MarketDataSubscriber (concept / trait / ref)

Compile-time contract and type-erased handle for components that consume market-data events.

// Concept
template <typename T>
concept MarketDataSubscriber =
    Subscriber<T> &&
    requires(T t,
             const BookUpdateEvent&  b,
             const TradeEvent&       tr,
             const CandleEvent&      c) {
      t.onBookUpdate(b);
      t.onTrade(tr);
      t.onCandle(c);
    };

Piece	Responsibility
MarketDataSubscriberTrait	Builds a static v-table combining Subscriber base info with three event callbacks (onBookUpdate, onTrade, onCandle).
MarketDataSubscriberRef	Two-pointer handle {void*, VTable*} that forwards calls without virtual inheritance.

MarketDataSubscriberRef API

SubscriberId   id()   const;
SubscriberMode mode() const;

void onTrade     (const TradeEvent&);      // trade prints
void onBookUpdate(const BookUpdateEvent&); // L2 updates
void onCandle    (const CandleEvent&);     // finished OHLCV

Purpose

• Uniform interface for strategies, loggers, metrics collectors to receive real-time market data.
• Decouples EventBus dispatch logic from concrete subscriber implementations via compile-time resolution.

Notes

• Zero runtime overhead beyond one pointer indirection per call.
• static_assert(concepts::MarketDataSubscriber<MarketDataSubscriberRef>) ensures the handle itself meets the concept for seamless composition.