What is the difference between char const and const char

Question

What s the difference between   char   const    and  const char

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const char  x Here X is basically a character pointer which is pointing to a constant value char  const x is refer to character pointer which is constant  but the location it is pointing can be change  const char  const x is combination to 1 and 2  means it is a constant character pointer which is pointing to constant value  const  char x will cause a compiler error  it can not be declared  char const   x is equal to point 1     the rule of thumb is if const is with  var name then the pointer will be constant but the pointing location can be changed   else pointer will point to a constant location and pointer can point to another location but the pointing location content can not be change

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The difference is that const char   is a pointer to a const char  while char   const is a constant pointer to a char    The first  the value being pointed to can t be changed but the pointer can be  The second  the value being pointed at can change but the pointer can t  similar to a reference    There is also a  const char   const   which is a constant pointer to a constant char  so nothing about it can be changed    Note   The following two forms are equivalent   const char     and  char const     The exact reason for this is described in the C   standard  but it s important to note and avoid the confusion  I know several coding standards that prefer   char const   over  const char    with or without pointer  so that the placement of the const element is the same as with a pointer const

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const   char is invalid C code and is meaningless   Perhaps you meant to ask the difference between a const char   and a char const    or possibly the difference between a const char   and a char   const  See also   What are const pointers  as opposed to pointers to const objects   Const in C Difference between const declarations in C   C   const question Why can I change the values of a const char  variable

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Some more complex constant variable pointer declaration     Observing cases when we get error and warning would help    understanding it better   int main void      char ca1 10    aaaa      char array 1   char ca2 10    bbbb      char array 2    char  pca1  ca1    char  pca2  ca2     char const  ccs  pca1    char   const csc  pca2    ccs 1   m       Bad - error  assignment of read-only location      ccs   1u       ccs  csc        Good    csc 1   n       Good   csc  ccs        Bad - error  assignment of read-only variable    csc       char const   ccss   amp ccs         Good   char const   ccss1   amp csc        Bad - warning  initialization from incompatible pointer type    char   const  cscs   amp csc        Good   char   const  cscs1   amp ccs       Bad - warning  initialization from incompatible pointer type    char    const cssc     amp pca1     Good   char    const cssc1    amp ccs      Bad - warning  initialization from incompatible pointer type   char    const cssc2    amp csc      Bad - warning  initialization discards    const                                                     qualifier from pointer target type     ccss 1    x      Bad - error  assignment of read-only location       ccss   8u        ccss  ccs        Good    ccss  csc        Good   ccss  ccss1       Good   ccss  cscs        Bad - warning  assignment from incompatible pointer type     cscs 1    y      Good    cscs  ccs        Bad - error  assignment of read-only location     cscs       cscs  csc        Bad - error  assignment of read-only location     cscs      cscs  cscs1       Good   cscs  cssc        Good     cssc 1    z      Good    cssc  ccs        Bad - warning  assignment discards    const                                       qualifier from pointer target type    cssc  csc        Good    cssc  pca2       Good   cssc  ccss        Bad - error  assignment of read-only variable    cssc      cssc  cscs        Bad - error  assignment of read-only variable    cssc      cssc  cssc1       Bad - error  assignment of read-only variable    cssc

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The const modifier is applied to the term immediately to its left  The only exception to this is when there is nothing to its left  then it applies to what is immediately on its right   These are all equivalent ways of saying  constant pointer to a constant char     const char   const const char const   char const   const char const const

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To avoid confusion  always append the const qualifier   int              mutable pointer to mutable int  int const        mutable pointer to constant int  int        const constant pointer to mutable int  int const  const constant pointer to constant int

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Here is a detailed explanation with code    const char   p  char   const p   const char   const p       these are the three conditions      const char  p const char   const p  pointer value cannot be changed     char   const p  pointer address cannot be changed     const char   const p  both cannot be changed    include lt stdio h gt     int main         const char   p     value cannot be changed     char z         p    c      this will not work     p    amp z      printf    c n   p       return 0         int main         char   const p     address cannot be changed     char z       p    c          p    amp z       this will not work     printf    c n   p       return 0           int main         const char   const p     both address and value cannot be changed     char z       p    c      this will not work     p    amp z     this will not work     printf    c n   p       return 0

