My response to Mr.Sebistian.
(Note: I use T^ just as an example of the name, the real name would again
be up to you guys because I can't make a judgment on syntax with limited
experience)
>To keep it simple and stay at the problem once again:



>From a std::tuple<T, U, V>

>you want to select an element by index (possibly runtime)

>it returns something like std::variant<T, U, V>, but perhaps with

 - better support of references

 - storing the index used for selection

 - custom operation wrappers



>That std::variant like type allows (common) operations to be called on.





>In current C++ I would create a class (UR is a custom class, not inside
the standard library):





class Dispatch

{

public:

    Dispatch(tuple<A, B, C> t, int index);



    void op1();

    void op2();



    tuple<A, B, C>& _tupleref;

    int _index;

}



>Then the compiler has anything it needs for optimization. What is missing
compared to your solution?





tuple<A, B, C> t = { a, b, c }; // initialize

Dispatch<A, B, C> d(t, i); // select index i

i.op2(); // call operations





>No need for new categories or value types or new syntax?

>Full flexibility for customization.

>Full optimization potential.
****ANSWER****
1. As I said the new T^ would allow for the user to see elements of type T
in heterogeneous list coming.
2. I updated my proposal (in my last email to Mr.thiago) to have a new type
other T^, that is different (and more type safe) than std::variant.
****Limitations in your example****
1. I can't pass an element from the tuple in your example to a function,
and expect an instantion from type_set(selector) to T^(as in my case) which
can further decay into T or T& or T&&. This is important because without
this feature your elements in tuple are stuck, and can be passed to a
function based on runtime indexes without switch statements that I am
guessing dispatch already has to use.
2. Compiler can't use additional book keeping for the switch case
statements that you are using in dispatch because of the zero overhead
Principe limiting tuples to be tuples instead of runtime indexed
heterogeneous lists.
3. You cant assign/move to the element that
d.index is pointing to. While in my case you can (refer to my last response
to Mr. Thiago: https://lists.isocpp.org/std-proposals/2026/04/17675.php),
to see the additional expression value type that I proposed.



On Mon, 6 Apr 2026, 4:04 pm Sebastian Wittmeier via Std-Proposals, <
std-proposals_at_[hidden]> wrote:

