How to get an object of a unknown class with given classname

Question

I am searching for a way to determine at runtime, which type of object should be alloced (based on a given class name, which is of type const char*).

Well the simplest way of course is to use loads of ifs /else ifs, but that doesnt seem applicable, because i have > 100 different classes(well at least they all derive from one base class), and i have to add new classes quite regularly aswell.

I already came up with a first draft, but sadly it doesnt compile yet (mingw & g++ 4.4)

template<typename TBase, typename TDerived, typename... TArgs>
Base* get_classobject(const char* classname)
{
    if(strcmp(classname,typeid(TDerived).name())==0)
        return new TDerived; //
    else if(sizeof...(TArgs)>0)
        return get_classobject<TBase,TArgs...>(classname);
    else
        return 0;
}


int main()
{
    Base* obj = get_classobject<Base,A,Foo,B,C>("Foo");
    // ^- Types A B C and Foo are all derived from Base
    delete obj; //of course we got an virtual dtor ;)
    return 0;
}

but that sizeof...(TArgs)>0 doesnt stop gcc from trying to generate code for get_classobject<TBase,const char*>(const char*) which fails

Do you have any idea, how to fix this, or any other idea ? Thanks.

EDIT: i solved it:

template<typename TBase, typename TDerived>
Base* get_classobject(const char* classname)
{
    if(strcmp(classname,typeid(TDerived).name())==0)
        return new TDerived;
    return 0;
}

template<typename TBase, typename TDerived, typename TArg, typename... TArgs>
Base* get_classobject(const char* classname)
{
    if(strcmp(classname,typeid(TDerived).name())==0)
        return new TDerived;
    return get_classobject<TBase,TArg,TArgs...>(classname);
}

EDIT For interested readers:
You should now that the implementation above is NOT compiler independent at all. The output of typeif(sometype).name() is compiler/implementation specific. Using a static const char* name variable or function inside all Derived classes, would fix this, but adds a bunch of work(of course you can use a macro for this, but if you are using macros already, you could aswell use another object factory method)

Smells like you need to implement a factory and `clone` methods. Using a `clone()` interface, you can create objects of a family and not need to know their class name. — Thomas Matthews, Jan 16 '10 at 01:04
I don't want to seem critical, but really... this smells bad. First of all the use of `typeid` is quite controversial, but I would also like to point out the inefficiency of such a method > you perform a search with linear complexity and you have to actually precise ALL the possible types that could get generated... and I don't even want to think about the nightmarish maintenance cost. — Matthieu M., Jan 16 '10 at 16:47
typeid can be easily substituted with a static ::name function, linear search is not an issure since i only call this method once (besides that any other object factory ive looked at also has a linear search or abuses macros) — smerlin, Jan 16 '10 at 19:03

Kornel Kisielewicz · Accepted Answer · 2010-01-15T22:40:40.750

3

Can't you just declare

template<typename TBase, typename TDerived, typename TArg, typename... TArgs>

?

Then you can specialize for the case of

typename TBase, typename TDerived, typename TArg

edited Jan 15 '10 at 22:40

answered Jan 15 '10 at 22:32

Kornel Kisielewicz

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wouldnt it even fail one recursion step earlier if i do that ? – smerlin Jan 15 '10 at 22:36
You're trying to get 1+ template parameters, instead of 0+ right? You can specialize the template for the 0 case. – Kornel Kisielewicz Jan 15 '10 at 22:40

score 2 · Answer 2 · edited May 23 '17 at 12:11

2

Read the answers over here, you likely need a factory.

edited May 23 '17 at 12:11

Community

1
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answered Jan 15 '10 at 22:36

Dmitry

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score 1 · Answer 3 · answered Jan 15 '10 at 22:37

1

How about making a specialized get_classobject() with no variadic temlates? That would stop the recursion.

You would then have one definition with variadic template, and another one of just template<typename TBase, typename TDerived>. Another idea is to make a non-template overload that accepts just const char*, and returns 0.

answered Jan 15 '10 at 22:37

Pavel Radzivilovsky

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Already tried that, still got `error: no matching function for call to 'get_classobject(const char*&)'|` – smerlin Jan 15 '10 at 23:07

score 0 · Answer 4 · edited May 23 '17 at 12:03

0

This sounds like you are looking for the classical object factory pattern. Have a look at this stackoverflow question. Personally I like this method

edited May 23 '17 at 12:03

Community

1
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answered Jan 15 '10 at 22:36

epatel

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when using that pattern you have to write the code yourself, using my way the compiler generates it for me ;) – smerlin Jan 15 '10 at 23:33
@smerlin If I understand *your* way you will need to chain/write all derived classes every time you need to call `get_classobject()`. So when adding a class or removing one this is an administrative/manual process. The way I describe is a little more *automatic*. Basically you only need to add two lines. METADECL in the class declaration and METAIMPL in a compile-unit (.cpp file), preferably close to a constructor. This automated feature of this solution is what really appeals to me, one can then even add classes in runtime by loading shared objects (.so/.dll files) Done that... – epatel Jan 16 '10 at 00:25
i only have to call this function once anyway, and if i had to call them more than once, i would add an (inline) wrapper function, so that i only have to write the classes once – smerlin Jan 16 '10 at 07:58

How to get an object of a unknown class with given classname

4 Answers4