How to Build a Static Array Initializer from Tuples or Variadic Templates in C ?

Creating an Array Initializer from a Tuple or Variadic Template Parameters

When dealing with persistent memory layouts that need to be embedded within the program code, there's a need for statically representing the description of the layout. This involves binding a particular ID to the layout, calculating offsets at compile time, and creating an array initializer that can be iterated at runtime without the restrictions of std::get(std::tuple).

Variadic Template Approach

To enable compile-time accumulation and sequence creation, consider using variadic templates. Each entry can consist of an identifier and either the size or type of a particular element. The top-level bundle of entries is defined as a "Layout" template, and each entry is represented by an "Entry" template.

Example Usage

<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>

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

To use this approach:

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

After implementing the prepend template to add an element to the front of a tuple, Layout::type would contain a tuple describing the data layout.

Additional Considerations

• Extracting the tuple into a std::array can be achieved using the "indexes" trick or custom methods that perform compile-time key searching and return offset and size information.
• Using aliases can help eliminate repeated type names, such as FooEnum.

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