Re: [boost] [parameter] named template parameters

Subject: Re: [boost] [parameter] named template parameters
From: Lorenzo Caminiti (lorcaminiti_at_[hidden])
Date: 2011-11-07 06:52:54

On Mon, Nov 7, 2011 at 5:56 AM, Daniel Wallin <daniel_at_[hidden]> wrote:
>>> On Wed, Nov 2, 2011 at 2:00 PM, Lorenzo Caminiti <lorcaminiti_at_[hidden]>wrote:
>>>> Do you see any problem in using extra template parameter to define the
>>>> argument pack and named template parameters?
>
> This is similar to what I want to do for the C++11 Boost.Parameter
> with functions:
>
>  template <
>    class... Args, class Pack = typename make_pack<parameters, Args...>::type
>  >
>  void f(Args&&... args) {
>    Pack pack(args...);
>    int x = pack[_x];
>  }
>
> In the case of class templates, the downside would be that the error
> messages would be pretty horrible if the user passes too many
> arguments. I guess it would be possible to guard a bit against that
> though, and possibly make the error messages better.
>
>>>> A couple more things:
>>>>
>>>> 1) I thing that BOOST_PARAMETER_TEMPLATE_KEYWORD(ClassType) macro
>>>> should define a symbol _ClassType to be used for the named parameter
>>>> so to be consistent with the BOOST_PARAMETER_NAME(x) which defined _x
>>>> to be used for the named function parameter. However, ClassType is
>>>> defined without the leading _. What is the reason for this asymmetry?
>
> We simply preferred making the names less ugly over consistency with
> function parameters.
>
>>> 2) I'd expect a macro that allows to specify tag and passing argument
>>>> name for template parameters
>>>> BOOST_PARAMETER_TEMPLATE_KEYWORD((PassClassType, mytag) ClassType) as
>>>> for BOOST_PARAMETER_NAME((pass_x, mytag) x) does for function
>>>> parameters. Why is such a usage of BOOST_PARAMETER_TEMPLATE_KEYWORD
>>>> not supported?
>
> Nobody implemented it. No real reason not to support it.

Thanks a lot Daniel! Then I will implement all these features in my
integration of Boost.Parameter and Boost.Contract.

Also, can you please take a look at why I can't use tag::graph::_ in
the DFS example? I really have _no_ idea on how to fix that and I'd
need your help...
http://boost.2283326.n4.nabble.com/boost-parameter-type-requirement-compiler-error-td3933830.html

For C++11 Boost.Parameter you might want to take a look at what I'll
try to do for constructors:
https://svn.boost.org/svn/boost/sandbox/contract/libs/contract/doc/html/contract__/examples.html#contract__.examples.__named_parameters___my_class__constructors_with_named_parameters

--Lorenzo