Re: [Texmacs-dev] Applied patches

[david]

On Sat, Jan 18, 2003 at 12:49:01PM +0100, Joris van der Hoeven wrote:
> 
> > > 951: Applied
> > 
> > This patch is wrong and should be unapplied.
> 
> OK, I did that.
>
> > The correct fix is to remove the uneeded (and seemingly wrong)
> > parenthesis in the type-id of new.
> 
> I am not sure that I understand what you propose.
> 
>   tree** rs= new (tree*)[nr_rows];
> 
> by
> 
>   tree** rs= new tree* [nr_rows];

Yes. In a recent answer to you on this mailing list, I said:

    > >   6. A few (2 or 3) explicit casts to type (X**) for the
    > >   result of a "new (X*) [...]" operation to work around an
    > >   apparent bug in our > > 7.4 compilers (I've also given
    > >   notice about this to our C++ front-end > > engineers). > >
    > >   No problem; we will correct that too.

    I am unsure that is really a compiler problem, and unsure the fix
    is right. For the record, the original code looks like:

       tree** rs= new (tree*)[nr_rows];

    ISO C++, section 5.3.4 [expr.new], clause 5:

      When the allocate object is an array (that is, the
      /direct-new-declarator/ syntax is used or the /new-type-id/ or
      /type-id/ denotes an array type, the /new-expression/ yields a
      pointer to the initial element (if any) of the array [Note: both
      'new int' and 'new int[10]' have type 'int*' and the type of
      'new int[i][10]' is 'int (*)[10]'.]

    That means that no cast (implicit or explicit) should be involved
    in this statement.

    In ISO C++ section 5.3.4 [expr.new], clauses 1, 2 et 3 one see
    that parenthesis in new-expressions can have suprising effects. It
    seems that in that code (tree*) may be interpreted as a
    parenthesised type-id and [nr_rows] as a new_initializer (which
    would yield a syntax error). Actually the standard seems a bit
    ambiguous on that point, which would explain that different
    compilers behave differently.

    What is the actual error you are getting from your compiler?

    Actually it seems that tree*[nr_rows] effectively means 'array of
    nr_rows pointers to tree'. So the following is correct:

      tree** rs= new tree*[nr_rows];

    A similar example is given in section 8.1 [dcl.name].

    Actually, it does not even seems that the seemingly unambiguous

      tree** rs= new ((tree*)[nr_rows]);

    is correct.
    
I admit that if you read mail *very* quickly, you may misunderstand
what I just said here.
    
    If one want to avoid type-id syntax, one should instead use
    typedef.

The "seemingly" came from some experiments I made with the program
c++decl:

c++decl> explain (int*[])i
cast i into array of pointer to int
c++decl> explain ((int*)[])i
parse error

-- 
David Allouche         | GNU TeXmacs -- Writing is a pleasure
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TeXmacs is NOT a LaTeX front-end and is unrelated to emacs.