Re: A few minor questions

From:
"Alf P. Steinbach" <alfps@start.no>
Newsgroups:
comp.lang.c++
Date:
Tue, 19 May 2009 04:48:53 +0200
Message-ID:
<gut6qh$g2c$1@news.eternal-september.org>
* blargg:

You cannot call constructors;


Of course one can, and that's the terminology employed both in practical
programming and by the standard.

the compiler does it as a part of constructing an object.


I think you mean that the compiler generates machine code (or other executable
code such as byte code) that calls a constructor.

And that is partially correct.

A source code constructor call may however not necessarily be translated to an
actual machine code call.

When constructing an object, the compiler never
calls more than one constructor for a given class.


That's literally meaningless, but I think you mean, "the generated machine code"
rather than "the compiler".

And if so then that's very incorrect.

But a correct description of it all, including calls to base class constructors,
the case of virtual inheritance, C++0x constructor call forwarding, and the
whole shebang, would be long.

A constructor cannot be virtual, since it creates an object out of storage
where there was none before.


I'm sorry but that's again incorrect, a misguided rationalization. The reason
that a constructor cannot be declared "virtual" in C++ is because the language
is designed that way. As discussed ad nauseam before it's not at all difficult
to find reasonable and useful semantics for a virtual-declared constructor, and
indeed standard terms such as "virtual constructor" (see the FAQ) wouldn't exist
if such reasonable and practically useful semantics did not exist.

And again, you cannot call a constructor directly;


I'm sorry but that's meaningless.

the compiler does it as a part of constructing an object.


In C++98 the compiler does not call anything in the program being compiled; it
just generates corresponding code (usually machine code).

You
can supply parameters for a base class constructor, which the compiler
calls before executing the body of the derived class consturctor. "derived
class" is the C++ term for subclass, and "base class" for the parent, BTW.

I imagine the C++ FAQ might answer related questions:
<http://www.parashift.com/c++-faq-lite/>


Yes, the FAQ is a good idea. :-)

Cheers & hth.,

- Alf

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