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From: Philip Gibbs <philip.gibbs@weburbia.com>
Subject: Re: Access control
Date: 1999/05/20
Message-ID: <VIOwJLAHoIR3Ewu0@194.222.186.18>#1/1
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Approved: stephen.clamage@sun.com (comp.std.c++)
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In article <7hvum9$4bb$1@nnrp1.deja.com>, Andrei Alexandrescu
<andrewalex@hotmail.com> writes
>
>Let's analyze this code:
>
>class A
>{
>       A();
>};
>
>class B : public A
>{
>public:
>       B();
>};
>
>B::B()
>{
>}
>
>B var;
>
>The code is in error, because B's default constructor will implicitly
>call A's default constructor, which is private.
>
>The question is: when should the compiler generate an error? This is a
>no-nonsense question, please take it seriously.
>
>1. At B::B()'s declaration
>2. At B::B()'s definition
>3. At var's definition

2. I would expect it at B::B()'s definition because this
uses the private constructor A::A() which is illegal here.

>
>Is there any guarantee on *where* the error will be generated?

I don't think there even has to be a concept of where in the code
a compile time error is generated.

>Put another way: if you comment out var's definition, will the code be
>valid? 

No, and with good reason. A similar variable declaration might
appear in another file where the definition of B::B() is not
included. Separate compilation would therefore be impossible
if the code was allowed to be valid.  

>If not, and if you comment B::B's definition, will the code be
>valid?

Yes. The definition of B::B() could be in another source file
and could be valid if it used the copy constructor of A instead
of the default constructor. The copy constructor would have to
construct from an object of a class derived from A (e.g. B)
because objects

I know the following is useless but it is well formed
code giving an alternative definition of B::B() which will
produce no compile time error

B b;
B::B():
A(b)
{
}

I think the result must be undefined since b is constructed
using itself. I can't see a way of producing a definition
of B::B() which would not have undefined behaviour but it
would be a clever compiler which could deduce as much from
the class definitions. 
 


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