[bind] Possible to use variadic templates with bind?

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Steve Lorimer

21 Jul 2010 21 Jul '10

2:29 p.m.

Hi Is it possible to pass variadic template arguments to bind, such that bind uses them as the placeholder arguments (_1, _2, _3 etc) I show my use case below. struct job_queue { // copy f into list for asynchronous envocation by another thread void enqueue(boost::function f); }; //----------------------------------------------------------------------- // helper class to define correct form of the member function callback template struct cb { typedef void (T::*type)(Args...); }; // a job defines a callback function which is any member function taking any number of arguments of any type template class job_t { boost::shared_ptr queue; boost::function arg_func; public: job_t(typename cb::type cb, boost::shared_ptr<T> that, boost::shared_ptr q) : arg_func(boost::bind(cb, that, Args...)), queue(q) {} // normally would be _1, _2, _3, etc void post(Args&... args) { boost::function f = boost::bind(arg_func, args...); queue->enqueue(f); } };

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Peter Dimov

21 Jul 21 Jul

4:23 p.m.

New subject: [bind] Possible to use variadic templates with bind?

Steve Lorimer wrote:

...

Hi

Is it possible to pass variadic template arguments to bind, such that bind uses them as the placeholder arguments (_1, _2, _3 etc)

I show my use case below. ... template struct cb { typedef void (T::*type)(Args...); }; ... job_t(typename cb::type cb, boost::shared_ptr<T> that, boost::shared_ptr q) : arg_func(boost::bind(cb, that, Args...)), queue(q) {} // normally would be _1, _2, _3, etc

An interesting puzzle. I tried to apply the 'Indexes' technique shown in the bind implementation section of http://osl.iu.edu/~dgregor/cpp/variadic-templates.pdf and ended up with the following: #include #include template struct int_tuple {}; // make indexes impl is a helper for make indexes template struct make_indexes_impl; template struct make_indexes_impl, T, Types...> { typedef typename make_indexes_impl, Types...>::type type; }; template struct make_indexes_impl > { typedef int_tuple type; }; template struct make_indexes: make_indexes_impl<0, int_tuple<>, Types...> { }; template< class T, class... Args, int... Indexes > boost::function< void(Args...) > make_func_helper( void (T::* pm) (Args...), T * that, int_tuple< Indexes... > ) { return boost::bind( pm, that, boost::arg()... ); } template< class T, class... Args > boost::function< void(Args...) > make_func( void (T::* pm) (Args...), T * that ) { return make_func_helper( pm, that, typename make_indexes::type() ); } struct X { void f( int, int ) { } }; int main() { X x; boost::function f = make_func( &X::f, &x ); f( 1, 2 ); } Hope this helps.

Larry Evans

6:41 p.m.

New subject: [bind] Possible to use variadic templates with bind?

On 07/21/10 11:23, Peter Dimov wrote: [snip]

...

An interesting puzzle. I tried to apply the 'Indexes' technique shown in the bind implementation section of

http://osl.iu.edu/~dgregor/cpp/variadic-templates.pdf

and ended up with the following:

#include #include

template struct int_tuple {};

// make indexes impl is a helper for make indexes template struct make_indexes_impl;

template struct make_indexes_impl, T, Types...> { typedef typename make_indexes_impl, Types...>::type type; };

template struct make_indexes_impl > { typedef int_tuple type; };

template struct make_indexes: make_indexes_impl<0, int_tuple<>, Types...> { }; [snip] That make_indexes was the inspiration for:

http://svn.boost.org/svn/boost/sandbox/variadic_templates/boost/mpl/package_... One additional feature of package_range_c is the the order of the indexes can be reversed. Useful if you want to reverse the order of a mpl sequence. -regards, Larry

Steve Lorimer

22 Jul 22 Jul

9:20 a.m.

New subject: [bind] Possible to use variadic templates with bind?

