🚀 go-pugleaf

jchristo@mit.edu (James Christodouleas) writes:

>fjh@munta.cs.mu.OZ.AU (Fergus Henderson) wrote:
>
>> Philippe Verdy <100105.3120@compuserve.com> writes:
>> 
>> >template <class T, class CT>
>> >class SmartPtr2  {
>> >   SmartPtr2(T* p) { mp = const_cast<CT>(p) ; }
>> >   CT & Dereference() {
>> >     return *mp ;
>> >   }
>> > private :
>> >   CT *mp ;
>> >} ;
>> >template <class T> class ConstSmartPtr : SmartPtr<T, T> {} ;
>> >template <class T> class FreeSmartPtr : SmartPtr<T, const T> {} ;
>> 
>> You forgot to delegate the constructors.
[...]
>What does it mean to "delegate constructors?"

Delegation is when the code to implement an method (in this case,
the constructor) in one class just "delegates" the work to someone
else by calling the same method in another class.

Perhaps the simplest way to explain what I meant is to write out the code:

	template <class T> class ConstSmartPtr : SmartPtr2<T, T> {
		// delegate the constructor
		CountSmartPtr(T* p) : SmartPtr2(p) {}
	};
	template <class T> class FreeSmartPtr : SmartPtr2<T, const T> {} ;
		// delegate the constructor
		FreeSmartPtr(T* p) : SmartPtr2(p) {}
	};

In C++, most operations are automatically delegated from a derived class
to its base class -- this is called inheritence.  However, C++ doesn't
provide any special support for delegating constructors.  That has to be
done manually.

I also wrote:

>> (Of course, if C++ supported template typedefs, there wouldn't be
>> any need to delegate constructors in examples like this.
>> Unfortunately it doesn't.)

In case that wasn't clear, I meant that instead of the above code, if
C++ supported template typedefs, you could write

	template <class T> typedef SmartPtr2<T, T> ConstSmartPtr;
	template <class T> typedef SmartPtr2<T, const T> FreeSmartPtr;

--
Fergus Henderson             	WWW: http://www.cs.mu.oz.au/~fjh
fjh@cs.mu.oz.au              	PGP: finger fjh@128.250.37.3
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