Const and pointer in c ++

In c/c ++, pointer itself is a difficult point. In addition, the combination with const often produces many confusing points. Here we will make a summary.

1. Define a const object
Const int buffsize = 512;
Since the constant definition cannot be modified, it must be initialized during definition.
Const I, j = 0; // error, I not initialized

2. the const object is the local variable of the file by default.
// File1.cc
Extern const int bufsize = 512; // define and initialize

// File2.cc
Extern const int bufsize; // Declaration
For (int index = 0; index! = Bufsize; ++ index)
{
//...
}

The default value of a non-const variable is extern. To make the const variable accessible in other files, it must be specified as extern.

3. pointer to the const object

Const int * p;
This p is a pointer to an int-type const object. const limits the type pointed to by the pointer p rather than p itself. That is, p

It is not a const. You do not need to initialize it during definition. You can also assign a value to p to point it to another const object.

However, you cannot use p to modify the value of the object to be pointed.
Example 1: int a = 0; p = & a; yes.
Example 2: * p = 20; No.
Conclusion: This pointer to a const object only limits the number of objects that p points to, rather than the object to which p points.

It is not acceptable to assign the address of a const object to a pointer that does not point to a const object.
Example 3: const int B = 10;
Int * p2 = & B; // error
Const int * p3 = & B; // OK
Conclusion: Variable B cannot be modified because it has const modification. However, pointer p2 is a common pointer and can be used to modify the value pointing to an object.
It is illegal to declare a conflict. The pointer to the const object cannot modify the value of the pointer to the object. Therefore, this method is valid.

You cannot use the void * pointer to save the address of the const object, but you must use the const void * type to save the address of the const object.

Const int a = 6;

Void * p = & a; // error

Const * cp = & a; // OK

Int const * p;
C ++ requires that the const keyword is equivalent before the type or variable name.

Const int n = 5; // same as below
Int const m = 10;

Const int * p; // same as below const (int) * p
Int const * q; // (int) const * p

4. const pointer

Int c = 20;
Int * const p4 = & c;

Pointer p4 is called a const pointer. It is the opposite of a pointer to a const object. It cannot modify the pointer to an object, but can modify the pointer to an object.

The value of the forward object. In addition, this pointer must be initialized during declaration.

5. const pointer to the const object

Const int d = 30;
Const int * const dp = & d;

The pointer dp can neither modify the object to which it points, nor modify the value of only the object.