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

[Geoffrey Keating <geoffk at apple dot com>]

On 13/10/2004, at 1:39 PM, Gabriel Dos Reis wrote:

> 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.
>
> ?

Yes.  The lvalue here is '(*p).fu', which has type 'int' and therefore 
is a modifiable lvalue.

Note that '*p' is not a modifiable lvalue, but all of '*p' is not being 
modified here.

> 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;
>    }

Yes.  Same reasoning.

--
Geoff Keating <geoffk@apple.com>

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