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




difference between pointer to constant and constant pointer with example in c++ (12)

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.


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.


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 initialise 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


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 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.


  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:


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 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;

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

const int* foo;
int *const bar; //note, you actually need to set the pointer 
                //here because you can't change it later ;)

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.


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

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.







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