MLExport.h

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#pragma once
// ============================================================
//  include/Malena/Core/MLExport.h
// ============================================================
//
//  MLExport<T> is an alternative to ML_EXPORT for users who prefer
//  explicit C++ syntax over macros. Place it in a .cpp file after
//  the class definition:
//
//    template class ml::MLExport<ScoreFireable>;
//
//  This forces instantiation of MLExport<T>::Registrar, whose
//  constructor calls Fireable::_register exactly once.
//
//  NOTE: Unlike ML_EXPORT, this cannot be placed in a header —
//  explicit template instantiation violates ODR if the header is
//  included in multiple translation units. Use ML_EXPORT for
//  header-compatible registration.
//
//  This header must be included AFTER Fireable.h so that Fireable
//  is a complete type. In practice this is guaranteed because
//  Fireable.h includes this file at its bottom.
//
// ============================================================
 
#include <Malena/Events/Fireable.h>
#include <type_traits>
 
namespace ml
{
    template<typename T>
    struct MLExport
    {
    private:
        struct Registrar
        {
            Registrar()
            {
                if constexpr (std::is_base_of_v<Fireable, T>)
                {
                    // Calls private Fireable::_register —
                    // accessible via: template<typename U> friend struct MLExport;
                    // declared in Fireable.h
                    Fireable::_register(new T());
                }
                else if constexpr (!std::is_base_of_v<Plugin, T>)
                {
                    // Neither Fireable nor Plugin — unsupported type
                    static_assert(!sizeof(T),
                        "[Malena] MLExport: type is not a recognised exportable "
                        "Malena type. Must derive from ml::Fireable. "
                        "For ml::Plugin types use ML_EXPORT instead.");
                }
                // Plugin case: extern "C" symbols are emitted by ML_EXPORT —
                // MLExport does not handle plugins. Use ML_EXPORT for plugins.
            }
        };
 
        // Constructed once when MLExport<T> is explicitly instantiated.
        // inline static ensures a single instance across translation units.
        inline static Registrar _registrar{};
 
        // Fireable::_register is private — MLExport accesses it via:
        // template<typename U> friend struct MLExport;  (declared in Fireable.h)
    };
 
} // namespace ml