Re: Is this a new syntax
* Marcel M?ller:
Alf P. Steinbach wrote:
{
(void*) new (h) Class_Name(xip, virobj, type, true);
}
The only thing you really know is that that code is most likely full
of bugs.
Sorry, that last assumes that it's written by an ordinary programmer.
It /could/ be that it's written by an expert.
But if not, then it's probably chock full of bugs.
I wonder a bit about the differentiation between ordinary and expert
programmer.
Experts know that they don't know all. The old definition of an expert is one
who has made almost every conveivable error in the field of expertise, and
learned from that. Still, also experts make mistakes, even /trivial/ newbie
mistakes; the main difference is that they know it and that they deal with it,
and that they tend to more often choose paths that turn out to not be dead-ends.
Most probably there are not that many people around, that
use this syntax without the adequate knowledge. In fact I have never
seen any buggy implementation with this syntax.
Any buggy implementation of what?
Most uses of placement new are ill-conceived attempts at premature optimization.
I have mostly only seen such uses of placement new, but that may be because I
don't usually delve into the innards of code that deals with memory allocation
proper (like std::vector code).
At some locations it is really useful. E.g. STL containers can reserve
space for new elements without calling the standard constructor
immediately.
STL containers are implemented by experts and very very thoroughly tested, so
that ordinary programmers don't have to deal with the things they do inside but
can rely on the safe functionality that's exposed.
As far as I remember std::vector works this way to deal
with the logical size versus the allocation size.
Or I have implemented a simple, highly efficient, Java like, immutable
string class, that takes only the memory footprint of char* per instance
and the string value plus two ints header per different string value
(reference counted).
I think that's an example of non-expert use, evil premature micro-optimization
using the most dangerous low-level tool available. At least, it sounds like
you're allocating 1 chunk storage for the string data plus two ints, and rely on
implicit knowledge (not dynamically represented) in the carrier. If so, then in
addition to possible alignment bugs, that means that you /inefficiently/ have to
dynamically allocate some variable amount of storage for every different string,
when the cost of allocation is precisely what you try to avoid. ;-)
So, the syntax is only rarely needed. But if so, it is very important.
Yes.
It is something like a back door for special purposes to call the
constructor directly.
No, placement new doesn't call a constructor "directly". The constructor
arguments you pass are forwarded to a constructor, not passed directly as in an
ordinary function call. In between there's a lot going on, including
preparation of cleanup in the event that the constructor throws.
Placement new gives some access to the class' allocator.
It forwards arguments to the allocation function and by that lets you choose the
allocation function (from those available), and that's it.
Calling a destructor explicitly is not less
dangerous and looks quite normal.
Don't mix "explicitly" and "directly": those are two very different notions.
Calling a destructor explicitly should never be done in normal high level code.
A "goto" also looks quite normal, but you know how it is... And a "goto" is far
more benign than an explicit destructor call.
Cheers, & hth.,
- Alf
PS: I think some of this is discussed in the FAQ. Can't hurt to look there.
--
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