Message Handlers

Actors can store a set of callbacks—usually implemented as lambda expressions—using either behavior or message_handler. The former stores an optional timeout, while the latter is composable.

Definition and Composition

As the name implies, a behavior defines the response of an actor to messages it receives.

message_handler x1{
  [](int32_t i) { /*...*/ },
  [](double db) { /*...*/ },
  [](int32_t a, int32_t b, int32_t c) { /*...*/ }

In our first example, x1 models a behavior accepting messages that consist of either exactly one int, or one double, or three int values. Any other message is not matched and gets forwarded to the default handler (see default-handler).

message_handler x2{
  [](double db) { /*...*/ },
  [](double db) { /* - unreachable - */ }

Our second example illustrates an important characteristic of the matching mechanism. Each message is matched against the callbacks in the order they are defined. The algorithm stops at the first match. Hence, the second callback in x2 is unreachable.

message_handler x3 = x1.or_else(x2);
message_handler x4 = x2.or_else(x1);

Message handlers can be combined using or_else. This composition is not commutative, as our third examples illustrates. The resulting message handler will first try to handle a message using the left-hand operand and will fall back to the right-hand operand if the former did not match. Thus, x3 behaves exactly like x1. This is because the second callback in x1 will consume any message with a single double and both callbacks in x2 are thus unreachable. The handler x4 will consume messages with a single double using the first callback in x2, essentially overriding the second callback in x1.


Defining message handlers in terms of callbacks is convenient, but requires a simple way to annotate messages with meta data. Imagine an actor that provides a mathematical service for integers. It receives two integers, performs a user-defined operation and returns the result. Without additional context, the actor cannot decide whether it should multiply or add the integers. Thus, the operation must be encoded into the message. The Erlang programming language introduced an approach to use non-numerical constants, so-called atoms, which have an unambiguous, special-purpose type and do not have the runtime overhead of string constants.

Atoms in CAF are tag types, i.e., usually defined as en empty struct. These types carry no data on their own and only exist to annotate messages. For example, we could create the two tag types add_atom and multiply_atom for implementing a simple math actor like this:

CAF_BEGIN_TYPE_ID_BLOCK(my_project, caf::first_custom_type_id)

  CAF_ADD_ATOM(my_project, add_atom)
  CAF_ADD_ATOM(my_project, multiply_atom)


 behavior do_math{
   [](add_atom, int32_t a, int32_t b) {
     return a + b;
   [](multiply_atom, int32_t a, int32_t b) {
     return a * b;

 // caller side: send(math_actor, add_atom_v, int32_t{1}, int32_t{2})

The macro CAF_ADD_ATOM defined an empty struct with the given name as well as a constexpr variable for conveniently creating a value of that type that uses the type name plus a _v suffix. In the example above, atom_value is the type name and atom_value_v is the constant.