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From: comeau@panix.com (Greg Comeau)
Newsgroups: comp.std.c++
Subject: Re: Strange new/new() problem
Date: Sat, 22 Jan 2005 03:47:55 GMT
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In article <41ec486b$1@solnews.wv.mentorg.com>,
Fedor G. Pikus <fedor_pikus@mentorg.com> wrote:
>I've run into a little problem with operator new.
>Consider this example:
>struct data
>{
>     int i;
>     int j;
>};
>int main()
>{
>     data** p = new (data*)[5];
>}
>
>What the programmer wanted is to allocate an array of pointers, of 
>course, and the parenthesis are unnecessary but arguably make reading 
>the code easier.

That does not request an array of pointers.  These don't work as decls:

(data *)p[5];
(data *p2)[5];

An array of pointers to data is 'data *[5]'

>GCC compiled the code just fine, and did what the programmer wanted, but 
>several other compilers refused. Error messages are of two types: either 
>it's "cannot assign data* to data**" or "[5] is unexpected". Comeau 
>falls into the first cathegory, and I usually trust Comeau to get it 
>right. Both messages suggest that these compilers interpret this code as 
>the placement new with "data *" being the location, even though it's a 
>type, not a variable.

It's not being interpreted as a placement new, but as a 'new data *'

>Just for kicks, I tried this modification:
>data* p1 = new (data*)[5];
>on some of the compilers which complained about pointer conversion, and 
>now the compilers are happy,

But the program is still wrong.

>program runs,

Well, at least does not fail.

>and p is NULL.

Appears to be thus far in your tests.

>So, is the above (topmost) example legal in C++ or parenthesis are 
>actually wrong, not just unnecessary?

Wrong IMO.

>Is the modification legal in C++ (Comeau says yes)? If the p1 line is 
>legal, what is it doing, how is it parsed? "data*" is a location 
>argument to placement new? What't the type then?

You've sub-expressions, starting with 'new data *'.
-- 
Greg Comeau / Comeau C++ 4.3.3, for C++03 core language support
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