Thanks for your input Peter and Larry. Larry, I'm fearful of using unreleased code. Any idea when your package_range_c could become part of an official boost release? With that in mind I'm interested in using Peter's approach (which I'm sure is very similar to the internals of your package_c hey Larry?) I have 3 questions please. 1. Please can you give an explanation of what the compiler is actually doing in your make_func_helper() and make_indexes_...() - This goes beyond my capabilities at the moment, and I'm always looking to improve my knowledge. 2. In attached file "attempt_1.cpp" at Line 44 - I'm getting a compiler error "expected primary-expression before '...' token" which I don't understand. How would I bind the variadic template arguments in order to create a functor of the form boost::function? void call(Args...) { boost::function< void() > bound_fn = boost::bind(f, Args...); bound_fn(); // in effect the bound_fn will be stored in a queue for later envocation by a different thread } 3. In attached file "attempt_2.cpp" at Line 27, there's a struct cb_ with a typedef "type" of the member function pointer of the form T::fn(Args...). template< class T, class... Args > struct cb_ { typedef void (T::*type)(Args...); }; As far as I can see, this is of the same form as Peter's void (T::*pm)(Args...) member function pointer is it not? How come the compiler can't match &X::f with typename cb_::type? TIA Steve

Steve Lorimer

9:29 a.m.

New subject: [bind] Possible to use variadic templates with bind?

Ok, I've figured out the answer to question 2: void call(Args... args) { boost::function< void() > bound_fn = boost::bind(f, args...); bound_fn(); // in effect the bound_fn will be stored in a queue for later envocation by a different thread } Interested in your comments on my other 2 questions? Thanks again Steve [snip]

...

1. Please can you give an explanation of what the compiler is actually doing

...

in your make_func_helper() and make_indexes_...() - This goes beyond my capabilities at the moment, and I'm always looking to improve my knowledge.

3. In attached file "attempt_2.cpp" at Line 27, there's a struct cb_ with a typedef "type" of the member function pointer of the form T::fn(Args...).

template< class T, class... Args > struct cb_ { typedef void (T::*type)(Args...); };

As far as I can see, this is of the same form as Peter's void (T::*pm)(Args...) member function pointer is it not? How come the compiler can't match &X::f with typename cb_::type? [/snip]

Larry Evans

11:36 a.m.

New subject: [bind] Possible to use variadic templates with bind?

On 07/22/10 04:20, Steve Lorimer wrote:

...

Thanks for your input Peter and Larry.

Larry, I'm fearful of using unreleased code. Any idea when your package_range_c could become part of an official boost release?

No idea. I've not gotten much feedback, maybe because there's no documentation??? However, most of the existing mpl docs would apply to the variadic mpl. Also, I'm having trouble getting the code to work with spirit. At present, I'm working on converting mpl:;and_ and mpl::or_ and variant to variadic versions in order to get the code to work with spirit. It's been very tough :(

...

With that in mind I'm interested in using Peter's approach (which I'm sure is very similar to the internals of your package_c hey Larry?)

Do you mean package_range_c? If so, then yes, it is essentially the same. The only reason for the extra features (arbitrary Value and Start and Finish template params) is that it seemed easy enough to implement.

...

I have 3 questions please.

[snip]

...

3. In attached file "attempt_2.cpp" at Line 27, there's a struct cb_ with a typedef "type" of the member function pointer of the form T::fn(Args...).

template< class T, class... Args > struct cb_ { typedef void (T::*type)(Args...); };

As far as I can see, this is of the same form as Peter's void (T::*pm)(Args...) member function pointer is it not? How come the compiler can't match &X::f with typename cb_::type?

My gcc4.5 gives error: attempt_2.cpp: In member function 'void func::call(Args ...)': attempt_2.cpp:47:65: error: expected primary-expression before '...' token call is not given any arguments, it's only given types. Try call(Args... args) Something similar to that is in section 3.2 of the reference Peter gave: template vector(const Value&... values) -regards, Larry

Larry Evans

12:08 p.m.

New subject: [bind] Possible to use variadic templates with bind?

On 07/22/10 04:20, Steve Lorimer wrote: [snip]

...

3. In attached file "attempt_2.cpp" at Line 27, there's a struct cb_ with a typedef "type" of the member function pointer of the form T::fn(Args...).

template< class T, class... Args > struct cb_ { typedef void (T::*type)(Args...); };

[snip] The attached revision of attempt_2.cpp compiles. The revisions involved adding args to func::call and rm'ing the struct cb_ and just using the typedef expression directly in make_function_helper. I *guess* (emphasis on "guess") the problem is using: cb_::type is something called non-deduced context. Try googling that phrase or look for it in the standard or wait for someone else on this list (David Abraham did that for me years ago) to confirm this diagnosis. HTH. -Larry

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Larry Evans
Peter Dimov
Steve Lorimer