const int = int const?

Question

For example, is

int const x = 3;

valid code?

If so, does it mean the same as

const int x = 3;

?

@Hamish: some compilers accept all sorts of invalid code; the only way to be sure that something's valid is to look in the standard, or ask. — Mike Seymour
– Mike Seymour, Commented Jul 14, 2010 at 14:54
Well, one is a constant integer and the other is an integer that's constant... :-) — Armstrongest
– Armstrongest, Commented Jul 14, 2010 at 15:02
I would not want to use such compiler then. In any case, a compiler is my first line of defense. — Hamish Grubijan
– Hamish Grubijan, Commented Jul 14, 2010 at 15:22
Check out this: c-faq.com/decl/spiral.anderson.html , it should make it clearer how to read a declaration. — Eugen Constantin Dinca
– Eugen Constantin Dinca, Commented Jul 14, 2010 at 16:39

Brian R. Bondy · Accepted Answer · 2010-07-14 14:48:21Z

162

They are both valid code and they are both equivalent. For a pointer type though they are both valid code but not equivalent.

Declares 2 ints which are constant:

int const x1 = 3;
const int x2 = 3;

Declares a pointer whose data cannot be changed through the pointer:

const int *p = &someInt;

Declares a pointer who cannot be changed to point to something else:

int * const p = &someInt;

answered Jul 14, 2010 at 14:48

Brian R. Bondy

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9 Comments

JAB Over a year ago

And ` const int * const p = &someInt;` would give you a pointer that cannot be changed to a value that cannot be changed, if I'm not mistaken.

stakx - no longer contributing Over a year ago

The pattern here becomes apparent if the definitions are read in reverse. For example: [const int *] = a pointer (*) to an int that is const. [int * const] = a const pointer (*) to an int.

T.E.D. Over a year ago

C syntax reads crappy no matter what you do. It wasn't designed to produce readable sources. You just have to learn the rules.

DevSolar Over a year ago

@ T.E.D. : Not quite. Just get into the habit to write const always after the thing it applies to, and you're done. (Always-trailing is the only style that could be applied consistently.)

Loki Astari Over a year ago

You should also include 'int const *' and just for a laugh 'int const * const'

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jmbpiano · Accepted Answer · 2019-04-08 01:03:10Z

43

Yes, they are the same. The rule in C++ is essentially that const applies to the type to its left. However, there's an exception that if you put it on the extreme left of the declaration, it applies to the first part of the type.

For example in int const * you have a pointer to a constant integer. In int * const you have a constant pointer to an integer. You can extrapolate this to pointer to pointers, and the English may get confusing but the principle is the same.

For another dicussion on the merits of doing one over the other, see my question on the subject. If you are curious why most folks use the exception, this FAQ entry of Stroustrup's may be helpful.

edited Apr 8, 2019 at 1:03

jmbpiano

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answered Jul 14, 2010 at 14:57

T.E.D.

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3 Comments

Cogwheel Over a year ago

Exactly what I was thinking. const int x is the exceptional form. int const x fits the same pattern as int * const x. If you need a constant pointer to a constant int, I usually write it as int const * const for consistency rather than const int * const.

T.E.D. Over a year ago

@Cogwheel - Precisely. Once you realise the "normal" form of putting the const to the extreme left is actually making use of an exceptional case, the question arises as to which way actually makes the code clearer in the long run. Hence my question stackoverflow.com/questions/988069/…

Talespin_Kit Over a year ago

@T.E.D. The rule in C++ is essentially that const applies to the type to its left. Is there any citation for this by known programming gurus or just kind of understood thing. Also in the "const int * p1". There is nothing to the left of const. How to apply the rule in this scenario.

T.E.D. · Accepted Answer · 2011-03-18 13:14:04Z

21

Yes, that is exactly the same. However, there is difference in pointers. I mean:

int a;

// these two are the same: pointed value mustn't be changed
// i.e. pointer to const value
const int * p1 = &a;
int const * p2 = &a;

// something else -- pointed value may be modified, but pointer cannot point
// anywhere else i.e. const pointer to value
int * const p3 = &a;

// ...and combination of the two above
// i.e. const pointer to const value
const int * const p4 = &a;

edited Mar 18, 2011 at 13:14

T.E.D.

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answered Jul 14, 2010 at 14:51

Archie

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Comments

Liu Boyu · Accepted Answer · 2023-01-06 17:19:36Z

15

From "Effective C++" Item 21

char *p              = "data"; //non-const pointer, non-const data
const char *p        = "data"; //non-const pointer, const data
char * const p       = "data"; //const pointer, non-const data
const char * const p = "data"; //const pointer, const data

const pointer: pointed value may be modified, but pointer cannot point
anywhere else

edited Jan 6, 2023 at 17:19

Liu Boyu

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answered Mar 18, 2011 at 3:22

Yuan

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Comments

ttchong · Accepted Answer · 2010-07-14 14:55:03Z

5

It is the same in meaning and validity.

As far as I know, const only get complex whenever it involves pointer.

int const * x;
int * const x;

are different.

int const * x;
const int * x;

are same.

answered Jul 14, 2010 at 14:55

ttchong

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Comments

Puelloc · Accepted Answer · 2022-07-01 15:14:05Z

0

I found that if always write const in right, it will becoming easier to read a complex pointer type, for fun.

For example:

// Just append const or pointer to
int x; // data
int const cx; // const data
int *px; // pointer to data
int const *pcx; //pointer to const data
int * const cpx; // const pointer to data
int const * const * * const cppcpcx; // const pointer to (pointer to (const pointer to const data))

answered Jul 1, 2022 at 15:14

Puelloc

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Comments

DannyLin · Accepted Answer · 2022-07-19 06:32:23Z

0

I have an idea how to interpret (non)const pointers and data. Split it with *.

Data type	Star sign	Pointer type	Combine	Data	Pointer
int	*	p	int *p	non-const	non-const
const int	*	p	const int *p	const	non-const
int	*	const p	int *const p	non-const	const
const int	*	const p	const int *const p	const	const

answered Jul 19, 2022 at 6:32

DannyLin

1

Comments

Zrn-dev · Accepted Answer · 2022-10-12 04:50:34Z

0

// 1.

int x1 = 1;
int x2 = 2;

const int* p = &x1;
//*p = 3;          // variable data is CAN NOT BE changed through pointer
p = &x2;           // pointer CAN POINT to other variable

cout << *p << endl;   // OUTPUT: 2


 
// 2.

int y1 = 1;
int y2 = 2;

int* const t = &y1;
*t = 3;            // variable data is CAN BE changed through pointer
//t = &y2;         // pointer CAN NOT POINT to other variable

cout << *t<<endl;   // OUTPUT: 3

answered Oct 12, 2022 at 4:50

Zrn-dev

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