On Thu, Nov 3, 2011 at 7:05 PM, Lorenzo Caminiti <lorcaminiti@gmail.com> wrote:
On Wed, Nov 2, 2011 at 6:46 PM, Jeffrey Lee Hellrung, Jr. <jeffrey.hellrung@gmail.com> wrote:
On Wed, Nov 2, 2011 at 2:00 PM, Lorenzo Caminiti <lorcaminiti@gmail.com>wrote:
Hello all (and especially Boost.Parameter's authors),
The Boost.Parameter docs suggest to implement parameter-enabled class templates using a series of typedes for the argument pack and the actual argument types. I think it is best to define extra template parameters with default values to old the argument pack Args and named template parameters ClassType, etc as shown below. This allows to use the named template parameters within the class declaration for example in a base class (boost::python::class_<ClassType, ...>) instead that just in the class definition.
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? [...] Yes but these all sound like implementation reasons to me. If the parameter name keywords need the leading _ for functions, as a library users I'd expect the template parameters to follow the same convention. Why should a user expect template parameter name keywords to follow a different convention than function parameter name keywords?
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. -- Daniel Wallin BoostPro Computing http://www.boostpro.com