Constant variables
Location
-
Courses
/
-
Quick C course
/
Constant variables
Summary
The keyword const
Using the keyword const, we can enforce that the value of a variable will not change by making any attempt to change it cause an error. This is useful as this may prevent accidental changes to a variable for which the value should stay the same (Ex. A variable storing the value of PI).
Constant variable
We define a constant variable the same as usual, but by adding the keyword const before its type:
const int constVar = 7;Once it is defined, we cannot change its value, so the following would cause an error:
constVar = 4; // Error.We can however still use its value:
int var = constVar;Pointer to a constant variable
Here we define a pointer to a constant variable (const keyword before the asterisk):
const int * ptr;We make it point to a variable:
ptr = &var;We can not change the value of the variable it points to:
*ptr = 13; // Error.But we can use its value:
int var2 = *ptr;Constant pointer
Here we define a constant pointer (const keyword after the asterisk):
int * const ptr = &var;We can not change which variable it points to:
ptr = &var2; // Error.But we can use and change the value of the variable it points to:
*ptr = 77;Constant pointer to a constant variable
We define a constant pointer to a constant variable:
const int * const ptr = &var;We can not change which variable it points to, nor its value:
ptr = &var2; // Error.
*ptr = 13; // Error.
We can only use the value of the variable it points to:
int var3 = *ptr;