C language beginner-const pointer

Note: initialization is required during the declaration.
Declaration method: type name * const variable name
1. You cannot point to another variable. The value of the pointer cannot be changed.
[Cpp]
# Include <stdio. h>
Void main (){
Int a = 89;
Int * const p = NULL;
P = &;
 
}

 
Error:
Error C2166: l-value specifies const object
2. assign values to the variable through the pointer (the variable value is not const)
[Cpp]
# Include <stdio. h>
Void main (){
Int a = 89;
Int * const cp = &;
* Cp = 188;
Printf ("a = % d \ n", );
}

 
Result:
A = 188
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3. the const Pointer Points to the const variable and changes the value of the variable (which can be implemented in LINIX)
[Cpp]
# Include <stdio. h>
Void main (){
Const int B = 89;
Int * const pp = & B;
* Pp = 100;
Printf ("B = % d \ n", B );
}

Method 2: const type name * variable name
1. pointer to constants pointer to a constant
Pointer to a constant to avoid modifying the value of a variable, regardless of whether the variable is a const variable.
[Cpp]
# Include <stdio. h>
Void main (){
Int x = 123;
Const int * pc = & x;
* Pc = 8989;
Printf ("% d \ n", x );
}

 
Result: it cannot be modified.
Rror C2166: l-value specifies const object
2. pointer to a constant. The pointer can point to another variable, and the value of the pointer can be changed.
[Cpp]
Void main (){
Int x = 123;
Int y= 321;
Const int * pc = & x;
Printf ("% p \ n", pc );
Pc = & y;
Printf ("% p \ n", pc );
}

 
3. A pointer to a constant cannot be assigned as a normal pointer, because a pointer to a constant cannot modify the value of a variable. If you assign it to a normal pointer, then www.2cto.com
It is not allowed to modify the value of a variable.
[Cpp]
# Include <stdio. h>
Void main (){
Int x = 123;
Const int * pc = & x;
Int * q = pc;
}

 
Error:
Error C2440: 'initializing': cannot convert from 'const int * 'to 'int *' Conversion loses qualifiers
 
Method 3: const int * const variable name
The value of the pointer variable and the value of the space pointed to by the pointer cannot be changed.
 
 
 
Deepen:
[Cpp]
# Include <stdio. h>
Void main (){
// The first basic type of const variable, where the const position can be
Const int x = 123;
Const int y = 321;
// Define a non-constbianl
Int z = 111;
// Define a pointer to a constant
Const int * p = & x;
// Define a constant pointer
Int * const q = & z;
// Define a non-const pointer
Int * w = & z;
 
X = y; // It is incorrect because the const variable cannot be modified, and the left value of the const variable cannot be used.
P = & z; // correct. The value of the pointer to a constant can be modified, that is, it can point to another variable, however, you cannot use this pointer to modify the memory space it points.
 
Value
* P = 10; // error. The pointer to a constant cannot be used to modify the value of a variable.
* P = y; // error, same as above
P = w; // correct. The value of the pointer to the constant can be modified.
Z = 5; // correct. z is a variable that can be changed.
Z = x; // correct. x can only be used as the right value as the const variable, not the left value.
* Q = x; // correct. The const pointer variable can be used to modify the value in the address to which it points.
Q = & x; // error, because x is a constant here and cannot be changed. The q pointer can be used to change the value of a variable.
// However, the principle is acceptable, and the value of the variable can be changed in Standard c.
W = & x; // error. Because x is a constant here, it cannot be changed, and w is a normal pointer that can be used to change the value of a variable.
// However, the principle is acceptable, and the value of the variable can be changed in Standard c.
W = p; // error, because the pointer to a constant cannot change the value in the memory space it points, the common pointer w can be used to change the value of a variable.
 
You cannot assign a pointer to a constant to a normal pointer.
W = q; // correct, because the q pointer can be used to change the value of the variable, and the w normal pointer can also be used.
 

}

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