Re: Runtime type selection

[Niklas Holsti <niklas.holsti@tidorum.invalid>]

xorque wrote:
> Hello.
> 
> I'd like to develop a program that processes a graph. The contents of
> the graph are a random selection of objects of different types (all
> elements may be of the same type or may all be different - assuming
> there are enough types defined). By 'type' in this case, I am
> referring
> to Ada types.
> 
> The basic problem is that I want to create an object of a type picked
> at
> random, at runtime. I'm not entirely sure what my options are here.
> 
> I could create a base tagged type and derive a set of types from that
> base, selecting at random. I could use variant records. It seems that
> in
> either case, I'd need to create an enumeration type with a 1:1 mapping
> between enumeration values and derived types.
> 
> Both of these solutions seem a little unwieldly. They also seem like
> they'd end up making the program difficult to extend (I'd ideally just
> like to derive a new type and have it automatically available to be
> selected at random for the graph).

Keep a list or array of (accesses to) a factory function for each 
possible (derived) type. When you derive a new type from the base 
graph-element type, add its factory function to this list (this is often 
called "registering" the new type/function). When you want to make a 
graph element of a "random" type, pick a random index into the list or 
array of factory functions and call the chosen function, which creates 
the new element and returns an access to it (type access all 
Base_Graph_Element_Type'Class).

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
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