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

31 Jul 2015

      On 31.07.2015 15:10, dariomt wrote:
...
Michael  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  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, mpl::pair, ... >
mapping;
template <typename F>
void g(useType w, F f)
{
   switch_apply( w, mapping, f );
}
Why not:

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

then:

   usemap.at(w)();

Or something similar ...

Cheers,

Leon

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

Leon Mlakar