How are signal signatures in `boost::signals2` implemented?

Question

I have been using boost::signals2 for some time now in my projects. To my shame, I still do not understand how they are implemented under the hood. My problems already start at the very definition of a signal. How is a definition such as

boost::signals2::signal< void (bool, double) >

handled? I can see from the implementation details that the signature becomes a template parameter that is aptly named Signature. However, I do not understand the syntax. Is this syntax permitted by the C++ standard? How can a signal "store" the function signature when it is provided in this form?

I already tried looking at the sources, but was unable to find an explanation for this syntax. Any help would be appreciated.

I'd guess `Signature` is used as function signature. google-mock uses the same thing to describe and bind function signatures of mocked interfaces, apparently it's valid syntax. — πάντα ῥεῖ, May 15 '13 at 07:17

score 2 · Accepted Answer · answered May 15 '13 at 07:44

2

Yes, this syntax is allowed; the type it denotes is a reference to function taking a bool and double and returning void. If we were to typedef it, it would be awkward, because the name would sit in the middle:

typedef void Signature(bool, double);

With the new alias syntax is becomes slightly more readable:

using Signature = void(bool, double);

The parallel with pointers to functions would be:

typedef void (*Pointer)(bool, double);

using Pointer = void (*)(bool, double);

Afterwards, there are template tricks to tease apart the various elements (extract the return type, and the type of each argument).

answered May 15 '13 at 07:44

Matthieu M.

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Thank you! That is what I was looking for. Maybe that's too broad a question, but how can I look up/learn this stuff myself? I tried reading the C++ standard document myself, but is there an "easier" way? – Gnosophilon May 21 '13 at 06:53
1

The C++ standard is perhaps the harder way about it. There are several "tutorial" or "advanced" books, if you look up at the description of the C++ tag you'll get a list of useful questions on this site. One, notably, is a [list of books](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) – Matthieu M. May 21 '13 at 06:58

score 0 · Answer 2 · edited May 23 '17 at 10:32

// In header: <boost/signals2/signal.hpp>
template<typename Signature, ... /* Omitted other args for clarity */> 
class signal : public boost::signals2::signal_base {
public:

  // In your case, typedef would be evaluated as
  //typedef void (bool, double) signature_type    
  typedef Signature  signature_type;

More info on how typedef can be used with function pointers: Typedef function pointer?

How are signal signatures in `boost::signals2` implemented?

2 Answers2