constant pointer vs pointer on a constant value

Question

What is the difference between the following declarations   char   const a  const char   a    In order to understand the difference I wrote this small program    include  lt stdio h gt   include  lt stdlib h gt    int main  int argc  char   argv        char a    x       char b    y        char   const pc1    amp a      const char   pc2    amp a       printf   Before n        printf   pc1  p n   pc1       printf    pc1  c n    pc1       printf   pc2  p n   pc2       printf    pc2  c n    pc2         pc1   b         pc1    amp b              pc2   b         pc2    amp b       printf    n n         printf   After n        printf   pc1  p n   pc1       printf    pc1  c n    pc1       printf   pc2  p n   pc2       printf    pc2  c n    pc2        return EXIT SUCCESS      I compiled the program  with gcc 3 4  and ran it  The output highlights the difference rather well   Before pc1 ffbfd7e7  pc1 x pc2 ffbfd7e7  pc2 x   After pc1 ffbfd7e7  pc1 y pc2 ffbfd7e6  pc2 x   However  I had to write the small program to get the answer  In case I m away from the machine  at an interview for instance   I wouldn t be able to answer the question   Can someone please explain  by commenting the above example  how the const keyword operates

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Trying to answer in simple way   char   const a     gt  a is  const  constant     pointer of type char  L  lt - R     gt   Constant Pointer   const char   a     gt  a is     pointer to char constant              L  lt - R     gt   Pointer to Constant    Constant Pointer   pointer is constant     i e  the address it is holding can t be changed  It will be stored in read only memory    Let s try to change the address of pointer to understand more   char   const a    amp b   char c  a    amp c     illegal   you can t change the address   a  is const at L-value  so can t change   a  is read-only variable    It means once constant pointer points some thing it is forever    pointer a points only b   However you can change the value of b eg   char b  a   char   const a   amp b   printf   n print a      c  n   a    a    c   printf   n now print a      c  n   a     Pointer to Constant   Value pointed by the pointer can t be changed    const char  a  char b    b   const char   a   amp b  char c  a  amp c    legal   a    c      illegal    a is pointer to constant can t change

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char   const a    means that the pointer is constant and immutable but the pointed data is not  You could use const cast in C    or c-style cast to cast away the constness in this case as data itself is not constant   const char   a    means that the pointed data cannot be written to using the pointer a  Using a const cast C    or c-style cast  to cast away the constness in this case causes Undefined Behavior

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The first is a constant pointer to a char and the second is a pointer to a constant char  You didn t touch all the cases in your code   char   const pc1    amp a     You can t make pc1 point to anything else    const char   pc2    amp a     You can t dereference pc2 to write       pc1    c     Legal      pc2    c     Illegal      pc1    amp b     Illegal  pc1 is a constant pointer     pc2    amp b     Legal  pc2 itself is not constant

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Above are great answers  Here is an easy way to remember this   a is a pointer   a is the value  Now if you say  const a  then the pointer is const   i e  char      const a    If you say  const  a  then the value is const   i e  const char   a

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To parse complicated types  you start at the variable  go left  and spiral outwards  If there aren t any arrays or functions to worry about  because these sit to the right of the variable name  this becomes a case of reading from right-to-left   So with char  const a  you have a  which is a const pointer     to a char  In other words you can change the char which a is pointing at  but you can t make a point at anything different   Conversely with const char  b  you have b  which is a pointer     to a char which is const  You can make b point at any char you like  but you cannot change the value of that char using  b          You can also of course have both flavours of const-ness at one time  const char  const c

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char   const a     a is writable  but a is not  in other words  you can modify the value pointed to by a  but you cannot modify a itself   a is a constant pointer to char   const char   a     a is writable  but  a is not  in other words  you can modify a  pointing it to a new location   but you cannot modify the value pointed to by a     Note that this is identical to  char const   a    In this case  a is a pointer to a const char

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The easiest way to understand the difference is to think of the different possibilities  There are two objects to consider  the pointer and the object pointed to  in this case  a  is the name of the pointer  the object pointed to is unnamed  of type char   The possibilities are    nothing is const the pointer is const the object pointed to is const both the pointer and the pointed to object are const    These different possibilities can be expressed in C as follows    char   a  char   const a  const char   a  const char   const a    I hope this illustrates the possible differences

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Now that you know the difference between char   const a and const char   a  Many times we get confused if its a constant pointer or pointer to a constant variable   How to read it  Follow the below simple step to identify between upper two   Lets see how to read below declaration  char   const a    read from Right to Left  Now start with a   1   adjacent to a there is const    char    const a     ---  So a is a constant           2   Now go along you get     char     const a      ---  So  a is a constant pointer to          3    Go along and there is char   char     const a       ---  a is a constant pointer to character variable  a is constant pointer to character variable     Isn t it easy to read   Similarily for second declaration  const char   a    Now again start with a   1   Adjacent to a there is    ---  So a is a pointer to         2   Now there is char  ---  so a is pointer character    Well that doesn t make any sense    So shuffle pointer and character  ---  so a is character pointer to          3   Now you have constant  ---  so a is character pointer to constant variable  But though you can make out what declaration means  lets make it sound more sensible   a is pointer to constant character variable

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const char   a    This states pointer to constant character  For eg   char b  s   const char  a    amp b    Here a   points to a constant char  s  in this case  You can t use a to change that value But this declaration doesn t mean that value it points to is really a constant it just means the value is a constant insofar as a is concerned  You can change the value of b directly by changing the value of b but you can t change the value indirectly via the a pointer    a  t        INVALID  b  t          VALID   char   const a  amp b   This states a constant pointer to char  It constraints a to point only to b however it allows  you to alter the value of b   Hope it helps

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You may use cdecl utility or its online versions  like https   cdecl org   For example    void    x  int           is a declare x as pointer to function  array of pointer to function returning int  returning void

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I will explain it verbally first and then with an example   A pointer object can be declared as a  const  pointer or a pointer to a  const  object  or both    A  const  pointer cannot be reassigned to point to a different object from the one it is initially assigned  but it can be used to modify the object that it points to  called the  pointee    Reference variables are thus an alternate syntax for  constpointers   A pointer to a  const  object  on the other hand  can be reassigned to point to another object of the same type or of a convertible type  but it cannot be used to modify any object   A  const pointer to a  const  object can also be declared and can neither be used to modify the pointee nor be reassigned to point to another object    Example   void Foo  int   ptr           int const   ptrToConst           int   const constPtr           int const   const constPtrToConst            ptr   0     OK  modifies the  pointee  data      ptr   0     OK  modifies the pointer        ptrToConst   0     Error  Cannot modify the  pointee  data      ptrToConst   0     OK  modifies the pointer        constPtr   0     OK  modifies the  pointee  data      constPtr   0     Error  Cannot modify the pointer        constPtrToConst   0     Error  Cannot modify the  pointee  data      constPtrToConst   0     Error  Cannot modify the pointer      Happy to help  Good Luck

[c] constant pointer vs pointer on a constant value

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