Class DirectExpressionStep

eval/eval/direct_expression_step.h:39–77

Represents a directly evaluated CEL expression. Subexpressions assign to values on the C++ program stack and call their dependencies directly. This reduces the setup overhead for evaluation and minimizes value churn to / from a heap based value stack managed by the CEL runtime, but can't be used for arbitrarily nested expressions.

// to / from a heap based value stack managed by the CEL runtime, but can't be
// used for arbitrarily nested expressions.
class DirectExpressionStep {
 public:
  explicit DirectExpressionStep(int64_t expr_id) : expr_id_(expr_id) {}
  DirectExpressionStep() : expr_id_(-1) {}

  virtual ~DirectExpressionStep() = default;

  int64_t expr_id() const { return expr_id_; }
  bool comes_from_ast() const { return expr_id_ >= 0; }

  virtual absl::Status Evaluate(ExecutionFrameBase& frame, cel::Value& result,
                                AttributeTrail& attribute) const = 0;

  // Return a type id for this node.
  //
  // Users must not make any assumptions about the type if the default value is
  // returned.
  virtual cel::NativeTypeId GetNativeTypeId() const {
    return cel::NativeTypeId();
  }

  // Implementations optionally support inspecting the program tree.
  virtual absl::optional<std::vector<const DirectExpressionStep*>>
  GetDependencies() const {
    return absl::nullopt;
  }

  // Implementations optionally support extracting the program tree.
  //
  // Extract prevents the callee from functioning, and is only intended for use
  // when replacing a given expression step.
  virtual absl::optional<std::vector<std::unique_ptr<DirectExpressionStep>>>
  ExtractDependencies() {
    return absl::nullopt;
  };

 protected:
  int64_t expr_id_;
};

// Wrapper for direct steps to work with the stack machine impl.
class WrappedDirectStep : public ExpressionStep {