What is the difference between const int*, const int * const, and int const *?
type pointer <-
* -> pointee
I like to think of
int *i as declaring "the dereference of
int"; in this sense,
const int *i means "the deref of
const int", while
int *const i means "deref of
const i is
(the one danger of thinking like this is it may lead to favoring
int const *i style of declaration, which people might hate/disallow)
A reference to a variable (here int), which is constant. We pass the variable as a reference mainly, because references are smaller in size than the actual value, but there is a side effect and that is because it is like an alias to the actual variable. We may accidentally change the main variable through our full access to the alias, so we make it constant to prevent this side effect.
int var0 = 0; const int &ptr1 = var0; ptr1 = 8; // Error var0 = 6; // OK
Once a constant pointer points to a variable then it cannot point to any other variable.
int var1 = 1; int var2 = 0; int *const ptr2 = &var1; ptr2 = &var2; // Error
Pointer to constant
A pointer through which one cannot change the value of a variable it points is known as a pointer to constant.
int const * ptr3 = &var2; *ptr3 = 4; // Error
Constant pointer to a constant
A constant pointer to a constant is a pointer that can neither change the address it's pointing to and nor can it change the value kept at that address.
int var3 = 0; int var4 = 0; const int * const ptr4 = &var3; *ptr4 = 1; // Error ptr4 = &var4; // Error
This question shows precisely why I like to do things the way I mentioned in my question is const after type id acceptable?
In short, I find the easiest way to remember the rule is that the "const" goes after the thing it applies to. So in your question, "int const *" means that the int is constant, while "int * const" would mean that the pointer is constant.
If someone decides to put it at the very front (eg: "const int *"), as a special exception in that case it applies to the thing after it.
Many people like to use that special exception because they think it looks nicer. I dislike it, because it is an exception, and thus confuses things.
I drew an image below to explain this, maybe helpful.
int const v and
const int v are identical.
For those who don't know about Clockwise/Spiral Rule: Start from the name of the variable, move clockwisely (in this case, move backward) to the next pointer or type. Repeat until expression ends.
Here is a demo:
The C and C++ declaration syntax has repeatedly been described as a failed experiment, by the original designers.
Instead, let's name the type “pointer to
Type”; I’ll call it
template< class Type > using Ptr_ = Type*;
Ptr_<char> is a pointer to
Ptr_<const char> is a pointer to
const Ptr_<const char> is a
const pointer to
Just for the sake of completeness for C following the others explanations, not sure for C++.
int const *p;
int * const p;
int const * const p;
int ** const pp;
int * const *pp;
int const **pp;
int * const * const pp;
int const ** const pp;
int const * const *pp;
int const * const * const pp;
// Example 1 int x; x = 10; int *p = NULL; p = &x; int **pp = NULL; pp = &p; printf("%d\n", **pp); // Example 2 int x; x = 10; int *p = NULL; p = &x; int ** const pp = &p; // Definition must happen during declaration printf("%d\n", **pp); // Example 3 int x; x = 10; int * const p = &x; // Definition must happen during declaration int * const *pp = NULL; pp = &p; printf("%d\n", **pp); // Example 4 int const x = 10; // Definition must happen during declaration int const * p = NULL; p = &x; int const **pp = NULL; pp = &p; printf("%d\n", **pp); // Example 5 int x; x = 10; int * const p = &x; // Definition must happen during declaration int * const * const pp = &p; // Definition must happen during declaration printf("%d\n", **pp); // Example 6 int const x = 10; // Definition must happen during declaration int const *p = NULL; p = &x; int const ** const pp = &p; // Definition must happen during declaration printf("%d\n", **pp); // Example 7 int const x = 10; // Definition must happen during declaration int const * const p = &x; // Definition must happen during declaration int const * const *pp = NULL; pp = &p; printf("%d\n", **pp); // Example 8 int const x = 10; // Definition must happen during declaration int const * const p = &x; // Definition must happen during declaration int const * const * const pp = &p; // Definition must happen during declaration printf("%d\n", **pp);
Just keep going, but may the humanity excommunicate you.
int x = 10; int *p = &x; int **pp = &p; int ***ppp = &pp; int ****pppp = &ppp; printf("%d \n", ****pppp);
Lot of people answered correctly I will just organize well here and put some Extra info which is missing in given Answers.
Const is keyword in C language also known as qualifier. Const can applied to the declaration of any variable to specify that it's value will not changed
const int a=3,b; a=4; // give error b=5; // give error as b is also const int you have to intialize while declaring itself as no way to assign it afterwards.
