Re: [Boost-users] C++ Template Metaprogramming - Exercise 2-0

From: Ben Hutchings (ben.hutchings_at_[hidden])
Date: 2005-01-10 10:08:58

Trask, Bruce (US SSA) wrote:
> Hello,
> 
> I am going through the book C++ Template Metaprogramming and wanted a 
> sanity check on Exercise 2-0 from the book.
> 
> My first question is:  What would be a motivating example for why one 
> would want a add_const_ref<T> metafunction that returned T if it was a 
> reference and T const& otherwise?

It's useful for forwarding function arguments.  This document gives 
several examples of cases where one may want to do that: 
<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1385.htm>. 
Forwarding must be done by reference, but implementations are required 
to treat T & as ill-formed if T is a reference type, rather than 
treating it as being the same as T.  add_const_ref<T> should yield the 
correct type to use for forwarding an argument of type T.

> My second question is:  How did I do? My guess is that the second crack 
> is more correct.
<snip>

Well the first attempt doesn't solve the problem because it requires an 
extra parameter.  The second attempt solves it but I don't think you're 
supposed to make use of other templates.  I think the expected solution 
would be more like this:

     template<typename T>
     struct add_const_ref
     {
         typename T const & type;
     };

     template<typename T>
     struct add_const_ref<T &>
     {
         typename T & type;
     };

Ben.