Compiling GCC With a C++ Compiler (g++)

[Gabriel Dos Reis]

Geoffrey Keating <geoffk@apple.com> writes:

| On 12/10/2004, at 8:42 PM, Gabriel Dos Reis wrote:
| 
| > The code in build_string
| >
| >   memset (s, 0, sizeof (struct tree_common));
| >   TREE_SET_CODE (s, STRING_CST);
| >   TREE_STRING_LENGTH (s) = len;
| >   memcpy ((char *) TREE_STRING_POINTER (s), str, len);
| >   ((char *) TREE_STRING_POINTER (s))[len] = '\0';
| >
| > is trying to do similar thing, but it is undefined behaviour.
| >
| >        [#5] If an attempt is made to modify an object defined  with
| >        a  const-qualified  type  through use of an lvalue with non-
| >        const-qualified type, the  behavior  is  undefined.
| 
| The object here is '*s', which was not defined with a const-qualified
| type.  It's not defined with any type at all, it was allocated through
| malloc which is not a typed interface.  (It would be different if it
| was allocated through 'new', but we do not have new in C.)

The undefined behaviour comes from different sources.
Consider the following:

   typedef struct {
      int fu;
      const int bhar;
   } S;

   void f(S *p)
   {
       (*p).fu = 90;            // #1
   }

Do you believe line #1 is valid in face of

       [#1]  An  lvalue  is an expression with an object type or an
       incomplete  type  other  than void;53) if an lvalue does not
       designate an object when it is evaluated,  the  behavior  is
       undefined.   When  an  object  is  said to have a particular
       type, the type is specified by the lvalue used to  designate
       the  object.  A modifiable lvalue is an lvalue that does not
       have array type, does not have an incomplete type, does  not
       have  a  const-qualified  type,  and if it is a structure or
       union, does not have any member (including, recursively, any
       member  or  element  of  all contained aggregates or unions)
       with a const-qualified type.

?

Notice that here, it is not a question of where the object pointed to 
by *p comes from.  

Next question is for

   void g(S *p)
   {
       p->fu = 90;
   }

-- Gaby