The boost "signature" question

Question

I'm trying to figure out how could many boost classes be able to accept a function signature as template argument and then "extract" from there the result type, first argument type and so on.

template <class Signature>
class myfunction_ptr
{
   Signature* fn_ptr;
public:
   typedef /*something*/  result_type;  // How can I define this?
   typedef /*something*/  arg1_type;  // And this?
};

I know that boost also provides a more portable implementation, using a template argument for each function argument and that's easy to understand. Can someone explain me the magic beyond this?

You mean, you want to know how [`function_traits`](http://www.boost.org/doc/libs/1_43_0/libs/type_traits/doc/html/boost_typetraits/reference/function_traits.html) is implemented? — kennytm, Jul 16 '10 at 07:42
@Kenny: Total side note, but I just spent forever working on my own `function_traits` and was just finishing, and now I found out Boost has it. Should of known. .-. — GManNickG, Jul 16 '10 at 07:49
@GMan as a total side side note, I thought that it's programmers who would have a finely tuned sense for grammatically sound structures, given the years we spend telling compilers what we want. `s/Should of/Should have/g`. Argh! Sorry, could not help it! ;) — Alex B, Jul 16 '10 at 07:58
@Alex: But there isn't a [compiler for English](http://stackoverflow.com/questions/282329/what-are-five-things-you-hate-about-your-favorite-language/282394#282394). — kennytm, Jul 16 '10 at 08:06
@Alex, @GMan If G.W.Bush can do it, so can GMan. http://www.slate.com/id/2071155 — cape1232, Jul 16 '10 at 15:58

score 6 · Accepted Answer · answered Jul 16 '10 at 07:54

At the core its just specialization and repetition:

template<class S> struct Sig;

template<class R> struct Sig<R ()> {
    typedef R result_type;
};

template<class R, class T0> struct Sig<R (T0)> {
    typedef R result_type;
    typedef T0 first_type;
};

// ...

The boost "signature" question

1 Answers1