"Delfin Rojas" wrote

I am having a problem with the mpl library and I wonder if someone could help me with it. Basically I am trying to get a joint_view of two mpl::vector types and I get an error message on mpl/clear.hpp saying

clear_traits< Tag > ::template algorithm< Sequence >

base class undefined.

I tried including several of the mpl hpp files, the ones that seemed could have something to do with this problem but that didn't help. I also get a similar problem trying to use mpl::push_back. In this case I get

push_back_traits< Tag > ::template algorithm< Sequence,T >

Base class undefined.

On the other hand mpl::push_front and mpl::insert compile beautifully.

The error messages you are getting basically say: "This algorithm is undefined for this type of sequence." I never used joint_view, but the push_back is currently not defined for the mpl::vector. AFAIK, this is going to be corrected in the next version of Boost (which is currently being put together). Also, if you are trying to use push_back in combination with, say, fold, this can be replaced with push_front in combination with fold_backward. HTH, Arkadiy

Hi David, I wasn't trying to add elements to a joint view. I am trying to make a joint sequence of two other sequences. I first tried to obtain that from joint_view but when that failed I tried using push_back to push each element of the second sequence at the end of the first one. I thought about trying mpl::copy but I am not sure if that is implemented either... Thanks -delfin -----Original Message----- From: boost-users-bounces@lists.boost.org [mailto:boost-users-bounces@lists.boost.org] On Behalf Of David Abrahams Sent: Monday, July 19, 2004 7:55 PM To: boost-users@lists.boost.org Subject: [Boost-users] Re: Mpl problem "Arkadiy Vertleyb" writes:

"Delfin Rojas" wrote

...

I am having a problem with the mpl library and I wonder if someone could help me with it. Basically I am trying to get a joint_view of two mpl::vector types and I get an error message on mpl/clear.hpp saying

clear_traits< Tag > ::template algorithm< Sequence >

base class undefined.

I tried including several of the mpl hpp files, the ones that seemed could have something to do with this problem but that didn't help. I also get a similar problem trying to use mpl::push_back. In this case I get

push_back_traits< Tag > ::template algorithm< Sequence,T >

Base class undefined.

On the other hand mpl::push_front and mpl::insert compile beautifully.

The error messages you are getting basically say:

"This algorithm is undefined for this type of sequence."

Exactly. Like all other views, joint_view is not extensible. How could you push_back onto a view, anyway? push_back is supposed to return the same "kind" of sequence as its input, but you can't add an element to a joint_view. -- Dave Abrahams Boost Consulting http://www.boost-consulting.com _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/boost-users

Thanks for the explanation Arkadiy. I had the impression this was probably a problem of stuff not been implemented yet. However since mpl documentation talks about vector and push_back I thought it was there... What I'm trying to do is make a variant that is a superset of another variant. For that, I make an initial mpl sequence with the types of the first variant and create it. Then I make a second list with the extra types of the second variant. I would then like to create the second variant passing the joint list of the first types sequence and second types sequence. That is why I wanted to use joint_view. Then I thought I would just push_back each of the second variant types to the first types sequence but that failed too. Finally I am doing an insert at the end (essentially a push_back but not quite). Thanks again -delfin -----Original Message----- From: boost-users-bounces@lists.boost.org [mailto:boost-users-bounces@lists.boost.org] On Behalf Of Arkadiy Vertleyb Sent: Monday, July 19, 2004 5:51 PM To: boost-users@lists.boost.org Subject: [Boost-users] Re: Mpl problem "Delfin Rojas" wrote

I am having a problem with the mpl library and I wonder if someone could help me with it. Basically I am trying to get a joint_view of two mpl::vector types and I get an error message on mpl/clear.hpp saying

clear_traits< Tag > ::template algorithm< Sequence >

base class undefined.

I tried including several of the mpl hpp files, the ones that seemed could have something to do with this problem but that didn't help. I also get a similar problem trying to use mpl::push_back. In this case I get

push_back_traits< Tag > ::template algorithm< Sequence,T >

Base class undefined.

On the other hand mpl::push_front and mpl::insert compile beautifully.

The error messages you are getting basically say: "This algorithm is undefined for this type of sequence." I never used joint_view, but the push_back is currently not defined for the mpl::vector. AFAIK, this is going to be corrected in the next version of Boost (which is currently being put together). Also, if you are trying to use push_back in combination with, say, fold, this can be replaced with push_front in combination with fold_backward. HTH, Arkadiy _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/boost-users

"Arkadiy Vertleyb" wrote

If you just need to concatenate two sequences, you can use fold_backward and push_front, something like:

template struct concatenate : mpl::fold_backward< V1, V2, mpl::push_front<_,_> // please verify this -- not sure about the order -- may be <_2, _1>

...

{};

After taking a look at the joint_view documentation:

Complexity Amortized constant time.

... I actually regret that I ever suggested using fold_backward/push_front for this purpose :) Arkadiy