Re: Is it a good way to use constructor?

From:

Scot T Brennecke <ScotB@Spamhater.MVPs.org>

Newsgroups:

microsoft.public.vc.language

Date:

Fri, 07 Aug 2009 01:27:39 -0500

Message-ID:

<OxQeqgyFKHA.3948@TK2MSFTNGP03.phx.gbl>

Ah, composition, not inheritance. Big difference. It changes the answer entirely.
During the class initializer of B(), the B object is not fully constructed, but 'this' does exist. It points to the B object that
is in the middle of being constructed. If A(B&) does not attempt to use the B object in its constructor, it should be safe to keep
a reference to it for later. If B has any base classes, their constructors will have been completed by the time A() is called.

Lorry Astra wrote:

Sorry, I make a mistake here, I rewrite them.
class A{
public:
    A(B& b){}
};

class B{
     A a;
public:
     B() : a(*this)
     {}
};

It should be an composition, not a inheritance, sorry about that. But I
have the same question, I think when object a is constructed using "*this", I
believe "*this" is not existed at that time. Is that right? Because
constructor B is not executed while a(*this) running.

"Victor Bazarov" wrote:

Lorry Astra wrote:

Dear all,
I have a question in today's code review, here is my code example:
class A{
public:
A(B& b)

'B' hasn't been defined or declared at this point.

{....}
};

class B{
public:
B() : A(*this)

This is ill-formed unless 'B' derives from 'A'.

{....}
};

I think here exists a risk in class B constructor.

A risk of what?

> Because I think when

constructor A is executed before constructon B ("B() : A(*this)"),
constructor A uses an object of class B as parameter.

So?

> But I believe the

object of class B is not generated while constructor A running.

You mean it hasn't been completely constructed yet, yes?

> So I think

"A(*this)" is a risk.

A risk of *what*?

So I think if this code is like that, the executed sequence will be:
1. A(*this) -> A(B& b)
2. B()

Is that right? Are there any errors in my description?

Yes, plenty. First off, the code is ill-formed. Please fix the code
and ask your question again then. In the mean time read the C++ FAQ,
section 10, question 10.7.

V
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