Re: Static reflection - a base for runtime reflection?

Mafj <>
Mon, 4 Aug 2008 15:56:42 CST
On Aug 1, 8:38 pm, Mathias Gaunard <> wrote:

On 31 juil, 22:28, Mafj <> wrote:

I am looking information aboutstaticreflectionin C++. What are the
key concepts. Whether it will be included in the upcoming C++

The next standard introduces two new forms of compile-timereflection:
  1) Concepts, which are most just syntactic sugar for (2)
  2) SFINAE with full expression support

The well-known SFINAE technique in C++03 only works withstaticmember
access depending on template parameters, but in C++0x you can use
whole expressions.

That is to say, if you write a function like this in C++0x

template<typename T1, typename T2>
auto add(const T1& t1, const T2& t2) -> decltype(t1 + t2)
     return t1 + t2;


The function will not be visible if t1 + t2 is not a valid operation
with the types that were provided.

It should also work with meta-functions:

template<typename T1, typename T2, typename Enable = void>
struct is_addable
     staticconst bool value = false;


template<typename T1, typename T2>
struct is_addable<T1, T2, decltype(T1() + T2())>
     staticconst bool value = true;


That seems really useful.

In my opinionstaticreflectionis to provide (only in compile time):

mechanism that would allows templates to iterate through members of a
type, i.e.
  - Iterate through fields, member functions and types of the class
    e.g. first< fields<ClassA> >::type to obtain type of the first
field of ClassA
  - iterate through bases of the class,
    e.g. base<ClassA> or next< first< bases<ClassA> > >::type

You can only do that, be it with C++03 or C++0x, by exposing that
information as compile-time data structures.

Well, I know I can define appropriate metafunctions/traits myself,
they would contain redundant information and they would require

A Boost.MPL map of MPL strings to functor types seems like the right
thing to do. I had wanted to do some preprocessor to do that myself
for a while.

  - iterate through qualifiers of fields (in fact it is sort of
possible now) - but why not it more neater to avoid generation of
intermediate types.

Type traits already do that.

I think type traits deal only with existing qualifiers (by their
definition they cannot work with others).
If there was a way to create custom qualifiers and a way to process
them then it will be really good.

  - iterate through parameters of function
    e.g. first< parameters<typeof(functionA)> >::type

A boost library already does that, it was called function traits if I
remember correctly.

  - obtaining string literals such as names of fields, name of types,
name of qualifiers, name of parameter variables etc
   e.g. type_literal( ClassA ) gives 'ClassA',
      for struct ClassA { int _myField; };
       name_literal< first< fields<ClassA> > > >::value gives
all in compile time

That information should obviously be available in the exposed data

Once again it is such a pain to maintain redundant data knowing that
compiler stores it all somewhere.
It would just a bit of good will to expose it for programmers.

  - additionally defining and using custom qualifiers/annotations in
type definitions, field and variable declarations and so on (in order
to be processed byreflectionrelated templates)

There is a way to put attribute on things in C++0x, but it's intended
for the compilers.
You'd have to expose those qualifiers along with the rest.

I know it is all possible by just defining extra traits for each
class. But once again maintenance.

Why all this information cannot be exposed by compiler in some neat
compiler-supported metafunctions.

  - uniform access to information such as current class type (from its
declaration and all member functions, e.g. 'this_class' ), current
function/method type (from body of a function, e.g. 'function_type')

That seems of no use.

Well, I agree there is typeof(*this), so 'this_class' keyword would
have no much use.

  - similarly definedreflectionfor templates themselves

Doesn't make sense.

Maybe one step at a time would be enough.

The only thing that makes sense is having access to a list of members
in a class, their name, their type (might be difficult to expose), and
a way to call them.

Of course we need to include in this place class bases (which are also
a kind of members).

I still think that having custom qualifiers, attributes or annotations
(however we call them)
Would be a good thing to affect the way how metafunctions could
process class members.
Maybe the qualifiers should be tied to fields/members rather than to
their types.
How would you call them annotations? Attributes?

I will come up with an example shortly.

Thanks for your thoughts.

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