How to read ?
just read from right to left every statement works smoothly
3 main things
type a. p is ptr to const int type b. p is const ptr to int type c. p is const ptr to const int
if * comes before int
1. const int * 2. const const int *
we look first
Major type 1. const int*
ways to arrange 3 things at 3 places 3!=6
i. * at start
*const int p [Error] *int const p [Error]
ii. const at start
const int *p type a. p is ptr to const int const *int p [Error]
iii. int at start
int const *p type a. int * const p type b. p is const ptr to int
Major type 2. const const int*
ways to arrange 4 things at 4 places in which 2 are alike 4!/2!=12
i. * at start
* int const const p [Error] * const int const p [Error] * const const int p [Error]
ii. int at start
int const const *p type a. p is ptr to const int int const * const p type c. p is const ptr to const int int * const const p type b. p is const ptr to int
iii. const at start
const const int *p type a. const const * int p [Error] const int const *p type a. const int * const p type c. const * int const p [Error] const * const int p [Error]
squeezing all in one
type a. p is ptr to const int (5)
const int *p int const *p int const const *p const const int *p const int const *p
type b. p is const ptr to int (2)
int * const p int * const const p;
type c. p is const ptr to const int (2)
int const * const p const int * const p
just little calculation
1. const int * p total arrangemets (6) [Errors] (3) 2. const const int * p total arrangemets (12) [Errors] (6)
int const * p,p2 ;
here p is ptr to const int (type a.) but p2 is just const int please note that it is not ptr
int * const p,p2 ;
similarly here p is const ptr to int (type b.) but p2 is just int not even cost int
int const * const p,p2 ;
here p is const ptr to const int (type c.) but p2 is just const int.
I had the same doubt as you until I came across this book by the C++ Guru Scott Meyers. Refer the third Item in this book where he talks in details about using
Just follow this advice
constappears to the left of the asterisk, what's pointed to is constant
constappears to the right of the asterisk, the pointer itself is constant
constappears on both sides, both are constant
const int*- pointer to constant
You can change the value of the pointer; you can not change the value of the
int object, the pointer points to.
const int * const- constant pointer to constant
You can not change the value of the pointer nor the value of the
int object the pointer points to.
int const *- pointer to constant
This statement is equivalent to 1.
const int* - You can change the value of the pointer but you can not change the value of the
int object, the pointer points to.
Actually, there is a 4th option:
int * const- constant pointer to
You can change the value of the object the pointer points to but you can not change the value of the pointer itself. The pointer will always point to the same
int object but this value of this
int object can be changed.
If you want to determine a certain type of C or C++ construct you can use the Clockwise/Spiral Rule made by David Anderson; but not to confuse with Anderson`s Rule made by Ross J. Anderson, which is something quite distinct.
I think everything is answered here already, but I just want to add that you should beware of
typedefs! They're NOT just text replacements.
typedef char *ASTRING; const ASTRING astring;
The type of
char * const, not
const char *. This is one reason I always tend to put
const to the right of the type, and never at the start.
This mostly addresses the second line: best practices, assignments, function parameters etc.
General practice. Try to make everything
const that you can. Or to put that another way, make everything
const to begin with, and then remove exactly the minimum set of
consts necessary to allow the program to function. This will be a big help in attaining const-correctness, and will help ensure that subtle bugs don't get introduced when people try and assign into things they're not supposed to modify.
Avoid const_cast<> like the plague. There are one or two legitimate use cases for it, but they are very few and far between. If you're trying to change a
const object, you'll do a lot better to find whoever declared it
const in the first pace and talk the matter over with them to reach a consensus as to what should happen.
Which leads very neatly into assignments. You can assign into something only if it is non-const. If you want to assign into something that is const, see above. Remember that in the declarations
int const *foo; and
int * const bar; different things are
const - other answers here have covered that issue admirably, so I won't go into it.
Pass by value: e.g.
void func(int param) you don't care one way or the other at the calling site. The argument can be made that there are use cases for declaring the function as
void func(int const param) but that has no effect on the caller, only on the function itself, in that whatever value is passed cannot be changed by the function during the call.
Pass by reference: e.g.
void func(int ¶m) Now it does make a difference. As just declared
func is allowed to change
param, and any calling site should be ready to deal with the consequences. Changing the declaration to
void func(int const ¶m) changes the contract, and guarantees that
func can now not change
param, meaning what is passed in is what will come back out. As other have noted this is very useful for cheaply passing a large object that you don't want to change. Passing a reference is a lot cheaper than passing a large object by value.
Pass by pointer: e.g.
void func(int *param) and
void func(int const *param) These two are pretty much synonymous with their reference counterparts, with the caveat that the called function now needs to check for
nullptr unless some other contractual guarantee assures
func that it will never receive a
Opinion piece on that topic. Proving correctness in a case like this is hellishly difficult, it's just too damn easy to make a mistake. So don't take chances, and always check pointer parameters for
nullptr. You will save yourself pain and suffering and hard to find bugs in the long term. And as for the cost of the check, it's dirt cheap, and in cases where the static analysis built into the compiler can manage it, the optimizer will elide it anyway. Turn on Link Time Code Generation for MSVC, or WOPR (I think) for GCC, and you'll get it program wide, i.e. even in function calls that cross a source code module boundary.
At the end of the day all of the above makes a very solid case to always prefer references to pointers. They're just safer all round.
