C++ There are many measures to protect the security of data , Such as private Data members of protection classes, etc . But for some common data , Such as function real participation formal parameters, etc , We can access the same data object in different ways on different occasions . Sometimes inadvertent misoperation will change the value of data .

One 、 Regular object

It is necessary to enable data to be shared among functions , And ensure that it is not arbitrarily modified , have access to const limit , That is, data is defined as read-only .
Use... When defining objects const limit , Call it Regular object .

The general form of the definition is :

    Class name const Object name （ Argument list ）;

or const Class name Object name （ Argument list ）;

Such as : const student stu1;
     student const stu2 (20060102,“ Li Si ”,90) ;
1) The data members in the constant object are const Of , There has to be an initial value . In any case , Data members in constant objects cannot be modified .

2) In addition to the default constructor and default destructor , Constant objects can only call constant member functions .
Such as :stu1.num=20060103;//error
   stu1.display( );//error
3) In actual programming , Sometimes Be sure to modify the value of a data member in the constant object. You can declare the data member as mutable( Variable ) To change its value .
The declaration form is :
mutable   Data member type   List of data member names ;

Such as : mutable int num;// Variable data members

Two 、 Constant data member

Use... When defining members const limit , Call it Constant data member .

The general form of the definition is :
const  Type name   Member name ;
Such as : const int num;
explain : Values of constant data members , Can only be initialized through the constructor's initialization table .

3、 ... and 、 Constant member function

Use... When defining member functions const limit , Call it Constant member function .
The general form of the definition is :
Return type Function name ( List of formal parameters ) const

{  The body of the function   }

explain :
1） Constant member functions can only reference data members in this class , You can't change its value .
2） Whether you declare or define a constant member function, you should add const keyword . Constant member functions can access const Data member , You can also visit non const Data members of .const Data members can be const Member function access , It can also be used by non const Member function access .

Data member	Non member function	Constant member function
Extraordinary data member	allow access to , You can modify	allow access to , Do not modify
Constant data member	allow access to , Do not modify	allow access to , Do not modify
Constant object data members	Access and modification are not allowed	allow access to , Do not modify

Description of constant member function :
1） In a class , If the values of some data members are allowed to be modified , The values of other data members cannot be modified , Some data members can be declared as const（ Constant data member ）, So that its value cannot be modified . Ordinary member functions can modify ordinary data members , But you can only access the values of constant data members .
2） If it is required that the values of all data members are not allowed to change , You can declare an object as const Of （ Regular object ), You can only use const Member functions access data . And its value cannot be modified
3) If you define a constant object , Only one of them can be called const Member functions , Instead of calling non const Member functions . If you need to access data members in constant objects , All member functions in a constant object can be declared as const Member functions , However, you should ensure that the data members in the object are not modified in the function .
4) Member functions in constant objects are not necessarily constant member functions . If the member function in the constant object does not add const Statement ,C++ Treat it as a non member function .
5) A constant member function cannot call another non constant member function .

Four 、 A constant pointer to an object

Constant pointer yes const Of You can't change the direction , but The object pointed to by the constant pointer is not necessarily const Of .

A constant pointer to an object defines the form ：

Class name *const Pointer variable name ;
Such as : student stu;
      student *const p= &stu ;
Usually , Use Constant pointer as formal parameter of function , The purpose is not to allow the value of pointer variables to be changed during function execution , Make it always point to the original object . If you try to modify the value of a constant pointer parameter during function execution , There will be compilation errors .

Pointer variable definition form pointing to constant object :
const  Class name   * Pointer variable name ;
Such as : const student*p;// The object pointed to by the pointer variable is const

explain : A pointer to a constant object , It cannot change the value of the object pointed to by it , But the value of the pointer variable itself can be changed , Therefore, you can define without initializing .

The difference between a constant pointer to an object and a pointer to a constant object :
student *const p =&d;
const student *p;
1） If an object has been declared as a constant object , You can only point to a constant object with a pointer variable .

2） If you define a pointer variable that points to a constant object , Even if it points to a non const The object of , The object it points to cannot be changed by pointer .

3) A pointer to a constant object is used as a function parameter , The purpose is to protect the object pointed to by the formal parameter pointer , Make it unmodified during function execution . Use constant pointers as formal parameters of functions , The purpose is not to allow the value of pointer variables to be changed during function execution , Make it always point to the original object . If you try to modify the value of a constant pointer parameter during function execution , There will be compilation errors .

5、 ... and 、 Common references to objects

stay C++ In the program , Constant pointers and references to objects are often used as function parameters . This can ensure data security , So that the data cannot be modified arbitrarily in the function , When calling a function, you don't have to pass a copy of the argument object , Greatly reduce the space and time overhead of function calls .

Objects often refer to the definition form :
const  Class name   & Reference variable name ;