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Two rules   If const is between char and    it will affect the left one  If const is not between char and    it will affect the nearest one    e g    char const    This is a pointer points to a constant char  char   const  This is a constant pointer points to a char

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char   const and const char      Pointing to a constant value   const char   p                 value cannot be changed   Constant pointer to a value   char   const p                 address cannot be changed   Constant pointer to a constant value   const char   const p           both cannot be changed

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I would like to point out that using int const    or  const int    isn t about a pointer pointing to a const int variable  but that this variable is const for this specific pointer   For example   int var   10  int const    p    amp var    The code above compiles perfectly fine   p points to a const variable  although var itself isn t constant

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const always modifies the thing that comes before it  to the left of it   EXCEPT when it s the first thing in a type declaration  where it modifies the thing that comes after it  to the right of it    So these two are the same   int const  i1  const int  i2    they define pointers to a const int   You can change where i1 and i2 points  but you can t change the value they point at   This   int  const i3    int   0x12345678    defines a const pointer to an integer and initializes it to point at memory location 12345678   You can change the int value at address 12345678  but you can t change the address that i3 points to

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Constant pointer  A constant pointer can point only to a single variable of the respective data type during the entire program we can change the value of the variable pointed by the pointer  Initialization should be done during the time of declaration itself    Syntax   datatype  const var    char  const comes under this case     program to illustrate the behaviour of constant pointer      include lt stdio h gt  int main      int a 10    int  const ptr  amp a     ptr 100    we can change the value of object but we cannot point it to another variable suppose another variable int b 20  and ptr  amp b  gives you error     printf   d   ptr     return 0       Pointer to a const value  In this a pointer can point any number of variables of the respective type but we cannot change the value of the object pointed by the pointer at that specific time    Syntax    const datatype  varor datatype const  var  const char  comes under this case      program to illustrate the behavior of pointer to a constant        include lt stdio h gt     int main           int a 10 b 20         int const  ptr  amp a         printf   d n   ptr               ptr 100 is not possible i e we cannot change the value of the object pointed by the pointer          ptr  amp b         printf   d   ptr            we can point it to another object          return 0

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First one is a syntax error  Maybe you meant the difference between  const char   mychar   and  char   const mychar   In that case  the first one is a pointer to data that can t change  and the second one is a pointer that will always point to the same address

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Rule of thumb  read the definition from right to left     const int  foo   Means  foo points     to an int that cannot change  const    To the programmer this means  I will not change the value of what foo points to      foo   123  or foo 0    123  would be invalid  foo    amp bar  is allowed      int  const foo   Means  foo cannot change  const  and points     to an int   To the programmer this means  I will not change the memory address that foo refers to      foo   123  or foo 0    123  is allowed  foo    amp bar  would be invalid      const int  const foo   Means  foo cannot change  const  and points     to an int that cannot change  const    To the programmer this means  I will not change the value of what foo points to  nor will I change the address that foo refers to      foo   123  or foo 0    123  would be invalid  foo    amp bar  would be invalid

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Another thumb rule is to check where const is     before      value stored is constant after      pointer itself is constant

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const char  is a pointer to a constant character char  const is a constant pointer to a character const char  const is a constant pointer to a constant character

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I presume you mean const char   and char   const    The first  const char    is a pointer to a constant character   The pointer itself is mutable   The second  char   const is a constant pointer to a character   The pointer cannot change  the character it points to can   And then there is const char   const where the pointer and character cannot change

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I remember from Czech book about C  read the declaration that you start with the variable and go left  So for  char   const a    you can read as   a is variable of type constant pointer to char    char const   a    you can read as   a is a pointer to constant variable of type char  I hope this helps   Bonus   const char   const a    You will read as a is constant pointer to constant variable of type char

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Lots of answer provide specific techniques  rule of thumbs etc to understand this particular instance of variable declaration  But there is a generic technique of understand any declaration      Clockwise Spiral Rule   A   const char  a    As per the clockwise spiral rule a is pointer to character that is constant  Which means character is constant but the pointer can change  i e  a    other string   is fine but a 2     c   will fail to compile  B   char   const a    As per the rule  a is const pointer to a character  i e  You can do a 2     c   but you cannot do a    other string

[c] What is the difference between char * const and const char *?

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