Re: Derived::Derived(const Base&)

"Alf P. Steinbach" <>
Tue, 22 May 2007 18:44:24 +0200

Hi folks,

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.

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)?

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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