Re: [boost] [intro] Preview: Introspection library

Subject: Re: [boost] [intro] Preview: Introspection library
From: Stefan Strasser (strasser_at_[hidden])
Date: 2010-06-28 11:13:29

Zitat von Sohail Somani <sohail_at_[hidden]>:

> On 10-06-28 8:00 AM, Stefan Strasser wrote:
>>
>> template<class Mapping>
>> friend void introspect(Mapping mapping,type<A>){
>>   mapping
>>     (base_class<A_base>()                  )
>>     (member<A,int             ,&A::m_a>()  )
>>     (member<A,float           ,&A::m_b>()  )
>>     (member<A,std::vector<int>,&A::m_vec>())
>>   ();
>> }
>
> I think a library like this would be useful.
>
> Isn't it simpler to specialize a template? Why does this need to be  
> done at run-time?

I assume you mean as opposed to something like

typedef mpl::vector<member<...>,member<...> > introspect;

there are several reasons for this:
  - I prefer the syntax above
  - although it is a runtime function it isn't really executed at  
runtime. an optimizing compiler generates no code.
  - if you need a MPL sequence, you can easily obtain one:

struct functor{
   template<class Sequence>
   void operator()(Sequence) const{
   }
};

intro::apply_local_sequence<A>(functor());

  - you can associate state with a member. for example:

mapping
   (member<...>(),"member_name")
();

which is useful e.g. for algorithms serializing to XML, or mapping a  
member to an SQL database field.

  - most algorithms only need to iterate members and don't need a MPL  
sequence, which saves lots of instantiations. they simply implement a  
functor that gets called for each member and call

intro::for_each<A>(functor());

> How do you introspect a type's constructors?

none of the algorithms need reflection of constructors or other member  
functions.
if you need it it can be added without changing the library. it would  
probably look something like:

template<class A0,class A1...>
struct constructor{};

mapping
   (constructor<arg_type1,arg_type2>())
();