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From: Simonson, Lucanus J (lucanus.j.simonson_at_[hidden])
Date: 2008-05-02 21:43:42

I have modified my example code to move the accessors out of the
templated traits class and into a non-templated concept struct as
Fernando suggested.


I also added compile time accessors that are parameterized by enum
values, which I decided would give me the benefits of type safety and
readability I was getting with my isotropic types as runtime parameters,
while allowing the much desired compile-time parameter for people who
prefer it. The runtime accessors are still there and call the
compile-time accessors by default.


The user can specialize the accessors to adapt them to their type, but
now it is significantly harder to do so, because previously the
accessors were members of a template class, and it was the class that
was specialized, allowing the functions to retain their template
parameter list. Now I am unable to partially specialize the accessor
functions, which means that I cannot make their arguments template
parameters of the function (because the user can't be expected to
specialize for every conceivable type.) The user also has to explicitly
specialize each version of the compile-time accessor, such as:


template <>

inline rectangle_traits<LayoutObject>::interval_type

rectangle_concept::get<HORIZONTAL>(const LayoutObject& rectangle) {

  return rectangle_traits<LayoutObject>::interval_type(rectangle.xl,



template <>

inline rectangle_traits<LayoutObject>::interval_type

rectangle_concept::get<VERTICAL>(const LayoutObject& rectangle) {

  return rectangle_traits<LayoutObject>::interval_type(rectangle.yl,



and can't do the following:


template <orientation_2d_enum orient>

inline rectangle_traits<LayoutObject>::interval_type

rectangle_concept::get(const LayoutObject& rectangle) {

            if(orient == HORIZONTAL) return




as might be their first inclination. I'm not sure to what extent that
makes specializing the accessors harder. User feedback I get is that
specialization (all kinds) is hard, and in practice I usually help them
out by providing the specializations they need for the types they have
to get them over that first hurdle and running with library.


I feel that there is still some more progress to be made here, please
have a look.




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