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
*/