Messages.rst

Type-Erased Tuples, Messages and Message Views

Messages in CAF are stored in type-erased tuples. The actual message type itself is usually hidden, as actors use pattern matching to decompose messages automatically. However, the classes message and message_builder allow more advanced use cases than only sending data from one actor to another.

The interface type_erased_tuple encapsulates access to arbitrary data. This data can be stored on the heap or on the stack. A message is a type-erased tuple that is always heap-allocated and uses copy-on-write semantics. When dealing with "plain" type-erased tuples, users are required to check if a tuple is referenced by others via type_erased_tuple::shared before modifying its content.

The convenience class message_view holds a reference to either a stack-located type_erased_tuple or a message. The content of the data can be access via message_view::content in both cases, which returns a type_erased_tuple&. The content of the view can be forced into a message object by calling message_view::move_content_to_message. This member function either returns the stored message object or moves the content of a stack-allocated tuple into a new message.

RTTI and Type Numbers

All builtin types in CAF have a non-zero 6-bit type number. All user-defined types are mapped to 0. When querying the run-time type information (RTTI) for individual message or tuple elements, CAF returns a pair consisting of an integer and a pointer to std::type_info. The first value is the 6-bit type number. If the type number is non-zero, the second value is a pointer to the C++ type info, otherwise the second value is null. Additionally, CAF generates 32 bit type tokens. These tokens are type hints that summarizes all types in a type-erased tuple. Two type-erased tuples are of different type if they have different type tokens (the reverse is not true).

Class type_erased_tuple

Note: Calling modifiers on a shared type-erased tuple is undefined behavior.

Observers
bool empty()	Returns whether this message is empty.
size_t size()	Returns the size of this message.
rtti_pair type(size_t pos)	Returns run-time type information for the nth element.
error save(serializer& x)	Writes the tuple to x.
error save(size_t n, serializer& x)	Writes the nth element to x.
const void* get(size_t n)	Returns a const pointer to the nth element.
std::string stringify()	Returns a string representation of the tuple.
std::string stringify(size_t n)	Returns a string representation of the nth element.
bool matches(size_t n, rtti_pair)	Checks whether the nth element has given type.
bool shared()	Checks whether more than one reference to the data exists.
bool match_element<T>(size_t n)	Checks whether element n has type T.
bool match_elements<Ts...>()	Checks whether this message has the types Ts....
const T& get_as<T>(size_t n)	Returns a const reference to the nth element.
Modifiers
void* get_mutable(size_t n)	Returns a mutable pointer to the nth element.
T& get_mutable_as<T>(size_t n)	Returns a mutable reference to the nth element.
void load(deserializer& x)	Reads the tuple from x.

Class message

The class message includes all member functions of type_erased_tuple. However, calling modifiers is always guaranteed to be safe. A message automatically detaches its content by copying it from the shared data on mutable access. The class further adds the following member functions over type_erased_tuple. Note that apply only detaches the content if a callback takes mutable references as arguments.

Observers
message drop(size_t n)	Creates a new message with all but the first n values.
message drop_right(size_t n)	Creates a new message with all but the last n values.
message take(size_t n)	Creates a new message from the first n values.
message take_right(size_t n)	Creates a new message from the last n values.
message slice(size_t p, size_t n)	Creates a new message from [p, p + n).
message extract(message_handler)	See extract.
message extract_opts(...)	See extract-opts.
Modifiers
optional<message> apply(message_handler f)	Returns f(*this).
Operators
message operator+(message x, message y)	Concatenates x and y.
message& operator+=(message& x, message y)	Concatenates x and y.

Class message_builder

Constructors
(void)	Creates an empty message builder.
(Iter first, Iter last)	Adds all elements from range [first, last).
Observers
bool empty()	Returns whether this message is empty.
size_t size()	Returns the size of this message.
message to_message( )	Converts the buffer to an actual message object.
append(T val)	Adds val to the buffer.
append(Iter first, Iter last)	Adds all elements from range [first, last).
message extract(message_handler)	See extract.
message extract_opts(...)	See extract-opts.
Modifiers
optional<message> apply(message_handler f)	Returns f(*this).
message move_to_message()	Transfers ownership of its data to the new message.

Extracting

The member function message::extract removes matched elements from a message. x Messages are filtered by repeatedly applying a message handler to the greatest remaining slice, whereas slices are generated in the sequence $[0, size)$, $[0, size-1)$, $...$, $[1, size-1)$, $...$, $[size-1, size)$. Whenever a slice is matched, it is removed from the message and the next slice starts at the same index on the reduced message.

For example:

auto msg = make_message(1, 2.f, 3.f, 4);
// remove float and integer pairs
auto msg2 = msg.extract({
  [](float, float) { },
  [](int, int) { }
});
assert(msg2 == make_message(1, 4));

Step-by-step explanation:

• Slice 1: (1, 2.f, 3.f, 4), no match
• Slice 2: (1, 2.f, 3.f), no match
• Slice 3: (1, 2.f), no match
• Slice 4: (1), no match
• Slice 5: (2.f, 3.f, 4), no match
• Slice 6: (2.f, 3.f), match; new message is (1, 4)
• Slice 7: (4), no match

Slice 7 is (4), i.e., does not contain the first element, because the match on slice 6 occurred at index position 1. The function extract iterates a message only once, from left to right. The returned message contains the remaining, i.e., unmatched, elements.

Extracting Command Line Options

The class message also contains a convenience interface to extract for parsing command line options: the member function extract_opts.

int main(int argc, char** argv) {
  uint16_t port;
  string host = "localhost";
  auto res = message_builder(argv + 1, argv + argc).extract_opts({
    {"port,p", "set port", port},
    {"host,H", "set host (default: localhost)", host},
    {"verbose,v", "enable verbose mode"}
  });
  if (! res.error.empty()) {
    // read invalid CLI arguments
    cerr << res.error << endl;
    return 1;
  }
  if (res.opts.count("help") > 0) {
    // CLI arguments contained "-h", "--help", or "-?" (builtin);
    cout << res.helptext << endl;
    return 0;
  }
  if (! res.remainder.empty()) {
    // res.remainder stors all extra arguments that weren't consumed
  }
  if (res.opts.count("verbose") > 0) {
    // enable verbose mode
  }
  // ...
}

/*
Output of ./program_name -h:

Allowed options:
  -p [--port] arg  : set port
  -H [--host] arg  : set host (default: localhost)
  -v [--verbose]   : enable verbose mode
*/