Double const declaration

Question

Hi.

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

This makes it a const pointer to a const value, rather than a mutable pointer to a const value or a const pointer to a mutable value.

Answer 2

It's a constant pointer to a constant unsigned char. You can't change the pointer nor the thing it points to.

Answer 3

const * unsigned char const buffer means that you cannot modify the pointer buffer nor the memory that buffer points to.

Answer 4

In a declaration like const * const T, the first const (before the *) means that what the pointer points at is const (i.e. it's a pointer to a const T). The const after the * means that the pointer itself is const (i.e. can't be modified to point at anything else).

Answer 5

A couple of articles to help you understand const correctness in C++:

• Wikipedia
• Possibility.com

Answer 6

The first const says that the data pointed to is constant and may not be changed, the second 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 7

assuming const unsigned char * const

Everyone is correct that its a const pointer to a const unsigned char.

C++ types read mostly right to left unless there are any modifiers on the far left then these read left to right.

Answer 8

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