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

Subject: [boost] [local] Bound types for concepts, etc?
From: Lorenzo Caminiti (lorcaminiti_at_[hidden])
Date: 2011-02-12 13:34:29

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;
}

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