Can 'operator T const& () const' do any harm?

"Alf P. Steinbach" <>
20 May 2006 17:00:51 -0400
I gather that with C++0x we'll be able to write

   class T { public: T(){} private: T( T const& ); };
   void foo( T const& ) {}

   int main() { foo( T() ); }

Currently this code is rejected by Visual C++ 7.1 (with option /Za),
Comeau Online version 4.3.3, and g++ 3.4.4.

However, this code:

   class NoCopy
   public: NoCopy() {}
   private: NoCopy( NoCopy const& ); NoCopy& operator=( NoCopy const& );

   class T: public NoCopy {};
   void foo( T const& ) {}

   int main() { foo( T() ); }

is accepted by two of the three compilers mentioned above; only g++
produces a compilation error (emits a diagnostic, barfs, whatever).

To make the code compile also with g++ one possibility is to do

   class T: public NoCopy
   public: operator T const& () const { return *this; }

This is IMO bad from a design point of view, because it relies on
convention (having this repeated in every class).

But can it do harm, e.g. unintentionally allow something Bad, or
disallow something Good?

A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
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