How do you calculate automatic array offsets using variadic templates in C ?

Creating an Array Initializer from a Tuple or Variadic Template Parameters

Variadic templates offer a solution to the need for automatic offset calculation in array initialization. By representing each element as an identifier and its size, a sequence can be created that drives the calculation of offsets.

A Layout structure is employed to hold the entries:

<code class="cpp">template<std::size_t offset, typename Key, typename... Entries>
struct LayoutHelper {
  typedef std::tuple<> type;
};
template<typename Key, typename... Entries>
struct Layout:LayoutHelper<0, Key, Entries...> {};</code>

Each entry has an identifier and a type or size:

<code class="cpp">template<typename Key, Key identifier, typename Data>
struct Entry {};</code>

To create a processed entry, we extend the concept:

<code class="cpp">template<std::size_t offset, typename Key, Key id0, typename D0, typename... Entries>
struct LayoutHelper<offset, Key, Entry<Key, id0, D0>, Entries...>
{
    typedef typename prepend
        < ProcessedEntry< Key, id0, D0, offset >
        , typename LayoutHelper<offset+sizeof(D0), Key, Entries...>::type
        > type;
};</code>

Usage looks like this:

<code class="cpp">Layout< FooEnum, Entry< FooEnum, eFoo, char[10] >, Entry< FooEnum, eFoo2, double > > layout;</code>

After finding a prepend implementation that adds elements to the front of a tuple, Layout::type will represent a tuple that expresses the data layout. This result can then be used to construct a std::array by unpacking the entries using the indexes technique.

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