What is the difference between const int const int const and int const

Question

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

User · Answer

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    amp 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

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The const with the int on either sides will make pointer to constant int   const int  ptr  amp i    or   int const  ptr  amp i    const after   will make constant pointer to int   int  const ptr  amp i    In this case all of these are pointer to constant integer  but none of these are constant pointer    const int  ptr1  amp i   ptr2  amp 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  amp i   const ptr2  amp j

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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  amp  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

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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

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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  amp 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    amp var1  ptr2    amp 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    amp 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    amp var3   ptr4   1         Error  ptr4    amp var4     Error

User · Answer

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

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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

User · Answer

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

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The C and C   declaration syntax has repeatedly been described as a failed experiment  by the original designers   Instead  let s name the type  ldquo pointer to Type rdquo   I rsquo ll call it Ptr    template lt  class Type  gt  using Ptr    Type     Now Ptr  lt char gt  is a pointer to char   Ptr  lt const char gt  is a pointer to const char   And const Ptr  lt const char gt  is a const pointer to const char   There

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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  amp  which prevents the object from being either modified or copied   Example    TYPE amp  TYPE  operator  const TYPE  amp rhs        return  this     But TYPE  amp   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  amp  reference   Example    bool TYPE  operator   const TYPE  amp rhs  const         There are common situations where both the return value and the method should be const   Example    const TYPE TYPE  operator  const TYPE  amp 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 amp  operator    unsigned i  const   then you ll probably also want the non-const version given by    inline T amp  operator    unsigned i      return const cast lt char amp  gt         static cast lt const TYPE amp  gt   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  amp  reference might witness the value to which it refers be changed elsewhere

User · Answer

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

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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

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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    amp x  int   pp   NULL  pp    amp p  printf   d n     pp       Example 2 int x  x   10  int  p   NULL  p    amp x  int    const pp    amp p     Definition must happen during declaration printf   d n     pp       Example 3 int x  x   10  int   const p    amp x     Definition must happen during declaration int   const  pp   NULL  pp    amp p  printf   d n     pp       Example 4 int const x   10     Definition must happen during declaration int const   p   NULL  p    amp x  int const   pp   NULL  pp    amp p  printf   d n     pp       Example 5 int x  x   10  int   const p    amp x     Definition must happen during declaration int   const   const pp    amp 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    amp x  int const    const pp    amp 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    amp x     Definition must happen during declaration int const   const  pp   NULL  pp    amp p  printf   d n     pp       Example 8 int const x   10     Definition must happen during declaration int const   const p    amp x     Definition must happen during declaration int const   const   const pp    amp 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    amp x  int   pp    amp p  int    ppp    amp pp  int     pppp    amp ppp   printf   d  n       pppp

User · Answer

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    amp a               assignment to where foo points to      dummy statement    foo   6               the value of a can  t get changed through the pointer   foo    amp b               the pointer foo can be changed     int  const bar    amp 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    amp 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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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 lt   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  amp 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  amp 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

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