Errors in CAF have a code and a category, similar to std::error_code and std::error_condition. Unlike its counterparts from the C++ standard library, error is plattform-neutral and serializable. Instead of using category singletons, CAF stores categories as atoms (see § :ref:`atom <#atom>`__). Errors can also include a message to provide additional context information.

Class Interface

(Enum x) Construct error by calling make_error(x)
(uint8_t x, atom_value y) Construct error with code x and category y
(uint8_t x, atom_value y, messa ge z) Construct error with code x, category y, and context z
uint8_t code() Returns the error code
atom_value category() Returns the error category
message context() Returns additional context information
explicit operator bool() Returns code() != 0

Add Custom Error Categories

Adding custom error categories requires three steps: (1) declare an enum class of type uint8_t with the first value starting at 1, (2) implement a free function make_error that converts the enum to an error object, (3) add the custom category to the actor system with a render function. The last step is optional to allow users to retrieve a better string representation from system.render(x) than to_string(x) can offer. Note that any error code with value 0 is interpreted as not-an-error. The following example adds a custom error category by performing the first two steps.

enum class math_error : uint8_t {
  division_by_zero = 1

error make_error(math_error x) {
  return {static_cast<uint8_t>(x), atom("math")};

std::string to_string(math_error x) {
  switch (x) {
    case math_error::division_by_zero:
      return "division_by_zero";
      return "-unknown-error-";

The implementation of to_string(error) is unable to call string conversions for custom error categories. Hence, to_string(make_error(math_error::division_by_zero)) returns "error(1, math)".

The following code adds a rendering function to the actor system to provide a more satisfactory string conversion.

class config : public actor_system_config {
  config() {
    auto renderer = [](uint8_t x, atom_value, const message&) {
      return "math_error" + deep_to_string_as_tuple(static_cast<math_error>(x));
    add_error_category(atom("math"), renderer);

With the custom rendering function, system.render(make_error(math_error::division_by_zero)) returns "math_error(division_by_zero)".

System Error Codes

System Error Codes (SECs) use the error category "system". They represent errors in the actor system or one of its modules and are defined as follows.

/// error codes used internally by CAF.
enum class sec : uint8_t {
  /// Indicates that an actor dropped an unexpected message.
  unexpected_message = 1,
  /// Indicates that a response message did not match the provided handler.
  /// Indicates that the receiver of a request is no longer alive.
  /// Indicates that a request message timed out.
  /// Indicates that requested group module does not exist.
  no_such_group_module = 5,
  /// Unpublishing or connecting failed: no actor bound to given port.
  /// Connecting failed because a remote actor had an unexpected interface.
  /// Migration failed because the state of an actor is not serializable.
  /// An actor received an unsupported key for `('sys', 'get', key)` messages.
  /// An actor received an unsupported system message.
  unsupported_sys_message = 10,
  /// A remote node disconnected during CAF handshake.
  /// Tried to forward a message via BASP to an invalid actor handle.
  /// Tried to forward a message via BASP to an unknown node ID.
  /// Middleman could not assign a connection handle to a broker.
  /// Middleman could not assign an acceptor handle to a broker.
  failed_to_assign_doorman_from_handle = 15,
  /// User requested to close port 0 or to close a port not managed by CAF.
  /// Middleman could not connect to a remote node.
  /// Middleman could not open requested port.
  /// A C system call in the middleman failed.
  /// A function received one or more invalid arguments.
  invalid_argument = 20,
  /// A network socket reported an invalid network protocol family.
  /// Middleman could not publish an actor because it was invalid.
  /// A remote spawn failed because the provided types did not match.
  /// Serialization failed because there was not enough data to read.
  /// Serialization failed because no CAF context is available.
  no_context = 25,
  /// Serialization failed because CAF misses run-time type information.
  /// Serialization of actors failed because no proxy registry is available.
  /// An exception was thrown during message handling.
  /// Linking to a remote actor failed because actor no longer exists.
  /// Adding an upstream to a stream failed.
  cannot_add_upstream = 30,

Default Exit Reasons

CAF uses the error category "exit" for default exit reasons. These errors are usually fail states set by the actor system itself. The two exceptions are exit_reason::user_shutdown and exit_reason::kill. The former is used in CAF to signalize orderly, user-requested shutdown and can be used by programmers in the same way. The latter terminates an actor unconditionally when used in send_exit, even if the default handler for exit messages (see § :ref:`exit-message <#exit-message>`__) is overridden.

enum class exit_reason : uint8_t {
  /// Indicates that an actor finished execution without error.
  normal = 0,
  /// Indicates that an actor died because of an unhandled exception.
  /// Indicates that the exit reason for this actor is unknown, i.e.,
  /// the actor has been terminated and no longer exists.
  /// Indicates that an actor pool unexpectedly ran out of workers.
  /// Indicates that an actor was forced to shutdown by a user-generated event.
  /// Indicates that an actor was killed unconditionally.
  /// Indicates that an actor finishied execution because a connection
  /// to a remote link was closed unexpectedly.
  /// Indicates that an actor was killed because it became unreachable.