Re: [boost] smart ptr changes

From: Greg Colvin (gcolvin_at_[hidden])
Date: 2001-08-06 19:31:59

From: John Max Skaller <skaller_at_[hidden]>
> Greg Colvin wrote:
> 
> > At this point I don't care what the syntax is, or whether it
> > is a specialization or an overload or something invented just
> > for the purpose.  But there has to be some way for users to
> > replace performance-critical components of the standard
> > library like swap.
> 
> Well, there doesn't necessarily _have_ to be a way,
> a fact which can be demonstrated merely by observing
> that at present there isn't :-)
> 
> If you mean: 'there is a deficiency in the C++
> Standard' you'd like to see corrected, then doesn't
> it make sense to _analyse_ possible solutions?

Yes, that is what I mean.  And yes, it makes lots of sense
to analyse possible solutions, but I don't want to do it
because it makes my brain feel old and tired.

> In particular, a general solution to a 
> class of problems is likely to be seen as
> better than a hack for a particular problem.
> For example, allowing function templates to be 
> partially specialised may solve the problem (not sure),
> and it may also solve a lot of related problems,
> which surely arise in client libraries as well,
> for the same reasons.
> 
> So if you really want a solution,
> and think partial specialisation of function
> templates might be a solution, then you need
> to be prepared to analyse the requirements.
> 
> I contend that 
> 
> a) the technology for such specialisations
> already exists in C++ compilers, at least to 
> some extent
> 
> b) the only issue of any significance
> is syntax
> 
> c) there is an obvious syntax, with
> an obvious problem: the obvious syntax is:
> 
> template<class X> 
> template<class U=X, class V=X>
> void f(U,V)
> 
> and the obvious problem is that by extension the
> correct form for a full specialisation is:
> 
> template<>
> template<class U=int, class V=long>
> void f(U,V)
> 
> which does not agree with the current <stupid> syntax:
> 
> void f(int,long);
> 
> which is, in fact, the correct syntax
> for an overload NOT A #$%^&*<g> specialisation.
> 
> The obvious fix is to break some existing
> code using full specialisations of function templates.
> Not all of it will be broken -- overloading will provide
> similar semantics in many cases I guess.
> 
> Sometimes, if you want to do something right,
> you have to accept the need to correct past errors.
> 
> Something as important as full generality for
> templates shouldn't be plauged with compatibility
> hacks which are likely to cause more problems
> for generic meta-programming than the problematic benefits
> of attempting to save badly structured legacy code.
> 
> I note that _exactly_ the same form could work for classes,
> but it isn't necessary since classes can't be overloaded:
> you can write
> 
> template <class X>
> class F<X,X>
> 
> and that can be expanded to 
> 
> template <class X>
> template <class U=X, class V=X>
> class F
> 
> and vice versa, since, because there is only one
> template F for a class, the order of arguments in
> 
> F<X,X>
> 
> is enough to determine the partial specialisation
> binding functor
> 
> U-->X
> V-->X
> 
> You CANNOT do that for functions, since:
> 
> void f<X,X>(X,X)
> 
> STILL isn't enough to determine which template 'f'
> you mean, given two templates with signatures
> 
> f(U,V)
> f(V,U)
> 
> Unlike overloads, it matters: you really must say
> which template you are specialising.
> 
> I note that two step resolution is required:
> one first decides which template to apply,
> THEN decided which specialisation to use to
> generate the instance. The first step 
> is already done for functions, the second is the
> same as is already done for classes.
> 
> Upgrading compilers to do both is non-trivial,
> but it can't be that hard either.
> 
> -- 
> John (Max) Skaller, mailto:skaller_at_[hidden] 
> 10/1 Toxteth Rd Glebe NSW 2037 Australia voice: 61-2-9660-0850
> New generation programming language Felix  http://felix.sourceforge.net
> Literate Programming tool Interscript     
> http://Interscript.sourceforge.net
> 
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