Re: Template question...

From:

"Ben Voigt" <rbv@nospam.nospam>

Newsgroups:

microsoft.public.vc.language

Date:

Mon, 12 Feb 2007 17:13:35 -0600

Message-ID:

<#B$pbsvTHHA.4956@TK2MSFTNGP04.phx.gbl>

"Victor Bazarov" <v.Abazarov@comAcast.net> wrote in message
news:epnobc$q41$1@news.datemas.de...

Andy wrote:

A template questions I'm hoping someone might be able to help me with.

First off, I have a template defined something like,

template <int Count>struct ACollectionOfThings
{
// Definition of a collection that contains exactly Count things.
};

I would like to be able to stop people using this class with certain
values of Count, as an example I also have a template,

template <int Count>struct ASmallCollectionOfThings
{
// Definition of a collection that contains exactly Count things.
};

And I would like people to use the Small collection for values of
Count less than some number, so I would like a compile time error to
be issued when someone did,

CollectionOfThings<ASmallNumber> ACollection;

How would I go about making this happen?

   template<int Count> struct ACollectionOfThings {
       typedef char not_allowed[Count > 16];
       not_allowed& foo(); // dummy member
   };

   template<int Count> struct ASmallCollectionOfThings {
       typedef char not_allowed[Count <= 16];
       not_allowed& foo(); // dummy member
   };

This assumes that your compiler forbids arrays of size zero, which isn't
true of all compilers. Better would be:

template<int Count> struct ACollectionOfThings {
    ~ACollectionOfThings() { // or any other member function that gets used
        const bool toosmall = Count < 16;
        switch { case toosmall: case true: break; }
    }
};

   int main()
   {
       ACollectionOfThings<123> c123; // OK
       ACollectionOfThings<10> c10; // causes an error
       ASmallCollectionOfThings<10> sc10; // OK
       ASmallCollectionOfThings<123> sc123; // causes an error
       return 0;
   }

V
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