Re: [Boost-users] Simple MPL question: Generate code from type sequence

4 Jan 2007

      Johan,

there is an mpl::for_each construct to do this. Unfortunately for_each is not documented, but it
is a part of mpl library. If you have the MPL book from Dave and Alexey you can read about it on
page 175. On page 176-179 you will find 3 differnt examples about type printing. Below is one of
the approaches converted to you example:

Your example should work with the following sample:

#include <iostreams>
#include 
#include 

namespace mpl = boost::mpl;

template<class T>
struct Type2Type {}; //this will prevent instantiation of every type
                     // and transport the type information only
                     // => Type2Type is only 1 byte big => cheaper as a reference or ptr

struct WriteSize
{
    template<class T>
    void operator() (Type2Type<T> t)
    {
        std::cout << sizeof(T) << " ";
    }
};

typedef mpl::vector SomeVector;

//now the call to for_each
mpl::for_eachmpl::_1 >(WriteSize());

With Kind Regards,

Ovanes Markarian

On Thu, January 4, 2007 11:34, Johan Torp wrote:
...
I would like to generate one call to a templated function for each type
in an MPL sequence. Which is the best way to do this?
E.g.:
typedef vector SomeVector;
template <typename T> write_size() { std::cout << sizeof(T) << " "; }
Should somehow expand to:
write_size<float>();
write_size<double>();
write_size<long double>();
I have managed to do this "manually" in a fairly general way, what I'm
looking for is some part of boost which replaces my ForEach struct below
or some nicer way of writing the same code:
==============================================
struct WriteSize
{
  template<typename T> void Call() {write_size<T>();}
};
template  struct ForEach
{
  template <class Func>  static void Call(Func& func)
  {
    func.Call<typename It::type>();
    ForEach::Call(func);
  }
};
template <class End> struct ForEach
{
  template <class Func>  static void Call(Func&) {}
};
// Then I can generate the calls by
WriteSize ws;
ForEach::type,  end::type>::Call(ws);
==============================================
--
System Developer
TouchTable AB
Tel: +46 (0)31 773 68 12
johan.torp@touchtable.se
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