Double const declaration

Question

I often see the following function declaration:

some_func(const unsigned char * const buffer)
{

}

Any idea why the const is repeated before the pointer name?

Thanks.

Answer 1

The first const says that the data pointed to is constant and may not be changed whereas the second const says that the pointer itself may not be changed:

char my_char = 'z';
const char* a = &my_char;
char* const b = &my_char;
const char* const c = &my_char;

a = &other_char; //fine
*a = 'c'; //error
b = &other_char; //error
*b = 'c'; //fine
c = &other_char; //error
*c = 'c'; //error

Answer 2

type declarations should(?) be read RTL. const modifies the thing on its left, but the rule is complicated by the fact that you can write both const T and T const (they mean the same thing).

• T * const is a constant pointer to mutable T
• T & const would be constant reference to mutable T, except references are constant by definition
• T const * is a mutable pointer to constant T
• T const & is a reference to constant T
• T const * const is constant pointer to constant T