[Boost-users] Re: Looking for lazy if_

From: David Abrahams (dave_at_[hidden])
Date: 2004-12-24 11:50:52

Agoston Bejo wrote:
> It seems to me that eval_if doesn't use lazy evaluation either. The error 
> message quite obviously refers to an attempt of the compiler to evaluate the 
> expression on the 'false' branch, whereas the condition (is_arithmetic<int>) 
> is true to my best knowledge.

eval_if uses lazy evaluation to the extent possible: it accepts two
nullary metafunctions and invokes one of them depending on how the
condition evaluates.  Your code computes T::value_type immediately,
before either metafunction can be invoked.
No library can change the properties of the C++ language.

You only need to make a metafunction that gets the value_type and use
that with eval_if:

  template <class T>
  struct get_value_type
  { typedef typename T::value_type type; };

  template <class T>
  struct value_type
    : eval_if<is_arithmetic<T>, identity<T>, get_value_type<T> >
  {};

HTH,
-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com