For me, the position of
const i.e. whether it appears to the LEFT or RIGHT or on both LEFT and RIGHT relative to the
* helps me figure out the actual meaning.
const to the LEFT of
* indicates that the object pointed by the pointer is a
const to the RIGHT of
* indicates that the pointer is a
The following table is taken from Stanford CS106L Standard C++ Programming Laboratory Course Reader.
Like pretty much everyone pointed out:
What’s the difference between
const X* p,
X* const p and
const X* const p?
You have to read pointer declarations right-to-left.
const X* pmeans "p points to an X that is const": the X object can't be changed via p.
X* const pmeans "p is a const pointer to an X that is non-const": you can't change the pointer p itself, but you can change the X object via p.
const X* const pmeans "p is a const pointer to an X that is const": you can't change the pointer p itself, nor can you change the X object via p.
If const is before * then value is constant.
If const is after * then address is constant.
if const are available both before and after * then both value and address are constant.
int * const var; //here address is constant.
int const * var; //here value is constant.
int const * const var; // both value and address are constant.
The const with the int on either sides will make pointer to constant int:
const int *ptr=&i;
int const *ptr=&i;
* will make constant pointer to int:
int *const ptr=&i;
In this case all of these are pointer to constant integer, but none of these are constant pointer:
const int *ptr1=&i, *ptr2=&j;
In this case all are pointer to constant integer and ptr2 is constant pointer to constant integer. But ptr1 is not constant pointer:
int const *ptr1=&i, *const ptr2=&j;
constis to the left of
*, it refers to the value (it doesn't matter whether it's
constis to the right of
*, it refers to the pointer itself
An important point:
const int *p does not mean the value you are referring to is constant!!. It means that you can't change it through that pointer (meaning, you can't assign $*p = ...`). The value itself may be changed in other ways. Eg
int x = 5; const int *p = &x; x = 6; //legal printf("%d", *p) // prints 6 *p = 7; //error
This is meant to be used mostly in function signatures, to guarantee that the function can't accidentally change the arguments passed.
Simple Use of
The simplest use is to declare a named constant. To do this, one declares a constant as if it was a variable but add
const before it. One has to initialize it immediately in the constructor because, of course, one cannot set the value later as that would be altering it. For example:
const int Constant1=96;
will create an integer constant, unimaginatively called
Constant1, with the value 96.
Such constants are useful for parameters which are used in the program but are do not need to be changed after the program is compiled. It has an advantage for programmers over the C preprocessor
#define command in that it is understood & used by the compiler itself, not just substituted into the program text by the preprocessor before reaching the main compiler, so error messages are much more helpful.
It also works with pointers but one has to be careful where
const to determine whether the pointer or what it points to is constant or both. For example:
const int * Constant2
Constant2 is variable pointer to a constant integer and:
int const * Constant2
is an alternative syntax which does the same, whereas
int * const Constant3
Constant3 is constant pointer to a variable integer and
int const * const Constant4
Constant4 is constant pointer to a constant integer. Basically ‘const’ applies to whatever is on its immediate left (other than if there is nothing there in which case it applies to whatever is its immediate right).
The general rule is that the
const keyword applies to what precedes it immediately. Exception, a starting
const applies to what follows.
const int*is the same as
int const*and means "pointer to constant int".
const int* constis the same as
int const* constand means "constant pointer to constant int".
Edit: For the Dos and Don'ts, if this answer isn't enough, could you be more precise about what you want?
Read it backwards (as driven by Clockwise/Spiral Rule):
int*- pointer to int
int const *- pointer to const int
int * const- const pointer to int
int const * const- const pointer to const int
Now the first
const can be on either side of the type so:
const int *==
int const *
const int * const==
int const * const
If you want to go really crazy you can do things like this:
int **- pointer to pointer to int
int ** const- a const pointer to a pointer to an int
int * const *- a pointer to a const pointer to an int
int const **- a pointer to a pointer to a const int
int * const * const- a const pointer to a const pointer to an int
And to make sure we are clear on the meaning of
int a = 5, b = 10, c = 15; const int* foo; // pointer to constant int. foo = &a; // assignment to where foo points to. /* dummy statement*/ *foo = 6; // the value of a can´t get changed through the pointer. foo = &b; // the pointer foo can be changed. int *const bar = &c; // constant pointer to int // note, you actually need to set the pointer // here because you can't change it later ;) *bar = 16; // the value of c can be changed through the pointer. /* dummy statement*/ bar = &a; // not possible because bar is a constant pointer.
foo is a variable pointer to a constant integer. This lets you change what you point to but not the value that you point to. Most often this is seen with C-style strings where you have a pointer to a
const char. You may change which string you point to but you can't change the content of these strings. This is important when the string itself is in the data segment of a program and shouldn't be changed.
bar is a constant or fixed pointer to a value that can be changed. This is like a reference without the extra syntactic sugar. Because of this fact, usually you would use a reference where you would use a
T* const pointer unless you need to allow
I always mess up how to use
const int * const, and
int const * correctly. Is there a set of rules defining what you can and cannot do?
I want to know all the do's and all don'ts in terms of assignments, passing to the functions, etc.