[c++] What is the difference between const int*, const int * const, and int const *?

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.

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.

1. A const to the LEFT of * indicates that the object pointed by the pointer is a const object.

2. 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.

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.

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?

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<char&>( static_cast<const TYPE&>(*this)[](i) ); }

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.

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

• 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.

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;

It's simple but tricky. Please note that we can swap the const qualifier with 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 p1 ==> *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.

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.

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 &param) 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 &param) 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

1. pp p data - int **pp;
2. pp p data - int ** const pp;
3. pp p data - int * const *pp;
4. pp p data - int const **pp;
5. pp p data - int * const * const pp;
6. pp p data - int const ** const pp;
7. pp p data - int const * const *pp;
8. 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);

1. 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
   

2. 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
   

3. 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
   

4. 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
   

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:

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.

1. 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.

2. 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.

3. 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:

4. 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.

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

1. If the word const appears to the left of the asterisk, what's pointed to is constant
2. If the word const appears to the right of the asterisk, the pointer itself is constant
3. If const appears on both sides, both are constant

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.

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.

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.

There.