Boost C++ Libraries: Ticket #4857: Boost.Parameter Constructor in Templates Not Supported?

attachment set

Dean Michael Berris — Tue, 16 Nov 2010 09:14:04 GMT

attachment → boost_parameter_template.cpp

Original test file to reproduce the problem.

Daniel Wallin — Tue, 16 Nov 2010 09:46:33 GMT

There are two problems here:

Type requirements should be parenthesized:
```
  (arg1, (int))
```

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 — Tue, 16 Nov 2010 09:58:59 GMT

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.