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From: =?ISO-8859-1?Q?J=F6rg?= Barfurth <joerg.barfurth@attglobal.net>
Subject: Re: templated Copy-Constructor?
Date: 2000/04/29
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Am 29.04.00, 02:41:36, schrieb "Stephan Keil" <kei@parsytec.de> zum Thema=
=20
templated Copy-Constructor?:


> Hi,

> consider the following example:

>  template<class T>
>  struct Blah
>  {
>   Blah(){}

>   template<class U>
>   Blah(const Blah<U>&)
>   {
>    cout << "Blah" << endl;
>   }
>  };

>  Blah<double> bd(Blah<int>());
>  Blah<int> bi(Blah<int>());

> With VC++ 6.0 the construction of 'bd' invokes the templated
> constructor where as the construction of 'bi' does not
> (i.e. apparently the compiler generated copy constructor
> is used instead). Is this correct behaviour according to
> the standard?

This is correct. The standard explicitly says that template constructors=20
(and assignment operators) wont be considered for copy construction (or=20
assignment).

Basically, the compiler-generated declarations are suppressed only if you=
=20
declare a non-template copy constructor (or assignment operator).

But then the (non-template) implicitly-declared overload takes precedence=
=20
over any matching template specializations.

Some more motivation: Otherwise the compiler, when seeing ...

template <class> struct Blah;
template <class T, class U> struct BlahDef;

template <class T>
struct Blah
{
	template <class U> Blah(Blah<U> const&);
	template <class U> Blah(U);

	template <class U> struct rebind { typedef Blah<U> other; }
	template <class U> Blah(typename rebind<U>::other const&);
	template <class U> Blah(typename Blah<U>::rebind<T>::other const&);

	template <class U> Blah(typename BlahDef<U, T>::type const&);
};

template <class T, class U> struct BlahDef
{=20
	typedef typename Blah<T>::rebind<U>::other type;
	// or simply: typedef T type;
};

... would have to try to find template arguments to instantiate a copy=20
constructor from the templates, in order to find out whether to=20
implicitly declare a copy constructor. Even restricting this to contexts=20
where the template arguments are deducible doesn't really improve the=20
situations.

As the standard is those template declarations might never be=20
instantiated.

> thanks for comments,

> Stephan

HTH

-- J=F6rg


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