Boost.Parameter Constructor in Templates Not Supported?

[Dean Michael Berris]

If I try to create a Boost.Parameter Constructor as shown in the following code listing, I get a handful of errors in the preprocessor code used by BOOST_PARAMETER_CONSTRUCTOR:

    #include <boost/parameter.hpp>

    namespace foo {

        BOOST_PARAMETER_NAME(arg1)
        BOOST_PARAMETER_NAME(arg2)

        template <class Tag>
        struct base {
            template <class ArgPack>
            base(ArgPack const & args)
            : val1(args[_arg1])
            , val2(args[_arg2])
            {}

            int val1,val2;
        };

        template <class Tag>
        struct derived : base<Tag> {
            BOOST_PARAMETER_CONSTRUCTOR(
                derived, (base<Tag>), tag,
                (optional (arg1,int,1) (arg2,int,2)))
        };

    } /* foo */

    struct default_ {};

    int main(int argc, char * arg[]) {
        foo::derived<default_> instance();
        return 0;
    }

With GCC 4.4 on Ubuntu Linux I get the following errors:

dean@dean-desktop:~/Source/spike$ g++ -o boost_parameter_template boost_parameter_template.cpp -I~/boost/
boost_parameter_template.cpp:24: error: macro "BOOST_PARAMETER_FOR_EACH_pred_aux2" passed 3 arguments, but takes just 2
boost_parameter_template.cpp:24: error: macro "BOOST_PP_SPLIT_0" requires 2 arguments, but only 1 given
boost_parameter_template.cpp:24: error: macro "BOOST_PP_SEQ_ELEM_III" requires 2 arguments, but only 1 given
boost_parameter_template.cpp:24: error: macro "BOOST_PP_SEQ_ELEM_III" requires 2 arguments, but only 1 given
boost_parameter_template.cpp:24: error: ‘BOOST_PP_IIF_0’ was not declared in this scope
boost_parameter_template.cpp:24: error: template argument 1 is invalid
boost_parameter_template.cpp:24: error: ‘BOOST_PP_REPEAT_1_BOOST_PP_TUPLE_ELEM_2_0’ does not name a type

[Dean Michael Berris]

Original test file to reproduce the problem.

[Daniel Wallin]

There are two problems here:

1. Type requirements should be parenthesized:

     (arg1, (int))
   

2. BOOST_PARAMETER_CONSTRUCTOR() is far less convenient than BOOST_PARAMETER_FUNCTION() in that it does not take default values for optional parameters, but rather expects the base constructor to do the right thing. This is shown in the docs at:

http://www.boost.org/doc/libs/1_44_0/libs/parameter/doc/html/index.html#parameter-enabled-constructors

but is missing from the reference documentation.

Changing your example accordingly:

         struct base {
             template <class ArgPack>
             base(ArgPack const & args)
-            : val1(args[_arg1])
-            , val2(args[_arg2])
+            : val1(args[_arg1 | 0])
+            , val2(args[_arg2 | 0])
             {}
 
             int val1,val2;
@@ -24,7 +24,7 @@
         struct derived : base<Tag> {
             BOOST_PARAMETER_CONSTRUCTOR(
                 derived, (base<Tag>), tag,
-                (optional (arg1,int,1) (arg2,int,2)))
+                (optional (arg1,(int)) (arg2,(int))))
         };

makes it compile.

[Dean Michael Berris]

Interesting, thanks Daniel. I hope the documentation gets updated with a simple explanation between the difference between BOOST_PARAMETER_CONSTRUCTOR() and BOOST_PARAMETER_FUNCTION().

I think this ticket can be closed now, hoping to see a different ticket filed for updating the documentation for the clarification.