While it is trivially simple to define some struct with callback functions,
such a definition turns out to be obnoxiously complex and hard to read in pracice,
thereby nullifying any benefit from the shorthand λ-based notation.
This kind of problem with the readability at client-side is quite common
when using callbacks for a policy-based design. To overcome this difficutly,
it would be necessary to define the policy and the callbacks in one
succinct builder expression. Unfortunately such a builder syntax
typically requires to write a lot of boilerplate template code.
After several failed experiments, I have found a bold new way to
exploit features of C++20 in conjunction with a macro, allowing
to generate a suitable data record directly form an C++ expression.
