Difference between char and const char

Question

What s the difference between   char  name   which points to a constant string literal  and   const char  name

User · Answer

In neither case can you modify a string literal, regardless of whether the pointer to that string literal is declared as char * or const char *.

However, the difference is that if the pointer is const char * then the compiler must give a diagnostic if you attempt to modify the pointed-to value, but if the pointer is char * then it does not.

User · Answer

char  is a mutable pointer to a mutable character string   const char  is a mutable pointer to an immutable character string  You cannot change the contents of the location s  this pointer points to  Also  compilers are required to give error messages when you try to do so  For the same reason  conversion from const char   to char  is deprecated   char  const is an immutable pointer  it cannot point to any other location  but the contents of location at which it points are mutable   const char  const is an immutable pointer to an immutable character string

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CASE 1   char  str    Hello   str 0     M     Warning may be issued by compiler  and will cause segmentation fault upon running the programme   The above sets str to point to the literal value  Hello  which is hard-coded in the program s binary image  which is flagged as read-only in memory  means any change in this String literal is illegal and that would throw segmentation faults   CASE 2   const char  str    Hello   str 0     M     Compile time error   CASE 3   char str      Hello   str 0     M      legal and change the str    Mello

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The question is what s the difference between char  name  which points to a constant string literal  and const char  cname  I e  given char  name    quot foo quot    and const char  cname    quot foo quot     There is not much difference between the 2 and both can be seen as correct  Due to the long legacy of C code  the string literals have had a type of char    not const char    and there are lots of older code that likewise accept char   instead of const char    even when they do not modify the arguments  The principal difference of the 2 in general is that  cname or cname n  will evaluate to lvalues of type const char  whereas  name or name n  will evaluate to lvalues of type char  which are modifiable lvalues  A conforming compiler is required to produce a diagnostics message if target of the assignment is not a modifiable lvalue  it need not produce any warning on assignment to lvalues of type char  name 0     x      no diagnostics  needed  cname 0     x      a conforming compiler  must  produce a diagnostic message  The compiler is not required to stop the compilation in either case  it is enough that it produces a warning for the assignment to cname 0   The resulting program is not a correct program  The behaviour of the construct is undefined  It may crash  or even worse  it might not crash  and might change the string literal in memory

User · Answer

Actually  char  name is not a pointer to a constant  but a pointer to a variable   You might be talking about this other question   What is the difference between char   const and const char

User · Answer

char  name   You can change the char to which name points  and also the char at which it points   const char  name   You can change the char to which name points  but you cannot modify the char at which it points  correction  You can change the pointer  but not the char to which name points to  https   msdn microsoft com en-us library vstudio whkd4k6a v vs 100  aspx  see  Examples    In this case  the const specifier applies to char  not the asterisk   According to the MSDN page and http   en cppreference com w cpp language declarations  the const before the   is part of the decl-specifier sequence  while the const after   is part of the declarator   A declaration specifier sequence can be followed by multiple declarators  which is why const char   c1  c2 declares c1 as const char   and c2 as const char   EDIT   From the comments  your question seems to be asking about the difference between the two declarations when the pointer points to a string literal     In that case  you should not modify the char to which name points  as it could result in Undefined Behavior  String literals may be allocated in read only memory regions  implementation defined  and an user program should not modify it in anyway  Any attempt to do so results in Undefined Behavior        So the only difference in that case  of usage with string literals  is that the second declaration gives you a slight advantage  Compilers will usually give you a warning in case you attempt to modify the string literal in the second case   Online Sample Example    include  lt string h gt  int main         char  str1    string Literal       const char  str2    string Literal       char source      Sample string        strcpy str1 source        No warning or error  just Undefined Behavior     strcpy str2 source        Compiler issues a warning      return 0      Output          cc1  warnings being treated as errors   prog c  In function    main       prog c 9  error  passing argument 1 of    strcpy    discards qualifiers from pointer target type        Notice the compiler warns for the second case but not for the first

User · Answer

The first you can actually change if you want to  the second you can t  Read up about const correctness  there s some nice guides about the difference   There is also char const   name where you can t repoint it

User · Answer

char mystring 101     My sample string   const char   constcharp   mystring      1  char const   charconstp   mystring      2  the same as  1  char   const charpconst   mystring      3   constcharp       ok charconstp       ok charpconst       compile error  constcharp 3      0      compile error charconstp 3      0      compile error charpconst 3      0      ok     String literals char   lcharp    My string literal   const char   lconstcharp    My string literal    lcharp 0     X           Segmentation fault  crash  during run-time lconstcharp 0     X      compile error      not  a string literal const char astr 101     My mutable string   astr 0     X               compile error   char  astr  0     X      ok

[c] Difference between char* and const char*?

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