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

[Gabriel Dos Reis <gdr at cs dot tamu dot edu>]

Zack Weinberg <zack@codesourcery.com> writes:

| Gabriel Dos Reis <gdr@cs.tamu.edu> writes:
| 
| > Geoffrey Keating <geoffk@apple.com> writes:
| > | The 'const' is there to indicate that the string should not be changed
| > | once the STRING_CST is created;
| >
| > And how do you create it?
| 
| By casting away the const, as is done in build_string.  Or by
| constructing the data in memory before giving it the type with the
| const qualifier.

In the message I sent, I listed:

   Because, if you mark a data member as const, then you have to
   initialize it somehow at object contruction time -- either through
   brace-enclosed initializer or real constructors.  But, now, you go on
   putting it in a union, and that union does not even happen to be the
   first member.

Constructing the data in memory before giving it the type with the const
qualifier, is morally what C++ constructors do.

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. 

| How else do you propose to enforce write-once semantics on a data
| structure that is, in fact, write-once?

Notice that the declaration was "const char *pointer" before Geoff's
change. 

| I am definitely with Geoff  here - this should be allowed (or, to

I don't think this is an issue of being "with somebody" as opposed to
writing a code with defined meaning.

| rephrase, if C++ has no  construct which allows what we want to do
| here, that is a language  deficiency).

C++ allows several ways out of this, some may or may not be applicable
to C.  Definitely, the const-casting away is undefined behaviour.
My own rule of thumb is that we one starts fiddling with bytes and
type punning and trickery, something is wrong it is time to think at
least twice about it.

-- Gaby