On Thursday, 8 September 2022 at 15:02:13 UTC, Paul Backus wrote:
On Thursday, 8 September 2022 at 03:18:08 UTC, Ali Çehreli
wrote:
I looked at how std.variant.VariantN prints the correct type
and failed to understand the magic there. :(
Then I came up with storing a lambda that is created when the
exact type is known. The following simple variant can carry
arbitrary set of data because the data is provided as sequence
template parameters (aka variadic).
This is actually pretty much exactly what VariantN does, except
instead of storing a pointer to a lambda, it stores a pointer
to an instance of a template function.
The member variable `fptr` [1] is the equivalent of your
`dataToStr`. It stores a pointer to an instance of the
`handler` template [2]. Whenever a new value is assigned to the
VariantN, `fptr` is updated to point to the template instance
corresponding to the new value's type [3].
[1]
https://github.com/dlang/phobos/blob/v2.100.1/std/variant.d#L217-L218
[2]
https://github.com/dlang/phobos/blob/v2.100.1/std/variant.d#L260-L645
[3]
https://github.com/dlang/phobos/blob/v2.100.1/std/variant.d#L731
My implement is similar but a pointer to template struct with
various functions. The advantage is that you can add various
attributes to those functions
https://github.com/apz28/dlang/blob/main/source/pham/utl/utl_variant.d#L904
https://github.com/apz28/dlang/blob/main/source/pham/utl/utl_variant.d#L1394
