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From: brianl@wenti.Berkeley.EDU (Brian Lee)
Newsgroups: comp.lang.c++
Subject: Re: Default constructor, unexpected call
Keywords: default constructor, copy constructor
Message-ID: <26580@pasteur.Berkeley.EDU>
Date: 30 Jul 90 22:52:36 GMT
Sender: news@pasteur.Berkeley.EDU
Reply-To: brianl@wenti.Berkeley.EDU (Brian Lee)
Lines: 65
X-Local-Date: 30 Jul 90 15:52:36 PDT

In Article 8713, leo@atcmp.nl (Leo  Willems) describes how
the default constructor for a member class is sometimes called
instead of the copy constructor.  I am also using AT&T C++
Release 2.0, and a similar thing happens for base classes.  I'm
relatively new to this newsgroup, so please forgive me if this
is old news.

Consider the following program

#include <iostream.h>

class X {
public:
    X()         { cout << "default X::X()\n"; }
    X(const X&) { cout << "copy X::X(const X&)\n"; }};

class Y : public X {
public:
    Y()         { cout << "default Y::Y()\n"; }
//    Y(const Y&)       { cout << "copy Y::Y(const Y&)\n"; }
};

class Z : public Y {
public:
    Z()         { cout << "default Z::Z()\n"; }
    Z(const Z&) { cout << "copy Z::Z(const Z&)\n"; }
};

main()
{
    cout << "\ndefault:\n";
    Z k;
    cout << "\ncopy:\n";
    Z l = k;
}

It produces the following output

default:
default X::X()
default Y::Y()
default Z::Z()

copy:
default X::X()
default Y::Y()
copy Z::Z(const Z&)

Note that the default constructor for Y is called instead of a
copy constructor being generated and that calling the default 
constructor for Y causes the default constructor for X to be called
instead of the copy constructor for X.

Assuming that the semantics of member-wise initialization extend
to base class initialization, this is wrong.

Try increasing the length of the inheritance chain and/or playing
with different combinations of defined/undefined copy constructors.

It appears that to be safe, one should define a default and copy
constructor for all classes.

    Brian Lee
    U.C. Berkeley Cadgroup
    brianl@ic.Berkely.EDU