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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c++
Subject: Re: "C++20 Coroutines" by Martin Bond
Date: Wed, 06 Oct 2021 13:19:12 -0700
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Ben Bacarisse <ben.usenet@bsb.me.uk> writes:

> Bonita Montero <Bonita.Montero@gmail.com> writes:
>
>> State-machines are _always_ spahgetti-code (while( !end )
>> switch( state ) { ... }), even when you write them conventionally.
>
> Obviously one person's spaghetti is another person's farfalle, but
> in my opinion, if the state is a (a pointer to a) function that
> returns (a pointer to) the next state, then you just get a clean
> loop calling a function:
>
>    data d = { ... };
>    State state = initial;
>    while (state) state = (State)state(&d);
>
> C++ has trouble writing the recursive type, so you need a cast, but
> its a safe cast.

In C++, needing a cast can be avoided by wrapping the function
pointer in a class or struct, as for example:

    class State {
        typedef State (*Go)( data & );
        Go go;

      public:
        State( Go g ) : go( g ) {}

        operator bool(){ return go; }

        State operator()( data &data ){
            return  go( data );
        }
    };


    void
    run_state_machine( State initial, data &data ){
        State s = initial;
        while(  s  )  s = s( data );
    }

The example above is specific to a partcular type of "data" being
acted on, but it's easy to generalize around that shortcoming by
using templates.