Re: [Boost-users] [mpl] insert not inserting?

From: Ovanes Markarian (om_boost_at_[hidden])
Date: 2007-03-28 17:59:09

I have one more suggestion, how to deal with such problems. If the code
compiles but does not do what you want it is somewhat difficult to
understand where the error is. I find the debugging ablity of MSVC extremely
useful. Even using the free edition one can instanciate the produced type
and see in the watch window of which type the instantiated object is.

In this case it woule be obvious that the type is mpl::insert<Constraints,
mpl::end<Constraints>, Base> and not the type to which compile time
insert<...>::type member would expand. I understand there are other tools
and possibilities like static assertions etc. which do the trick, but this
one is visual...

Best Regards,
Ovanes

P.S. would be great to have smth. like a C++ Metaprogramm Debugger ;)

-----Original Message-----
From: Ovanes Markarian [mailto:om_boost_at_[hidden]] 
Sent: Mittwoch, 28. März 2007 23:34
To: boost-users_at_[hidden]
Subject: Re: [Boost-users] [mpl] insert not inserting?

I think the problem is in the typedef statement below:

typedef mpl::insert<Constraints,
                       mpl::end<Constraints>,
                       Base> BaseClasses;

mpl::insert is an own type but you need the new sequence type with is
accessed via:
mpl::insert<...>::type

Best Regards,
Ovanes
 

-----Original Message-----
From: Scott Meyers [mailto:usenet_at_[hidden]]
Sent: Donnerstag, 29. März 2007 00:28
To: boost-users_at_[hidden]
Subject: [Boost-users] [mpl] insert not inserting?

In my continuing effort to demonstrate my inability to get much of anything
working with the MPL, I'm having trouble inserting a type into an existing
set of types.  The code compiles and runs, but the revised type sequence is
empty. 
Here's the code:

#include <boost/mpl/set.hpp>
#include <boost/mpl/for_each.hpp>
#include <typeinfo>
#include <iostream>

namespace mpl = boost::mpl;

struct Base { virtual ~Base(){} };
struct A { virtual ~A(){} };
struct B { virtual ~B(){} };

struct Print {
   template<typename T>
   void operator()(const T&)
   {
     std::cout << typeid(T).name() << ", ";
   }
};

int main()
{
   typedef mpl::set<A, B> Constraints;

   // print contents of Constraints (should be the equivalent
   // of "[A, B,]")
   std::cout << '[';
   mpl::for_each<Constraints>(Print());
   std::cout << "]\n";

   // add Base to Constraints, call result BaseClasses
   typedef mpl::insert<Constraints,
                       mpl::end<Constraints>,
                       Base> BaseClasses;

   // print contents of BaseClasses (should be the equivalent
   // of "[A, B, Base, ]")
   std::cout << '[';
   mpl::for_each<BaseClasses>(Print());
   std::cout << "]\n";
}

Can anybody spot what I'm doing wrong?  I know that I could use the
two-argument form of insert with a set, but I'm trying to write code that
will work with a vector, too, and at any rate, I get the same results with
the two-argument form of set.

Thanks,

Scott

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