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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c++
Subject: Re: Tricky!
Date: Tue, 30 Jul 2019 08:03:24 -0700
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Xref: csiph.com comp.lang.c++:67205

"Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> writes:

> On 29.07.2019 15:30,  <<D6 F6 [05b6]>>  Tiib wrote:
>
>> On Monday, 29 July 2019 14:49:58 UTC+3, Bonita Montero  wrote:
>>
>>> I need a class-function / method that returns a function-pointer
>>> to the same type of class-function.  How do I implement this?  I
>>> need this to have a kind of state-machine where each state-func-
>>> tion returns a pointer to the next state function.  I.e I would
>>> write:  "while( (fn = state->*fn()) != nullptr );".  I see no way
>>> to define the type of fn since it is recursive.
>>
>> Perhaps there are no way to make a function that return function
>> pointer of its own type.  However I do not understand the need
>> of it.
>
> I think she is inspired by (possibly someone else who is inspired
> by) an article by the Go language creator.
>
> As I recall his examples work nicely in Go, or assembly language, or
> a more dynamic language like Python, but have to be rewritten with
> some indirection for a language with strong static type checking
> like C++.

The problem here is not strong static type checking but the lack
of recursive types in C++.  A language can have recursive types
and also have strong static type checking.

> The reason for doing it that way, in languages where it's simple, is
> the textual simplicity of the source code.  Since it doesn't give
> that textual simplicity in C++, it's not a good way in C++.  It's
> doable, just not in any practical way.

That depends on what is meant by "practical".  It can be done
like this:

    #include <iostream>

    class State {
	using Nexter = ...;

      public:
	State(){}
	Nexter r0(){  return  &State::r1;  }
	Nexter r1(){  return  &State::r2;  }
	Nexter r2(){  return  &State::rn;  }
	Nexter rn(){  return  0;  }
    };

    void
    run_state_machine(){
	State *s = new State;

	for(  auto f = &State::r0;  f;  f = (s->*f)()  ){
	    ::std::cout << "Got one\n";
	}
    }

with a fairly short (five-ish lines) elaboration for the "...".
Personally I think this approach offers a fairly nice textual
simplicity, and is eminently practical.

If someone thinks five lines of overhead is too much, the "..."
can be made into a template, giving

        using Nexter = WrapMemberFunctionPointer< State >;

which provides an even nicer source text.