Removing a const qualifier might circumvent the read-only status of an object and result in it being modified;
Removing a const qualifier might result in an exception when the object is accessed;
Removing a volatile qualifier might result in accesses to the object being optimized away.

Note: Removal of the C99 restrict type qualifier is benign.

Example

         uint16_t x;
         uint16_t * const cpi = &x;     /* const pointer             */
         uint16_t * const *pcpi;        /* pointer to const pointer  */
         uint16_t * *ppi;
   const uint16_t *pci;                 /* pointer to const          */
volatile uint16_t *pvi;                 /* pointer to volatile       */
         uint16_t *pi;
pi = cpi;                               /* Compliant - no conversion
                                                  no cast required   */
pi  = (uint16_t *)pci;                  /* Non-compliant             */
pi  = (uint16_t *)pvi;                  /* Non-compliant             */
ppi = (uint16_t * *)pcpi;               /* Non-compliant             */

MISRA-C 2004 Rule 11.5 (required): A cast shall not be performed that removes any const or volatile qualification from the type addressed by a pointer.

Cast operation removes const or volatile modifier from a pointer or reference.

[Undefined 39, 40]

Any attempt to remove the qualification associated with the addressed type by using casting is a violation of the principle of type qualification. Notice that the qualification referred to here is not the same as any qualification that may be applied to the pointer itself.

Example

uint16_t               x;
uint16_t * const       cpi = &x;  /* const pointer               */
uint16_t * const    * pcpi;       /* pointer to const pointer    */
const uint16_t *    * ppci;       /* pointer to pointer to const */
uint16_t *          * ppi;
const uint16_t      * pci;        /* pointer to const            */
volatile uint16_t   * pvi;        /* pointer to volatile         */
uint16_t            * pi;
...
pi = cpi;                         /* Compliant - no conversion
                                                no cast required */
pi = (uint16_t *)pci;             /* Not compliant               */
pi = (uint16_t *)pvi;             /* Not compliant               */
ppi = (uint16_t * *)pcpi;         /* Not compliant               */
ppi = (uint16_t * *)ppci;         /* Not compliant               */

MISRA C++ 2008 Rule 5-2-5 (required): A cast shall not remove any const or volatile qualification from the type of a pointer or reference.

[Undefined 7.1.5.1(4, 7)]

Rationale

Removal of the const or volatile qualification may not meet developer expectations as it may lead to undefined behaviour.

Example

void f ( const char_t * p )
{
   *const_cast< char_t * >( p ) = '\0';   // Non-compliant
}
int main ( )
{
   f ( "Hello World!" );
}