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.

Hatchery

It should be const unsigned char* const – Flatus

iirc unsigned char const * const also is valid – Brittney

No, you don't see such declarations "often". The declaration is, in fact, invalid and thus makes no sense. Please, be more careful when posting code with your questions. – Kelsiekelso

Reading the declaration from right to left will give you a better understanding. const unsigned char * const is a constant pointer (*const) to an constant unsigned char (const unsigned char). – Olympe

Sorry, corrected to right syntax. The right to left reading advice is very useful, thanks! (I actually remembered studying something about the compiler parsing in RTL fashion during CS courses :) ) – Hatchery

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

Flatus

+1 But might I suggest adding examples demonstrating that const pointers may not be reassigned. (i.e., char* const x = NULL; x = &some_char;) – Imamate

@Imamate I believe I already included that b = &other_char; //error – Flatus

I believe I need to pay more attention :) My apologies – Imamate

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

Concentre

Thanks, the RTL reading advice is useful for such feature cases. – Hatchery

Answer 3

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

Apostle

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).

You can read the declaration from the object being declared outward, so const unsigned char * const buffer is read as: "buffer is a const pointer to a const unsigned char" (this is why const should always being placed after what it modifies--with it before, you have to rearrange things to make the sentence--with it declared as unsigned char const * const buffer, translation to English is simple and straighforward (or perhaps "straightbackward", since you actually read from right to left in this case).

Filiate

Answer 5

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.

Brittney

Answer 6

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.

Escharotic

Answer 7

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

Boyette

Answer 8

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

• Wikipedia
• Possibility.com

Grouchy