[boost] Call traits, Compressed Pair

From: scleary_at_[hidden]
Date: 2000-01-17 14:15:48

On call_traits, we need to define some strict requirements; for example,
which of the following are valid for _any_ T:
  void func(call_traits<T>::param_type a)
  {
    call_traits<T>::value_type v = a; // assuming T has a copy constructor
    call_traits<T>::reference r1 = a;
    call_traits<T>::reference r2 = v;
    call_traits<T>::const_reference cr1 = a;
    call_traits<T>::const_reference cr2 = r1;
    v = a; // assuming T allows assignment; maybe we could have a
const_value_type?
    a = v; // assuming T allows assignment; maybe we could have a
const_param_type?
    // should we look at a return_type?  Or is it not necessary?
  }

IMO, we should allow, as much as possible, for reference, etc. to be proxy
types for a given T, so well-defined restrictions on how the types in
call_traits may be used is a must.

-

Howard: on your implementation of compressed_pair, I would add the ability
to derive from both types if they're both empty, thus allowing
compressed_pair to be an empty class as well.  (I already have some classes
that allow up to 3 contained templated classes to be empty -- a
compressed_pair of a compressed_pair :), and other classes that are empty if
their contained template classes are empty).  Of course, if both types are
the same type (watch out for cv-qualification), then we would have to
contain one and derive from the other, like you have it now.

Also, it's just personal preference, but I like to hide (in 'details'), as
much as possible, anything that is not documented, like the default template
parameters on compressed_pair.  For example, instead of:
  template <class T1, class T2,
      bool FirstEmpty  = is_empty<T1>::value,
      bool SecondEmpty = is_empty<T2>::value>
  class compressed_pair;
I prefer:
  namespace details {
    template <class T1, class T2,
        bool FirstEmpty  = is_empty<T1>::value,
        bool SecondEmpty = is_empty<T2>::value>
    class compressed_pair_helper;
    // implementation identical to original compressed_pair
  } // namespace details
  template <class T1, class T2> class compressed_pair
      : public details::compressed_pair_helper<T1, T2>
  { ... (constructors) };
  // OK to derive public because the base class is
  // in 'details', which the user shouldn't use

It's a bit more work, but will work if user code requires a compressed_pair
to have only two template parameters (think of a user class with a template
template parameter trying to be satisfied by compressed_pair -- not likely,
but possible).

        -Steve