[Boost-users] Designing a table-like data structure with Fusion

Subject: [Boost-users] Designing a table-like data structure with Fusion
From: Matthias Vallentin (vallentin_at_[hidden])
Date: 2010-01-14 12:50:24

I am in the process of designing a table-like data structure and would
like to hear your feedback on the choice of Fusion for this task. In the
context of databases, a table contains multiple columns that exhibit
different types (short, unsigned int, long long, etc...).  Columns must
be added at runtime. The table is column-oriented in that it stores each
column individually, yet provides a row-oriented interface to add
data. The table structure (aka. schema) remains immutable once all
columns have been added.

To benefit from the heterogeneous nature of Fusion, I would consider a
table row a fusion::vector, maybe created using fusion::vector_tie from
some arbitrary data.

At the very high level, I think of something along the lines of:

    template <typename T>
    struct column
    {
        void write(const T& x)
        {
            // Store x in some fashion (memory or disk).
        }
    };

    struct table
    {
-->     fusion::vector<???> cols;

        template <typename C>
        void add_column(C* c)
        {
            fusion::push_back(cols, c);
        }

        template<typename R>
        void write(const R& row)
        {   
            // Writer simply calls c->write(x).
            fusion::for_each(fusion::zip(row, cols), writer());
        }
    };

    int main()
    {
        table t;

        ...

        auto* ic = new column<int>();
        auto* dc = new column<double>();

        t.add_column(ic);
        t.add_column(dc);

        ...
    }

As you can see (at the mark -->), I need to cross at some point the
compile-time/runtime boundary but I am having trouble designing that
interface. Perhaps boost::variant might help here to bound the potential
number of types? Any kind of thoughts are appreciated.

   Matthias
-- 
Matthias Vallentin
vallentin_at_[hidden]
http://www.icir.org/matthias