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esphome/core/template_lambda.h

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+#pragma once
+
+#include "esphome/core/optional.h"
+
+namespace esphome {
+
+/** Lightweight wrapper for template platform lambdas (stateless function pointers only).
+ *
+ * This optimizes template platforms by storing only a function pointer (4 bytes on ESP32)
+ * instead of std::function (16-32 bytes).
+ *
+ * IMPORTANT: This only supports stateless lambdas (no captures). The set_template() method
+ * is an internal API used by YAML codegen, not intended for external use.
+ *
+ * Lambdas must return optional<T> to support the pattern:
+ *   return {};    // Don't publish a value
+ *   return 42.0;  // Publish this value
+ *
+ * operator() returns optional<T>, returning nullopt when no lambda is set (nullptr check).
+ *
+ * @tparam T The return type (e.g., float for sensor values)
+ * @tparam Args Optional arguments for the lambda
+ */
+template<typename T, typename... Args> class TemplateLambda {
+ public:
+  TemplateLambda() : f_(nullptr) {}
+
+  /** Set the lambda function pointer.
+   * INTERNAL API: Only for use by YAML codegen.
+   * Only stateless lambdas (no captures) are supported.
+   */
+  void set(optional<T> (*f)(Args...)) { this->f_ = f; }
+
+  /** Check if a lambda is set */
+  bool has_value() const { return this->f_ != nullptr; }
+
+  /** Call the lambda, returning nullopt if no lambda is set */
+  optional<T> operator()(Args... args) {
+    if (this->f_ == nullptr)
+      return nullopt;
+    return this->f_(args...);
+  }
+
+  /** Alias for operator() for compatibility */
+  optional<T> call(Args... args) { return (*this)(args...); }
+
+ protected:
+  optional<T> (*f_)(Args...);  // Function pointer (4 bytes on ESP32)
+};
+
+}  // namespace esphome