I always mess up how to use const int*
, 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.
int *(*)(char const * const)
. Start to the right of the parenthesized *
then we have to move left: pointer
. Outside the parens, we can move right: pointer to function of ...
. Then we have to move left: pointer to function of ... that returns pointer to int
. Repeat to expand the parameter (the ...
): pointer to function of (constant pointer to constant char) that returns pointer to int
. What would the equivalent one-line declaration be in a easy-reading language like Pascal?
function(x:^char):^int
. There function types are imply a pointer to a function so no need to specify it, and Pascal doesn't enforce const correctness. It can be read from left to right.
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 const
:
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 NULL
pointers.
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:
https://i.stack.imgur.com/sT6ng.png
https://i.stack.imgur.com/Zt0G2.png
https://i.stack.imgur.com/kXH8P.png
https://i.stack.imgur.com/UeqZO.png
https://i.stack.imgur.com/f5ftV.png
void (*signal(int, void (*fp)(int)))(int);
from archive.is/SsfMX
I think everything is answered here already, but I just want to add that you should beware of typedef
s! They're NOT just text replacements.
For example:
typedef char *ASTRING;
const ASTRING astring;
The type of astring
is 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.
typedef int* PINT
(I assume its something that came from practices in C and many developers kept doing it). Great, I replaced that *
with a P
, it doesn't speed up typing, plus introducing the issue you mention.
PINT
is indeed a rather dumb usage of a typedef, especially cuz it makes me think that the system stores uses beer for memory. typedef s are pretty useful for dealing with pointers to functions, though.
PVOID
, LPTSTR
stuff in Win32 api!
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* p means "p points to an X that is const": the X object can't be changed via p. X* const p means "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 p means "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.
const X* p;
== X const * p;
as in "p points to an X that is const": the X object can't be changed via p.
Constant reference: 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 Constant pointers 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
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* const is the same as int const* const and 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?
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.
const T*
and it has become more natural. How often do you ever use a T* const
anyways, usually a reference will do just fine. I got bit by all this once when wanting a boost::shared_ptr<const T>
and instead wrote const boost::shared_ptr<T>
. Same issue in a slightly different context.
const
is the type of that which is const, and everything to its right is that which is actually const. Take int const * const * p;
as an example. No I don't normally write like that, this is just an example. First const
: type int, And the int that is const is the contents of the const pointer that is the contents of p
. Second const: type is pointer to const
int, const oblect is the contents of p
Simple Use of const
.
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
declares that Constant2
is variable pointer to a constant integer and:
int const * Constant2
is an alternative syntax which does the same, whereas
int * const Constant3
declares that Constant3
is constant pointer to a variable integer and
int const * const Constant4
declares that 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).
ref: http://duramecho.com/ComputerInformation/WhyHowCppConst.html
It's simple but tricky. Please note that we can apply the const
qualifier to any data type (int
, char
, float
, etc.).
Let's see the below examples.
const int *p
==> *p
is read-only [p
is a pointer to a constant integer]
int const *p
==> *p
is read-only [p
is a pointer to a constant integer]
int *p const
==> Wrong Statement. Compiler throws a syntax error.
int *const p
==> p
is read-only [p
is a constant pointer to an integer]. As pointer p
here is read-only, the declaration and definition should be in same place.
const int *p const
==> Wrong Statement. Compiler throws a syntax error.
const int const *p
==> *p
is read-only
const int *const p
==> *p
and p
are read-only [p
is a constant pointer to a constant integer]. As pointer p
here is read-only, the declaration and definition should be in same place.
int const *p const
==> Wrong Statement. Compiler throws a syntax error.
int const int *p
==> Wrong Statement. Compiler throws a syntax error.
int const const *p
==> *p
is read-only and is equivalent to int const *p
int const *const p
==> *p
and p
are read-only [p
is a constant pointer to a constant integer]. As pointer p
here is read-only, the declaration and definition should be in same place.
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 const
.
Just follow this advice
If the word const appears to the left of the asterisk, what's pointed to is constant If the word const appears to the right of the asterisk, the pointer itself is constant If const appears on both sides, both are constant
To remember in easy way :
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.
e.g.
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 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 Ptr_
:
template< class Type >
using Ptr_ = Type*;
Now Ptr_<char>
is a pointer to char
.
Ptr_<const char>
is a pointer to const char
.
And const Ptr_<const char>
is a const
pointer to const char
.
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.
A const to the LEFT of * indicates that the object pointed by the pointer is a const object. A const to the RIGHT of * indicates that the pointer is a const pointer.
The following table is taken from Stanford CS106L Standard C++ Programming Laboratory Course Reader.
https://i.stack.imgur.com/aijhB.png
There are many other subtle points surrounding const correctness in C++. I suppose the question here has simply been about C, but I'll give some related examples since the tag is C++ :
You often pass large arguments like strings as TYPE const & which prevents the object from being either modified or copied. Example : TYPE& TYPE::operator=(const TYPE &rhs) { ... return *this; } But TYPE & const is meaningless because references are always const.
You should always label class methods that do not modify the class as const, otherwise you cannot call the method from a TYPE const & reference. Example : bool TYPE::operator==(const TYPE &rhs) const { ... }
There are common situations where both the return value and the method should be const. Example : const TYPE TYPE::operator+(const TYPE &rhs) const { ... } In fact, const methods must not return internal class data as a reference-to-non-const.
As a result, one must often create both a const and a non-const method using const overloading. For example, if you define T const& operator[] (unsigned i) const;, then you'll probably also want the non-const version given by : inline T& operator[] (unsigned i) { return const_cast
Afaik, there are no const functions in C, non-member functions cannot themselves be const in C++, const methods might have side effects, and the compiler cannot use const functions to avoid duplicate function calls. In fact, even a simple int const &
reference might witness the value to which it refers be changed elsewhere.
The const with the int on either sides will make pointer to constant int:
const int *ptr=&i;
or:
int const *ptr=&i;
const
after *
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;
if const is to the left of *, it refers to the value (it doesn't matter whether it's const int or int const)
if const is to the right of *, it refers to the pointer itself
it can be both at the same time
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.
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 const
s 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.
Function parameters:
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 nullptr
in param
.
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.
Just for the sake of completeness for C following the others explanations, not sure for C++.
pp - pointer to pointer
p - pointer
data - the thing pointed, in examples x
bold - read-only variable
Pointer
p data - int *p;
p data - int const *p;
p data - int * const p;
p data - int const * const p;
Pointer to pointer
pp p data - int **pp; pp p data - int ** const pp; pp p data - int * const *pp; pp p data - int const **pp; pp p data - int * const * const pp; pp p data - int const ** const pp; pp p data - int const * const *pp; pp p data - 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);
N-levels of Dereference
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);
const int* - pointer to constant int object.
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 int object.
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 int object.
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 int object.
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.
simple mnemonic:
type
pointer <- *
-> pointee name
I like to think of int *i
as declaring "the dereference of i
is int
"; in this sense, const int *i
means "the deref of i
is const int
", while int *const i
means "deref of const i
is int
".
(the one danger of thinking like this is it may lead to favoring int const *i
style of declaration, which people might hate/disallow)
I drew an image below to explain this, maybe helpful.
int const v
and const int v
are identical.
https://i.stack.imgur.com/JwWMu.jpg
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
[Error]
if * comes before int
two types
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)
little Extra
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.
Finished
Success story sharing
const int x = 0; const int *const px = &x; const int *const *const p = &px;
works just fine.