BOOST_FUSION_ADAPT_TPL_STRUCT and template array size

Question

I am trying to iterate over a C++ template structure thanks to BOOST_FUSION_ADAPT_TPL_STRUCT. My structure contains fixed-size multidimensional arrays whose sizes are template parameters. If we consider Boost's example modified to fit my problem:

#include <iostream>
#include <string>
#include <boost/fusion/adapted/struct/adapt_struct.hpp>
#include <boost/fusion/include/adapt_struct.hpp>

// Example:
// http://www.boost.org/doc/libs/1_53_0/libs/fusion/doc/html/fusion/adapted/adapt_tpl_struct.html

namespace demo
{
    template<typename T, unsigned int SIZE1, unsigned int SIZE2, typename Name, typename Age>
    struct employee
    {
        Name name;
        Age age;
        T ar[SIZE1][SIZE2];
    };
}

// Any instantiated demo::employee is now a Fusion sequence
BOOST_FUSION_ADAPT_TPL_STRUCT(
    (T)(SIZE1)(SIZE2)(Name)(Age),
    (demo::employee) (T)(SIZE1)(SIZE2)(Name)(Age),
    (Name, name)
    (Age, age)
    (T, ar[SIZE1][SIZE2]))

int main()
{
    demo::employee<float, 2, 2, std::string, int> e;
    e.name = "Bob";
    e.age = 25;
    e.ar[0][0] = e.ar[1][0] = 0.1;
    e.ar[0][1] = e.ar[1][1] = 0.2;
}

The compilation fails. Moreover, it also fails if we only add an integer template parameter without even using it for the array size.

Is that even possible with BOOST_FUSION_ADAPT_TPL_STRUCT? If not, how should I go about this?

Answer 1

From doc:

The sequence (template_param0)(template_param1)... declares the names of the template type parameters used.

While you have non-type template parameter SIZE:

template<typename T, unsigned int SIZE, typename Name, typename Age>
struct employee

You may convert it to type template parameter and use boost::mpl::int_ as wrapper for carry size.

Now, your code is compiled.

template<int Size>
struct Array
{
    template<typename T>
    struct Of
    {
        typedef T type[Size];
    };
};

namespace demo
{
    template<typename T, typename SIZE, typename Name, typename Age>
    struct employee
    {
        Name name;
        Age age;
        T ar[SIZE::value];
    };
}

// Any instantiated demo::employee is now a Fusion sequence
BOOST_FUSION_ADAPT_TPL_STRUCT(
    (T)(SIZE)(Name)(Age),
    (demo::employee) (T)(SIZE)(Name)(Age),
    (Name, name)
    (Age, age)
    (typename Array<SIZE::value>::template Of<T>::type, ar))

 //...
 demo::employee<float, int_<2>, std::string, int> e;