value of is set to public ;
6. Change the attribute value through the object point ;

7. Print a Value

Carry out friend

1.2 Member function friend ：

1. Friend function is not a member function of a class , Accessing members of an object in the body of a function , You must add an operator to the object name "∵" Add object member name . But friend functions can access all members of the class , General functions can only access public members in a class .
2. Friend functions are not restricted by the access key in the class , You can put it in the public of the class 、 private 、 The protection part , But the result is the same .
3. The scope of a friend function of a class is not the scope of that class . If the friend function is a member function of another class , The scope is another — Scope of class , Otherwise, it is the same as the general function .
   
   With Base Statement , But I don't know which attributes are included in the declaration , Will 12 Line function post compilation , Plus member function friends .
   

1.3 Like friends

An entire class can be a friend of another class . Each member function of a friend class is a friend function of another class , Can access all members in another class , share , Protect or private data members .

2 Static keywords

By keyword static Decorate a member of a class body , Become a static member of a class (static class member). A static member of a class is shared by all its objects , No matter how many objects there are , Only one copy of a static member is stored in public memory . Static data members are treated as global object pairs of this type .

1. When adding static keywords to local variables , Visibility has not changed , Life span has changed .
2. When adding static keywords to global variables , The lifetime has not changed , Visibility has changed （ Available only in this document , Not visible in other files ）

2.1 Static attribute

In class design , With keywords static The decorated data members are static data members . All objects instantiated with this type , A storage space allocated by the shared system for static members , And this storage space is for program execution main Function , No longer allocate space for static members when instantiating objects （ Static member data is not in object space ).
2.1.1. Static data members are designed for information sharing ;


_pi For shared , therefore , Print out only 4 Bytes .
Example 2：

Example 3： Record the number of current objects created

Special static members ：

Example 3： In its own type design , Cannot be designed as either a value type or a reference . The type of a static data member can be its class, not static Data members can only be declared as pointers to this class .

summary :

1. Static data members are designed for information sharing , And information exchange .
2. Static data members of a class are shared by all class objects , It doesn't belong to a specific instance .
3. Static data members of a class must be defined outside the class , Do not add... When defining static Keyword cannot be created in the initialization list of a constructor .
4. The static data member type of the class is int,short, char, long long , And is const , Can be initialized directly in the class , It can also be initialized outside the class .
5. Use static data members in member functions of classes , Static data members have not been previously this.
6. When the static data members of a class are public, they can be used in external functions : Class name : Static data member name or object . Static data member name . You can define your own statically typed objects in the class body .

3 Static methods ：

Function member description is static , Will be independent of the different objects of this class . Calls to static function members , Outside the object, you can do the following : Class name :: Function name or object name . Function name .
As opposed to static data members , For ease of use , Most static function members are public .

The static member function does not this The pointer , So in static member functions, you explicitly use this Pointers will cause compilation time errors . Attempt to access implicit use this The non static data member pointed to by the pointer will also cause compilation time errors .
summary :
A regular member function declaration describes three things that are logically different from each other :

1. This function can access the private part of the class declaration .
2. This function is in the scope of the class .
3. This function must be activated via an object （ There is one this The pointer ).
   Declaring a function as a friend can make it have only the first property .
   Declare a function as static It can be made to have only the first and second properties

4 Object to object relationship

In a system , An object may be related to different objects , Here are the different relationships .
. rely on (Dependency)( Use one )
· relation (Association)( Use one )· polymerization (Aggregation)( There is one )
· Combine (Composition )( There is one ,“ use … To achieve ”)· Inherit (Inheritance)( It's a )
· Class template (Class template)
4.1 relation (Association)： Use one
For two independent objects , Be the object A With another object B When there is a fixed correspondence , The two objects are related . An association is a special case of a dependency .
In type design , The association relationship is mainly reflected in the pointer or reference of the target type , As a property member of the current type . There is no relationship between whole and part , It just matters .

When designed as a pointer , Is a weak correlation （ A teacher can teach multiple courses at the same time ）; When designed as a reference , For strong correlation （ Once designed , Then the teacher is bound to this course ）