RE: [boost] Several MPL questions...

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2002-07-25 04:49:53

Jaap Suter wrote:
> just when I thought I've seen it all in C++, the MPL pops up. 
> Typelists where cool and all, but the way you have created 
> an STL-ish MPL that seperates algorithm and container is 
> stunning to say the least. 

Thank you!

> Is the current MPL release (July 17th) designed to work with 
> MSVC 6.0 Service Pack 5? 

Yes. You can even use lambda expression, although (obviously) there are some
limitation.

> Does the code-generation facilities of MPL allow something 
> like the following (pseudocode):
> 
> template< typename t_TypeList >
> class Tree
> {
>     // Constructor;
>     Tree( t_TypeList[ 0 ]* a_Child_0,
>             t_TypeList[ 0 ]* a_Child_1,
>             t_TypeList[ 0 ]* a_Child_2,
>                             ......
>             t_TypeList[ 0 ]* a_Child_n );
> 
>     // Attributes;
>     t_TypeList[ 0 ]*     m_Child_0;
>     t_TypeList[ 1 ]*     m_Child_1;
>     t_TypeList[ 2 ]*     m_Child_2;
>                    ........
>     t_TypeList[ n ]*     m_Child_n;
> }

Constructor is a problem (preprocessor can help there, though), members -
easily:

    template< typename Base, typename Iterator >
    struct tree_node
        : Base
    {
        typedef typename Iterator::type T;
        typedef typename Iterator::pos pos;
        typedef tree_node type; // makes 'tree_node' a metafunction

        using Base::node;
        T& node(mpl::int_c<pos::value>);
        T m_node; 
    };

    struct empty_tree
    {
        void node();
    };

    template< typename Types >
    struct tree
        : mpl::iter_fold<
              Types
            , empty_tree
            , tree_node<_,_>
            >::type
    {
    };

    int main()
    {
        tree< mpl::vector<int,long,bool> > t;
        t.node(mpl::int_c<1>()) = -1l;
        assert(t.node(mpl::int_c<1>()) == -1l);
    }

Aleksey