Re: [Boost-users] Map runtime value to type

Subject: Re: [Boost-users] Map runtime value to type
From: dariomt (dariomt_at_[hidden])
Date: 2015-07-31 09:10:02

Michael <mwpowellhtx <at> gmail.com> writes:
> 
> 
> On July 31, 2015 6:01:04 AM EDT, dariomt <at> gmail.com wrote:
> >Hi all,
> >
> >Say I have a compile-time map of <value,type> pairs, and I need to
> >access
> >the correct element given a value at runtime and do something with 
that
> >type. That something would be the same for any of the mapped types
> >(e.g.
> >calling an overloaded function that would then do the right thing
> >depending
> >on the type).
> >
> >The value would typically be an enum.
> >
> >Is there anything in the Boost libraries to help with this?
> >
> >If I used C++11, would there be a simple solution to this?
> 
> Not really a boost question, perhaps.
I thought asking if Boost has tools to help with certain C++ problem was 
allowed in this list. Please correct me if I'm wrong. 

> 
> First, I'm not sure why anyone wouldn't leverage at least C++11+. 
Especially considering compiler
> advancements in recent history. Optimization technology being what it 
is and all.
> 
I'd rather use a well tested Boost component than a C++11 solution I have 
to implement, test and document.

> That said, if it was me, I might have a look at a std::function (or 
boost::function) "type factory" that knew
> how to interface with an instance repository.
> 
> Could be a functor instance, as well, for that matter.
> 
> >Regards
> 
> Cheers.
> 
> >PS: I cannot use Boost.Variant
> 
> Why not?
> 
Sorry, it is clear I didn't explain my problem well enough.

Say I have this code.
struct A {};
struct B {};
enum useType {useA, useB};

template <typename F>
void g(useType w, F f)
{
  switch (w)
  {
    case useA: A a; f(a);
    case useB: B b; f(b);
  }
}

I need to generalize this to any number of enum values and any number of 
corresponding types.

Something like this (pseudocode):

typedef compile_time_map< mpl::pair<useA,A>, mpl::pair<useB,B>, ... > 
mapping;

template <typename F>
void g(useType w, F f)
{
  switch_apply( w, mapping, f );
}