Re: Passing pointer-to-member as arguments in variadic template.

From:
peter koch larsen <peter.koch.larsen@googlemail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Mon, 27 Oct 2014 09:26:44 CST
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Den mandag den 27. oktober 2014 07.40.17 UTC+1 skrev ?? Tiib:

On Monday, 27 October 2014 02:40:11 UTC+2, peter koch larsen wrote:

How do I effectively - and elegantly if this is possible - pass
pointers-to-members in a template?

I can do something like this:

template<typename T,int T::*mpi> void f(T& t) { t.*mpi = 42;}

and a more generic variant would be

template<typename T,typename Q,Q T::*mpi> void g(T& t) { t.*mpi = 42; }

but this variant is more difficult to specify.


What is difficult about it and why such thing is needed?


The need is for traversing the struct. I use the traversal to e.g.
serialise the structures, to determine layout on a screen and also in
an introspection-like manner where I might have no instance available.
So I have my visitor in three varieties: const, non-const and static.
(The difficulty in having the (type,value) template is in specifying
the pairs in a variable template).

What I would like to do is something like

template<typename T,typename... Args> void g(T& t)
where each arg is a pointer-to-member.


That can't be because pointer to member is not type, it is value.
You have to specify type of those to have a variadic list.
Something like:

    template<typename T,int T::*... Mpis> void g(T& t)


Yes, this is the core problem. Perhaps some sort of duck-typing would
solve that?

The use is for generic traversal of structures - e.g.:

template<typename S,typename Functor,typename... Fields>
void visit(S s,Functor f);
(Those who saw Howard Hinnants "Types do not know #" will have a use-case.
Mine is different - I use it for e.g. serialization).


I haven't looked the lecture. I feel that you should perhaps see how
tuple is made and works. It may help you with something.
Like ... can you describe ... what that does?:

    auto Type_members = std::make_tuple(&Type::mem1, &Type::mem2);


I have something like that in my current solution, but it is not so
beautiful in my eyes.

Something of it might be useful or may be such tuple itself may be useful
to you. If it misses the issue totally then please complain.

Is there anyone with a solution to this problem?


There was some confusion in your problem description between types
and values. So it is likely that I did not even understand your
problem, but I tried to be helpful. :)


I appreciate your help. Let me try to rephrase the problem:

You have a struct and would like to be able to apply a function to each
of the member values. The functor could serialise the item, calculate a
checksum or a hash-value or whatever. A not so useful example:

struct printer
{
    template<typename T>
    void operator() (T const& t) { std::cout << t << '\n'; }
};

I have a struct:

struct Person { std::string name; std::string address; double salary; };

Now I would like to combine the two (not working because Fields is not
a type):

template<typename S,typename... Fields> struct visitor
{
    // This would be performed by partial specialization of visitor
    // This is just to give you an idea.
    template<typename F>
    static void visit(S const& s,F f) { f(s.*first_field);f(s.*rest); }
}

If I could create the visitor, I would have no problem "binding"
a struct and its corresponding visitor - e.g. using a traits class.
That would enable me to have code like

template<typename T> void basic_print(T const& t)
{
     printer p;
     std::visitor<T>::visit(t,p);
}

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