Re: [boost] [local] Bound types for concepts, etc?

Subject: Re: [boost] [local] Bound types for concepts, etc?
From: Lorenzo Caminiti (lorcaminiti_at_[hidden])
Date: 2011-02-12 23:00:40

On Sat, Feb 12, 2011 at 1:34 PM, Lorenzo Caminiti <lorcaminiti_at_[hidden]> wrote:
> On Sun, Feb 6, 2011 at 2:25 PM, Mathias Gaunard
> <mathias.gaunard_at_[hidden]> wrote:
>> As I said, it does. With Boost.Local, you can still call polymorphic
>> functions on the variables captured from scope without specifying the types
>> of the arguments.
>
> I am thinking to provide the type of the bound parameters within the
> local function body as `<BOUND_PARAM_NAME>_type`. This way you can use
> such types to check concepts, etc...
>
> Is there value in this?
>
> #include <boost/local/function.hpp>
> #include <boost/concept_check.hpp>
> #include <iostream>
>
> template<typename T>
> class Addable {
>    T x;
>    T y;
>    void return_type(T); // Used to check addition returns type `T`.
> public:
>    BOOST_CONCEPT_USAGE(Addable) {
>        return_type(x + y); // Check addition `T operator+(T x, T y)`.
>    }
> };
>
> struct pod {};
>
> int main() {
>    pod sum; // Can't be added.
>    int factor = 10;
>
>    BOOST_LOCAL_FUNCTION(
>    (void) (add)( (double)(num) (const bind)((factor)) (bind)((&sum)) )
>    ) {
>        // In here `sum_type` and `factor_type` are the types of the
> respective bound parameters.
>        BOOST_CONCEPT_ASSERT((Addable<sum_type>));
>
>        sum = sum + factor * num; // Error because `sum`'s type can't
> be added :(
>    } BOOST_LOCAL_FUNCTION_END(add)
>
>    add(100.0);
>
>    return 0;
> }

Sorry, I forgot to mention... of course, with the new simplified
Boost.Local syntax and using C99 variadic macros, the above example
will actually become:

int main() {
    pod sum;
    int factor = 10;

    void BOOST_LOCAL_FUNCTION_PARAMS(double num, const bind factor, bind& sum) {
        BOOST_CONCEPT_ASSERT((Addable<sum_type>));

        sum = sum + factor * num;
    } BOOST_LOCAL_FUNCTION_NAME(add)

    add(100.0);

    return 0;
}

> On GCC:
>
> $ g++ -Wall -Werror -I../../.. concepts.00.cpp
> concepts.00.cpp: In static member function ‘static void
> main()::boost_local_auxXfunctorXadd::boost_local_auxXbody(main()::boost_local_auxXfunctorXadd&,
> int, main()::boost_se_capture_t_1_add35&, double)’:
> concepts.00.cpp:39: error: no match for ‘operator+’ in ‘sum +
> (#‘float_expr’ not supported by dump_expr#<expression error> * num)’
> concepts.00.cpp: In destructor ‘Addable<T>::~Addable() [with T = pod]’:
> /usr/include/boost/concept/detail/general.hpp:29:   instantiated from
> ‘static void boost::concept::requirement<Model>::failed() [with Model
> = Addable<pod>]’
> concepts.00.cpp:37:   instantiated from here
> concepts.00.cpp:22: error: no match for ‘operator+’ in
> ‘((Addable<pod>*)this)->Addable<pod>::x +
> ((Addable<pod>*)this)->Addable<pod>::y’
>
> Note: In this specific case even the GCC error without the concept
> assert (error on line 39) is pretty clear but in general allowing
> programmer to add concept specifications might lead to better error
> reporting and a better specification of the local function.

-- 
Lorenzo