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

Subject: Re: [Boost-users] Map runtime value to type
From: Leon Mlakar (leon_at_[hidden])
Date: 2015-07-31 09:54:12

On 31.07.2015 15:10, dariomt wrote:
> 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 );
> }

Why not:

std::map<useType, std::function<void(void)>> use_map = {
   { useA, [] { A a; f(a); } },
   { useB, [] { B b, f(b); } }
};

then:

   usemap.at(w)();

Or something similar ...

Cheers,

Leon