Re: template template parameters not working with STL containers

From:

David Abrahams <dave@boostpro.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 19 Feb 2009 12:55:39 CST

Message-ID:

<87hc2qwdnh.fsf@mcbain.luannocracy.com>

on Wed Feb 18 2009, vl106 <vl106-AT-hotmail.com> wrote:

The code sample below is not working. It is taken from
Nicolai Josuttis "C++ Templates: The Complete Guide".
In the chapter 5.4 Template Template Parameters it reads
[slightly stripped]:

#include <deque>

template <typename T>
class MyVector {
public:
void push_back(T const&) {}
};

template <typename T, template <typename> class CONT = /*MyVector*/> std::deque >

<snip>

Others have explained the problem; I can offer some alternative
solutions:

1. Metafunction Class:

   // Create one of these for each container type
   struct make_deque
   {
       template <class T>
       struct apply
       {
           typedef std::deque<T> type;
       };
   };

   template <typename T, typename GenCont = make_deque>
   class Stack {
   private:
        typename GenCont<T>::type elems; // elements
      public:
        void push(T const&); // push element
   };

2. MPL Lambda Expression:

   #include <boost/mpl/apply.hpp>
   #include <boost/mpl/placeholders.hpp>
   namespace mpl = boost::mpl;

   template <typename T, typename GenCont = std::deque<mpl::_> >
   class Stack {
   private:
        typename mpl::apply<GenCont,T>::type elems;
      public:
        void push(T const&);
   };

See http://www.boost.org/libs/mpl and http://www.boostpro.com/mplbook
for more info.

Regards,

--
Dave Abrahams
BoostPro Computing
http://www.boostpro.com

      [ See http://www.gotw.ca/resources/clcm.htm for info about ]
      [ comp.lang.c++.moderated. First time posters: Do this! ]