Re: [Boost-users] [fusion][?] a dictionary with compile time (or types as) keys

Subject: Re: [Boost-users] [fusion][?] a dictionary with compile time (or types as) keys
From: Gordon Woodhull (gordon_at_[hidden])
Date: 2014-02-17 12:20:07

> On Feb 17, 2014, at 8:20 AM, Larry Evans <cppljevans_at_[hidden]> wrote:
> 
>> On 02/16/14 12:30, Nick Stokes wrote:
>> The problem is the following:
>> 
>> class TypeDictionary {
>> public:
>>    // implementation be here -- and the question is about this
>> };
>> 
>> struct Foo;
>> struct Bar;
>> 
>> void userOfTypeDictionary() {
>> 
>>     TypeDictionary td;
>>     td.insert( Foo() );
>>     td.insert( Bar() );
>>     td,insert( double(3.14) );
>>     // and other unknown (to TypeDictionary)
>>     // list of types attached
>> 
>>     // later on, in a different scope perhaps
>>     Foo& f = td.get<Foo>() ;
>>     Bar& f = td.get<Bar>();
>>     double pi = tg.get<double>();
>>     // ...
>> }
>> 
>> 
>> Now the question so far seems like right up in the alley of
>> Boost.Fusion, perhaps with a boost::fusion::set, but I am unsure how to
>> manipulate a set of types unknown ahead of time as in code above, and
>> with a  -- rather obvious -- extension whereby different instances of
>> TypeDictionary   would be used to contain a different set of types,
>> potentially in different compilation units.
>> 
>> 
>> As for a realistic motivating use case, think of a  plugin manager. At
>> any given time of its life an arbitrary heterogeneous set of  objects
>> are attached to it, and the it is the manager's job to manage the life
>> time of the attached objects and return (a reference) to them when
>> queried in a type-safe manner.
>> 
>> 
>> Any ideas on whether such a thing is possible, and how, would be deeply
>> appreciated.
>> 
>> Thanks many,
>> Nick
> Wouldn't:
> 
> http://www.boost.org/doc/libs/1_55_0/libs/fusion/doc/html/fusion/container/map.html
> 
> work?

I think the OP is looking for something runtime, i.e. type-erased. I know I have seen such a thing but I don't recall seeing one in Boost.

Cheers,
Gordon