On Thu, 5 Jan 2023 at 01:02, Jason McKesson via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> On Wed, Jan 4, 2023 at 5:37 PM Frederick Virchanza Gotham via
> Std-Proposals <std-proposals_at_[hidden]> wrote:
> >
> > On Wed, Jan 4, 2023 at 10:04 PM Andrey Semashev via Std-Proposals
> > <std-proposals_at_[hidden]> wrote:
> > > Even if you never call the
> > > operator<< in your program that uses std::any, the compiler still has
> to
> > > generate the implementation for it - for every type that you ever store
> > > in std::any. Eventually this makes std::any too expensive to use for
> > > more and more people.
> >
> >
> > If none of the source files in your project have:
> >
> > #include <any_ostream>
> >
> > then your program will never have the extra classes and functions I
> wrote.
> >
> > And even if you _do_ include my header file and never use anything in
> > it, your compiler will eradicate unreachable code.
> >
> > I don't know why you think std::any has to become any more complicated
> > or less efficient.
>
> Because that's not how that works.
>
> Type erasure erases the type. But as part of type erasure, it has to
> instantiate a template somewhere that will perform all of the erased
> operations for that type erasure. `any` is copyable, and therefore if
> you shove an object into it, the type erasure *must* generate a copy
> constructor/assignment operator, even if you never copy that
> particular `any`. The same goes for `operator<<`: the type erasure
> process must generate, at the time you insert the object into `any`,
> whatever functionality is needed to use it.
>

I would like to reiterate that the original proposal here does not perform
type erasure; it requires external registration. That has its own
downsides, of course.

Even if you never call that code, the implementation of `any` must
> assume that you will. And therefore, the compiler and type-erasure
> machinery must build whatever is needed to do so. Even if it never
> gets emitted, it still has to do the work.
>
> And that work cannot be in a separate header file.
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