C + + Const qualifier

• Basic Features

1. A const object must be initialized once it has been created and its value cannot be modified.
2. You can initialize a const object with a non-const object, or you can assign a const object to a non-const object

• By default, const objects are only valid within a file

1. When a const variable with the same name appears within multiple files, it is equivalent to defining a separate variable in a different file
2. To share a const object between multiple files, add the extern keyword before the definition of the variable

• & Type and const

1.The & (Reference) type differs from the * (pointer) type,The declaration, definition, and initialization of a reference type are together, and no other variables can be referenced after initialization            there is no limit to the declaration, definition, initialization, and subsequent assignment of pointer types.

  
 

   
  

  
 int a = 1;
   
  

  
 int b = 2;
   
  

  

   
  

  
 int &c = a;
   
  

  
 int *p;
   
  

  
 p = &a;
  
 

2, with the const collocation

  
 

   
  

  
 int a = 1;
   
  

  
 int b = 2;
   
  

  

   
  

  
 /**< 因为引用在被初始化之后就不能在引用其它变量，所以没有int & const c = a这种写法 */
   
  

  
 const int &c = a;
   
  

  
 /**
   
  

  
 * 不能给c赋值，对a无要求(a可以是const或者非const类型，a也可以重新赋值)
   
  

  
 * 不能将c赋值给非const类型
   
  

  
 */
   
  

  
 
   
  

  
 const int *p1 = &a;//等同于int const *p1 = &a;
   
  

  
 /**
   
  

  
 * 底层const
   
  

  
 * 不能通过p1修改a的值，对a无要求
   
  

  
 * 不能将p1赋值给非const类型
   
  

  
 * p1可以指向其他变量
   
  

  
 */
   
  

  
 
   
  

  
 int * const p2 = &a;
   
  

  
 /**
   
  

  
 * 顶层const
   
  

  
 * 可以通过p2修改a的值
   
  

  
 * p2不能指向其他变量
   
  

  
 */
   
  

  
 
   
  

  
 const int * const p3 = &a;
  
 

If the const is located in*, the const is used to modify the variable pointed to by the pointer, that is, the pointer to a constant;
If the const is located in*on the right, const is the modifier pointer itself, that is, the pointer itself is a constant.


From for notes (Wiz)

C + + Const qualifier