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Re: [std-proposals] Detection of whether a determined user-defined literal is declared

From: veronica alphonso <verodeving_at_[hidden]>
Date: Tue, 15 Aug 2023 23:50:36 +0100
I'm aware about the potential pitfalls of userexpr, the main one being how
to determine whether t`s value comes from a literal, but i think this could
be sorted in a similar way as how
std::reference_constructs/converts_from_temporary work from the compiler's
perspective; if t's type isn't one of the list declared at
https://eel.is/c++draft/over.literal#3, is constructed from a
non-literal/temporary, and it's accessed after its lifetime begins(dynamic
branches included) and before its usage in the userexpr(the concept of
access being defined by https://eel.is/c++draft/intro.defs#defns.access),
then the t's usage on a userexpr is ill.formed.

And sure, userexpr could be used for this endeavour, but i don't want to
promote it as the main solution until i fully study all of its potential
details, that's why my intention was to submit an intermediate

El mar, 15 ago 2023 a las 22:19, Jens Maurer (<jens.maurer_at_[hidden]>)

> On 15/08/2023 20.05, veronica alphonso wrote:
> > An example of using this proposed feature(and userexpr, which is another
> feature yet to propose) would be:
> So, you're asking for a new feature to support yet another novel feature.
> > template<class R, class T, const char* suffix, bool sized = true>
> > requires std::user_defined_literal_r<R, types_to_params<T,
> std::conditional_t<sized, std::size_t, void>>, suffix>
> > constexpr R instantiate_usl(T t)
> > {
> > return userexpr(t, sized, suffix);
> > // userexpr converts the given instance, sized or not, and suffix to its
> literal representation
> > // in the form of an expression
> > }
> Why can't you use
> requires { userexpr(t, sized, suffix }
> in your concept, then? All you want to check is whether the given
> "userexpr"
> is valid or not.
> (I'm not offering an opinion whether "userexpr" is a good idea to start
> with.)
> Jens

Received on 2023-08-15 22:50:49