KillSwitch (concept / trait / ref)

Compile-time contract and type-erased handle for components that enforce risk limits and can halt trading instantly.

// Concept
template <typename T>
concept KillSwitch = requires(T ks, const Order& o, const std::string& reason) {
  ks.check(o);           // validate order, maybe trigger
  ks.trigger(reason);    // force-trigger
  { ks.isTriggered() } -> std::same_as<bool>;
  { ks.reason()     } -> std::same_as<std::string>;
};

Piece	Responsibility
KillSwitchTrait	Builds a static v-table with check, trigger, isTriggered, reason.
KillSwitchRef	Two-pointer handle {void*, VTable*} that forwards calls without virtual inheritance.

KillSwitchRef API

void check(const Order&);             // call before each order submit
void trigger(const std::string& msg); // external forced shutdown
bool isTriggered() const;             // query state
std::string reason() const;           // reason string

Purpose

• Provide a pluggable circuit-breaker that strategies and executors can consult before sending orders.
• Centralise logic for position/size caps, PnL limits, rate limits, etc.

Notes

• Zero runtime overhead beyond one pointer indirection.
• Extend by implementing a concrete class satisfying concepts::KillSwitch and pass it where a KillSwitchRef is expected.