> To keep it simple and stay at the problem once again:
>
>
>
> From a std::tuple<T, U, V>
>
> you want to select an element by index (possibly runtime)
>
> it returns something like std::variant<T, U, V>, but perhaps with
>
> - better support of references
>
> - storing the index used for selection
>
> - custom operation wrappers
>
>
>
> That std::variant like type allows (common) operations to be called on.
>
>
>
>
>
> In current C++ I would create a class (UR is a custom class, not inside
> the standard library):
>
>
>
>
>
> class Dispatch
>
> {
>
> public:
>
> Dispatch(tuple<A, B, C> t, int index);
>
>
>
> void op1();
>
> void op2();
>
>
>
> tuple<A, B, C>& _tupleref;
>
> int _index;
>
> }
>
>
>
> Then the compiler has anything it needs for optimization. What is missing
> compared to your solution?
>
>
>
>
>
> tuple<A, B, C> t = { a, b, c }; // initialize
>
> Dispatch<A, B, C> d(t, i); // select index i
>
> i.op2(); // call operations
>
>
>
>
>
> No need for new categories or value types or new syntax?
>
> Full flexibility for customization.
>
> Full optimization potential.
>
>
>
>
>
>
>
>
> -----Ursprüngliche Nachricht-----
> *Von:* Muneem via Std-Proposals <std-proposals_at_[hidden]>
> *Gesendet:* Mo 06.04.2026 03:35
> *Betreff:* Re: [std-proposals] Fwd: Extension to runtime polymorphism
> proposed
> *An:* std-proposals_at_[hidden];
> *CC:* Muneem <itfllow123_at_[hidden]>;
> My answer to Mr.Thiago Marciena
>
> >So far, all I've understood is that this is a new syntax for simplifying:
> a) the creation of a std::variant<references, ...>
> b) visitation
>
> That is, if you had:
> std::vector<Foo>
> std::map<Foo, Bar>
>
> You would implicitly declare a
> std::variant<std::vector<Foo> &, std::map<Foo, Bar> &>
>
> populate it with either a runtime or compile time choice
>
> and visit it, calling a function or more in it.
>
> It would be up to the compiler to determine that every use of this type
> compiles for every type in the variant.
>
> ****answer****
> You are mostly right but with some major details left out:
> 1.the user can overload based of the new expression value type, which
> allows the user to reason better.
> 2. This new type can be passed to functions and the compiler will
> instantiate each function that was found in the overload resolution of each
> type for this. Which might sound like std::visit but is much more different:
> 1.for a compile time index, the compiler gurrenties inlining.
> 2. The compiler gurrenties that returning a heterogeneous set only leads
> to a pointer move (mechanism is just like exceptions can be captured using
> references for performance).
> 3. Unlike std::array or vectors of std::variants, the representation of
> this list would be completely up to the compiler, hence the code instnatied
> for each one would be as well.
> 4. Std::visit can only return a variant, while In my case, for any func()
> that you pass an element of the heterogeneous list to, the overload of
> func() chosen for each possible type(of elements in the list) can return:
> 1.T
> 2. Or just that type as T^
> In both cases, unless the user casts the return value of func to specific
> value T or T^, you can only assign the return value of func tons variable
> if the variable is variant, unless the index is constexpr, in which case
> you can assign it to T and the compiler will make sure you get an error
> unless the type is correct. If any of the functions return T, then the ones
> returning T^ would be decayed into T or conversely, we could tell the
> compiler to throw an error (upto comcencess). Std::variants can further
> enchance this by allowing the usage of variants of type T^, for each
> std::variant<T^, U^, V^>, this could help function overloads see
> heterogeneous element accesses coming and handle them. T^ for any T is
> gurrentied to be the size of a simple pointer.
>
>
> >I don't see how you're solving this problem. There are two issues that
> cause
> object slicing: first, it's the calling of a function to implement it.
> This is
> easy to do with a virtual function, so a solved problem, but different from
> copy/move semantics.
>
> ***Answer***
> Problem isn't solved using virtual functions, for example:
> Base *ptr= new Base{};
> Dereieved obj;
> Obj= virtual_clone(ptr);
> //Slicing so problem is not solved
>
>
> >Second and most importantly: the destination memory area must be of
> sufficient
> size to accommodate any of the runtime full objects, which by definition
> cannot
> be known at compile time. You are not addressing this problem. And if you
> come
> up with a solution for it, then it is also available for the case of
> calling a
> virtual function.
>
> ***Answer***
> The destination in this case would either be std::variant or T(if index is
> constexpr or a explicit cast specifically for this job is created). The
> specific cast job would also be of a massive help to implement this.
>
>
> >And again: what do virtual functions and object slicing have to do with
> anything?
>
> ***Answer***
> Because it shows virtual functions are incomplete, say you have want a
> list of integers, floats, doubles, and you want to do arithmetic, now you
> have to use either wrap each in variant, in which case you want pass them
> to overloads expecting this specific case. The compiler isn't given a free
> hand for the list representation, but rather wrapped in the constraints of
> a pre existing container that provides gurrenties that pivoted to other
> motivations. In Mr. Steve's case, it was arrays holding "wrapper clases" to
> multiple different container type objects. In my case it would be probably
> very similar to what I talked about in here:
> https://lists.isocpp.org/std-proposals/2026/04/17641.php.
>
>
> >Same as std::variant then.
> ***ANSWER***
> Not the same as variant for the reasons that talked about in the paragraph
> above. Basically a new heterogeneous list would be pivoted towards
> representation thats best for itself rather than being in the constraints
> of the gurrentied behaviours of some other container. Though there would be
> no dynamic allocating what so ever.
>
> >And I don't see how you're solving this problem. This is still the "magic
> happens" portion of what you've been trying to convey for a week and it
> seems
> no one understands.
>
> You seem to be saying that one needs to have an overload for each of the
> possible types, but your solution removes this need. How?
> ***Answer***
> The magic portion is that you can provide overloads specifically for these
> value types T^ that see elements of heterogeneous coming. A even better one
> would be functions accepting std::variants of these T^ or T types. So the
> solution to the magic exists, but to take advantage of it, we need that
> solution to not be constraint by the gurrentied behaviours of current
> container(in order to make them into lists), and to see the elements of
> those heterogeneous coming so we can provide overloads for them.
>
> >We don't know if it is type-safe, because we can't tell what the type is
> and
> how the compiler can reason anything at compile time to prove safety,
> because
> we don't understand what you're proposing.
> ***Answer***
> 1. It is type safe just like std::visit in that everything is produced at
> compile time, but also not like solutions based of std::visits:
> 1. it's not constraint by containers meant used with std::variants.
> 2. provides more gurrenties like the one I talked about here:
> https://lists.isocpp.org/std-proposals/2026/04/17641.php.
> 3.unlike an array of variants, each element has a fixed type, so you can't
> change it. This makes it more type safe and contained, which also makes it
> easy to optimize, and provide gurrentied for (constexpr indexes).
> 4. Because each index has a fixed type, the compiler can further optimize
> for that aspect.
>
>
> Basically think of it as the better of both variants and tuples.
>
>
>
>
>
> Regards, Muneem
>
> On Mon, 6 Apr 2026, 4:51 am Thiago Macieira via Std-Proposals, <
> std-proposals_at_[hidden]> wrote:
>
> On Sunday, 5 April 2026 10:49:34 Pacific Daylight Time Steve Weinrich via
> Std-
> Proposals wrote:
> > He seems to be fixated on a container of some type that returns
> references
> > to different type of containers. For the life of me, I can't figure out
> why
> > this would be needed or how to make it type-safe!
>
> So far, all I've understood is that this is a new syntax for simplifying:
> a) the creation of a std::variant<references, ...>
> b) visitation
>
> That is, if you had:
> std::vector<Foo>
> std::map<Foo, Bar>
>
> You would implicitly declare a
> std::variant<std::vector<Foo> &, std::map<Foo, Bar> &>
>
> populate it with either a runtime or compile time choice
>
> and visit it, calling a function or more in it.
>
> It would be up to the compiler to determine that every use of this type
> compiles for every type in the variant.
>
> --
> Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
> Principal Engineer - Intel Data Center - Platform & Sys. Eng.
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