Re: [Boost-users] Delegating to boost::function with arbitrary signature

On Sat, Dec 14, 2013 at 10:53 AM, TONGARI J wrote:

2013/12/15 Alexander Lamaison

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TONGARI J writes:

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2013/12/14 Alexander Lamaison <span dir="ltr">mailto:awl03@doc.ic.ac.uk</span> I've asked this question on Stack Overflow but not found a solution yet: http://stackoverflow.com/q/20578575/67013. It's Boost.Function/MPL-related so I'll ask here too.

I'm trying to create something like a `boost::function`, but with an extra 'smart handle' tucked inside, which controls the lifetime of a resource the functions needs to execute correctly.

If I didn't need the function signature to be completely generic, this would be easy. I would just create a functor with the correct signature, and keep a `boost::function` and the handle as members.

However, I do need generic signatures. The code below is as close as I've got but gets a compile error, presumably, because I'm not able to unpack a list of arguments like this:

template<typename Signature> class library_function { public: library_function( library_handle handle, boost::function function) : m_handle(handle), m_function(function) {}

typename result_type operator()(arg1_type arg1, arg2_type arg2) { return m_function(arg1, arg2); }

// I don't think I can declare the 'params' like this ... typename boost::function_types::result_type<Signature>::type operator()( boost::function_types::parameter_types<Signature> params) { return m_function(params); // ... nor unpack them like this }

private: boost::function<Signature> m_function; library_handle m_library; };

I think `params` is an MPL list, so what I've actually done is declare a call operator with a single MPL-list parameter. My MPL-foo is not good. Is there a way to convert the list to a legal definition of multiple function paramters?

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Why not just derive from boost::function so you get the call operator for free?

I avoided publicly-inheriting from `boost::function` because that's just asking for slicing problems. Private inheritance seemed like a good idea but I then run into a problem passing the resulting object to anything expecting a `boost::function`. Instead of treating as any regular callable object, the compiler knows that it inherits from a `boost::function` and complains that the conversion is hidden. Maybe there's some way round this?

Another thought is to use boost::function directly, and provide a make function:

There you go. Simple decorator pattern. Nice and transparent and tucked away.

template<typename Signature> boost::function<Signature> make_library_function(shared_ptr<HMODULE> library, Signature* f) { return library_function<Signature>(library, f); }

And library_function becomes:

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template<typename Signature> class library_function { public: library_function( shared_ptr<HMODULE> library, Signature* library_funtion) : m_library(library), m_function(library_function) {}

operator Signature*() const { return m_function; }

private: shared_ptr<HMODULE> m_library; Signature* m_function; };

Note: code not tested

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