TITLE: example of traits usage

(Newsgroups: comp.lang.c++.moderated, 12 Mar 2000)

STUEKEN: Dieter Stueken <stueken@conterra.de>

> this is a simple function to copy a const array:
> 
> const int *copy(const int *src, int len)
> {
>   int *tmp = new int[len];
>   for(int i=0; i<len;i++)
>     tmp[i] = src[i];
> 
>   return tmp;
> }
> 
> OK. looks fine. Now I want to turn it into a template:
> 
> template<class T> T *copy(T src, int len)
> 
> You will find, that tmp turns into const too
> and cannot be initialized any more :-(
> 
> The only solution I found is to define an other const_copy
> template, to get hands on the non const T, like:
> 
> template<class T> const T *const_copy(const T src, int len)
> 
> Unfortunately I don't have just a tiny function but a huge
> template class instead. So, do I have to copy the whole thing
> again for the const case, or is there an other solution for that?

ADLER: Darin Adler <darin@bentspoon.com>

There's a solution with traits:

   template <typename T> struct non_const_pointer;

   template <typename T>
   struct non_const_pointer<T*> { typedef T* type; };

   template <typename T>
   struct non_const_pointer<const T*> { typedef T* type; };

   template<class T>
   typename non_const_pointer<T*>::type
   copy(T* src, int len) {
      typename non_const_pointer<T*>::type tmp = new T[len];
      for (int i=0; i<len; i++)
         tmp[i] = src[i];
      return tmp;
   }

There are probably more elegant variants of this solution as well.



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