Re: Derived::Derived(const Base&)

"Alf P. Steinbach" <>
Wed, 23 May 2007 11:32:43 +0200
* James Kanze:

On May 22, 6:44 pm, "Alf P. Steinbach" <> wrote:


Can anybody shed some light on this problem?

class Interface {
  Interface() { ...}
  virtual ~Interface() { ...}
  virtual method() = 0;

class Impl1: public Interface {
  Impl1() { ...}
  Impl1(const Interface&); // problem 1
  virtual ~Impl1() { ... }
  Impl1& operator=(const Interface&); // problem 2

The problem is that the compiler insists on generating the following
  Impl1(const Impl1&); // copy constructor
  Impl1& operator=(const Impl1&); // assignment operator
for me.
I do not need these methods.
I do not want these methods.
I would have thought the compiler would call one of my explicit
methods since every Impl1 is also an Interface.

No. Both the copy constructor and the copy assignment operator are very
special member functions (thus, listend under "Special member
functions"). They're generated if they're used and not declared.

Is there some simple trick I am missing here?

At the technical C++ level: just declare them.

I don't think that will do what he wants. If I understand him
correctly, he wants Impl1( Interface const& ) to be used when
copying an Interface. In that case, the only solution he has is
to als define his Impl1( Impl1 const& ) to do exactly the same

First off, technicality: a definition is a declaration, so in a C++
technical interpretation that solution is included in what I said.

But just declaring them with no definition is, contrary to (the natural
and most sensible interpretation of) your statement, sufficient to
guarantee they'll not be invoked.

Instead of using static_cast it's then convenient to equip the Interface
class with an explicit asInterface() member function:

     #include <iostream>
     #include <ostream>

     void say( char const s[] ) { std::cout << s << std::endl; }

     class Interface
         Interface() {}
         virtual ~Interface() {}
         virtual void method() = 0;

         virtual Interface& asInterface() { return *this; }

     class Impl1: public Interface
         Impl1( Impl1 const& );
         Impl1& operator=( Impl1 const& );

         Impl1() {}

         Impl1( Interface const& )
         { say( "Copying interface" ); }

         Impl1& operator=( const Interface& )
         { say( "= interface" ); return *this; }

         void method() {}

     int main()
         Impl1 a;
         Impl1 b( a.asInterface() );

         a = b.asInterface();

But at the design level, having polymorphic assignment is almost never a
good idea.

Have you really thought through the consequences, how to handle all
combinations of destination and source (e.g., run time errors)?

Maybe he's implementing the letter/envelope idiom. (But
somehow, I don't think so, and I think you're right, copy and
assignment aren't going to work like he wants.)

Yes. Instead of copying to existing objects, he should probably be
considering cloning. And with a restriction to dynamic allocation the
asInterface function wouldn't be needed because all objects would be
handled via pointers or references to interfaces.

A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

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