type_index | Compile time copyable type info | SDK library

Simply:

You can get your code to work providing only two overloads of does_have:

Unsigned integer overflow is not undefined behaviour, however UBSan still has an option to check for it because it is often still a bug. But not in this case. This warning is provoked by a totally innocent hash function. It is supposed to have unsigned integer overflows.

You can silence this warning.

You can also report a bug to libc++ maintainers. They probably should add

I never used this library before, but some poking here and there in the source code and it seems I can make it work (it is written in a nice fashion - so I was lucky, pretty much). I have no idea if there are better ways, but here you go:

I'll try to answer the question about that using line:

• case_ is a variable of type hana::pair created by hana::make_pair(hana::type_c, f) (the first parameter is a wrapper around a type)
• hana::first(case_) returns the first item of the pair (the hana::type_c wrapper around the type)
• +hana::first(case_) uses the unary plus to convert the value from an lvalue to an rvalue (see https://www.boost.org/doc/libs/1_68_0/libs/hana/doc/html/structboost_1_1hana_1_1type.html)
• decltype(+hana::first(case_)) evaluates to the type of the first item of the pair (that hana::type_c wrapper)
• decltype(+hana::first(case_))::type returns the actual type of the first item of the pair (whatever the type was that was constructed inside hana::type_c)
• using T = typename decltype(+hana::first(case_))::type; names that original type as T (the typename bit is needed because C++ is a complicated language and sometimes the compiler needs a hint about whether a thing is a type or not)

You need some machinery to extract that original type that was passed to hana::make_pair - if you were building something to solve only your particular problem you would make it simpler, but they need to make the library so generic that it will solve everybody's problems and that adds complexity.

As for that second return line:

The whole premise of the example is that switch_ is passed a boost::any and it calls the right lambda with the contents of the boost::any.

hana::second(case_) is one of the lambdas originally given to switch_ so if you use hana::second(case_)(a) then a boost::any gets passed to your lambda but the code inside the lambda isn't expecting a boost::any so the error message says std::to_string doesn't accept a boost::any.

You could actually use hana::second(case_)(a) and then cast the boost::any parameter back to the original type inside the lambda. That would actually work fine, but I think that is something switch_ should be doing for you so that the lambda gets the type you expect.

It's just unfortunate that boost::any requires such a terrible cast syntax.

You should sperate two sets of template parameters: one for the enclosing class template, and another one for the member function template itself. E.g.

I suppose, you think of this as the alternative way of creating your objects

Below code is modified for your requirement as per my understanding for your requirements: