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.