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

On 07/31/2015 05:01 AM, dariomt@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?

Regards

PS: I cannot use Boost.Variant

I've tried an alternative variant declared as: template < typename... Keys , typename... Vals

struct map < key_val_var...

: key_val_var... { private: typename std::aligned_union::type my_storage; std::size_t my_which; ... }' where key_val_var is: template struct key_val_var { public: using key=Key; protected: template static void construct ( void*storage , Args&&... args ) { new (storage) Val(std::forward<Args>(args)...); } static void destruct ( void*storage ) { auto p=static_cast(storage); p->~Val(); } static Val* get_val_ptr ( void*storage ) { auto p=static_cast(storage); return p; } When assigning a new value to this variant, the old value has to be destroyed based on the *runtime* value of my_which. This is done by indexing into a temp vector of the static destructs in the superclasses: void destroy_which() { typedef void(*destructor_t)(void*); destructor_t destructors[]= {key_val_var::destruct...} ; destructors[my_which](storage()); } I'm guessing you could do something similar for the get_val_ptr's in the superclass. If you think it would help, I could post the code. -regards, Larry