Constant pointer vs Pointer to constant

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I want to know the difference between   const int  ptr    and   int   const ptr     and how it works   It is pretty difficult for me to understand or keep remember this  Please help

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int i  int j   int   const ptr1    amp i    The compiler will stop you changing ptr1   const int   ptr2    amp i    The compiler will stop you changing  ptr2   ptr1    amp j     error  ptr1   7     ok  ptr2    amp j     ok  ptr2   7     error   Note that you can still change  ptr2  just not by literally typing  ptr2   i   4  printf  before   d n    ptr2      prints 4 i   5  printf  after   d n    ptr2      prints 5  ptr2   6     still an error   You can also have a pointer with both features   const int   const ptr3    amp i   ptr3    amp j     error  ptr3   7     error

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Referncing from  This Thread    Constant Pointers  Lets first understand what a constant pointer is  A constant pointer is a pointer that cannot change the address its holding  In other words  we can say that once a constant pointer points to a variable then it cannot point to any other variable   A constant pointer is declared as follows     lt type of pointer gt    const  lt name of pointer gt   An example declaration would look like    int   const ptr   Lets take a small code to illustrate these type of pointers     include lt stdio h gt   int main void        int var1   0  var2   0      int  const ptr    amp var1      ptr    amp var2      printf   d n    ptr        return 0       In the above example     We declared two variables var1 and var2 A constant pointer    ptr    was declared and made to point var1 Next  ptr is made to point var2  Finally  we try to print the value ptr is pointing to      Pointer to Constant  As evident from the name  a pointer through which one cannot change the value of variable it points is known as a pointer to constant  These type of pointers can change the address they point to but cannot change the value kept at those address   A pointer to constant is defined as   const  lt type of pointer gt    lt name of pointer gt  An example of definition could be    const int  ptr  Lets take a small code to illustrate a pointer to a constant      include lt stdio h gt   int main void        int var1   0      const int  ptr    amp var1       ptr   1      printf   d n    ptr        return 0       In the code above     We defined a variable var1 with value 0 we defined a pointer to a constant which points to variable var1 Now  through this pointer we tried to change the value of var1 Used printf to print the new value

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const int  ptr    is a pointer to constant  content   You are allowed to modify the pointer  e g  ptr   NULL  ptr    but modification of the content is not possible   int   const ptr    Is a constant pointer  The opposite is possible  You are not allowed to modify the pointer  but you are allowed to modify what it points to e g   ptr    5

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Please refer the following link for better understanding about the difference between Const pointer and Pointer on a constant value   constant pointer vs pointer on a constant value

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const int  ptr    declares ptr a pointer to const int type  You can modify ptr itself but the object pointed to by ptr shall not be modified  const int a   10  const int  ptr    amp a     ptr   5     wrong ptr          right    While int   const ptr     declares ptr a const pointer to int type  You are not allowed to modify ptr but the object pointed to by ptr can be modified  int a   10  int  const ptr    amp a     ptr   5     right ptr          wrong  Generally I would prefer the declaration like this which make it easy to read and understand  read from right to left   int const   ptr     ptr is a pointer to constant int  int  const ptr      ptr is a constant pointer to int

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const int   ptr    means that the pointed data is constant and immutable but the pointer is not   int   const ptr    means that the pointer is constant and immutable but the pointed data is not

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1  Constant Pointers   These type of pointers are the one which cannot change address they are pointing to  This means that suppose there is a pointer which points to a variable  or stores the address of that variable   Now if we try to point the pointer to some other variable  or try to make the pointer store address of some other variable   then constant pointers are incapable of this   A constant pointer is declared as   int  const ptr   the location of  const  make the pointer  ptr  as constant pointer   2  Pointer to Constant   These type of pointers are the one which cannot change the value they are pointing to  This means they cannot change the value of the variable whose address they are holding   A pointer to a constant is declared as   const int  ptr  the location of  const  makes the pointer  ptr  as a pointer to constant   Example  Constant Pointer   include lt stdio h gt   int main void        int a      10 11       int  const ptr   a        ptr   11       printf   n value at ptr is      d  n   ptr       printf   n Address pointed by ptr      p  n   unsigned int  ptr        ptr        printf   n Address pointed by ptr      p  n   unsigned int  ptr        return 0      Now  when we compile the above code  compiler complains    practice   gcc -Wall constant pointer c -o constant pointer constant pointer c  In function    main     constant pointer c 13  error  increment of read-only variable    ptr      Hence we see very clearly above that compiler complains that we cannot changes the address held by a constant pointer   Pointer to Constants   include lt stdio h gt   int main void        int a   10      const int  ptr    amp a        printf   n value at ptr is      d  n   ptr       printf   n Address pointed by ptr      p  n   unsigned int  ptr         ptr   11       return 0      Now  when the above code is compiled  the compiler complains    practice   gcc -Wall pointer to constant c -o pointer to constant pointer to constant c  In function    main     pointer to constant c 12  error  assignment of read-only location     ptr      Hence here too we see that compiler does not allow the pointer to a constant to change the value of the variable being pointed   Quotation

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const int  ptr    here think like  ptr is constant and  ptr can t be change again   int   const ptr   while here think like ptr as a constant and that can t be change again

[c] Constant pointer vs Pointer to constant

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