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?
My second question is: How did I do? My guess is that the second crack
is more correct.
Here is crack one:
My first crack at the exercise is below (my 2nd crack is below that)
#include <iostream>
#include ::value_type t) {
// .. do something
}
class BigClass
{
};
int main()
{
int i = 0;
BOOST_STATIC_ASSERT((boost::is_same ::template result_<T>::value_type value_type; };
template<typename T>
void func(typename add_const_ref<T>::value_type t)
{
// .. do something
}
class BigClass
{
};
int main()
{
int i = 0;
BOOST_STATIC_ASSERT((boost::is_same
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
Ben Hutchings wrote:
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.
Any solutions is allowable, though that one is not particularly elegant.
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
{ typename T & type; };
Actually there's a much more concise solution using the type traits library. That is, after all, the focus of the chapter. -- Dave Abrahams Boost Consulting http://www.boost-consulting.com
David Abrahams wrote:
Ben Hutchings wrote:
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.
Any solutions is allowable, though that one is not particularly elegant.
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; };
Actually there's a much more concise solution using the type traits library. That is, after all, the focus of the chapter.
Remind me not to talk about books I haven't read in future... Ben.
participants (3)
-
Ben Hutchings -
David Abrahams -
Trask, Bruce (US SSA)