I have an interesting problem. Consider this class hierachy:
class Base
{
public:
virtual float GetMember( void ) const =0;
virtual void SetMember( float p ) =0;
};
class ConcreteFoo : public Base
{
public:
ConcreteFoo( "foo specific stuff here" );
virtual float GetMember( void ) const;
virtual void SetMember( float p );
// the problem
void foo_specific_method( "arbitrary parameters" );
};
Base* DynamicFactory::NewBase( std::string drawable_name );
// it would be used like this
Base* foo = dynamic_factory.NewBase("foo");
I've left out the DynamicFactory definition and how Builders are
registered with it. The Builder objects are associated with a name
and will allocate a concrete implementation of Base. The actual
implementation is a bit more complex with shared_ptr to handle memory
reclaimation, but they are not important to my problem.
ConcreteFoo has class specific method. But since the concrete instances
are create in the dynamic factory the concrete classes are not known or
accessible, they may only be declared in a source file. How can I
expose foo_specific_method to users of Base*?
I'm adding the solutions I've come up with as answers. I've named
them so you can easily reference them in your answers.
I'm not just looking for opinions on my original solutions, new ones
would be appreciated.
The cast would be faster than most other solutions, however:
in Base Class add:
void passthru( const string &concreteClassName, const string &functionname, vector<string*> args )
{
if( concreteClassName == className )
runPassThru( functionname, args );
}
private:
string className;
map<string, int> funcmap;
virtual void runPassThru( const string &functionname, vector<string*> args ) {}
in each derived class:
void runPassThru( const string &functionname, vector<string*> args )
{
switch( funcmap.get( functionname ))
{
case 1:
//verify args
// call function
break;
// etc..
}
}
// call in constructor
void registerFunctions()
{
funcmap.put( "functionName", id );
//etc.
}
The CrazyMetaType solution.
This solution is not well thought out. I was hoping someone might
have had experience with something similar. I saw this applied to the
problem of an unknown number of a known type. It was pretty slick. I
was thinking to apply it to an unkown number of unknown type***S***
The basic idea is the CrazyMetaType collects the parameters is type
safe way, then executing the concrete specific method.
class Base
{
...
virtual CrazyMetaType concrete_specific( int kind ) =0;
};
// used like this
foo->concrete_specific(foo_method_id) << "foo specific" << foo_specific;
My one worry with this solution is that CrazyMetaType is going to be
insanely complex to get this to work. I'm up to the task, but I
cannot count on future users to be up to be c++ experts just to add
one concrete specific method.
Add special functions to Base.
The simplest and most unacceptable solution is to add
foo_specific_method to Base. Then classes that don't
use it can just define it to be empty. This doesn't work because
users are allowed to registers their own Builders with the
dynamic_factory. The new classes may also have concrete class
specific methods.
In the spirit of this solution, is one slightly better. Add generic
functions to Base.
class Base
{
...
/// \return true if 'kind' supported
virtual bool concrete_specific( int kind, "foo specific parameters" );
};
The problem here is there maybe quite a few overloads of
concrete_specific for different parameter sets.
Just cast it.
When a foo specific method is needed, generally you know that the
Base* is actually a ConcreteFoo. So just ensure the definition of class
ConcreteFoo is accessible and:
ConcreteFoo* foo2 = dynamic_cast<ConcreteFoo*>(foo);
One of the reasons I don't like this solution is dynamic_casts are slow and
require RTTI.
The next step from this is to avoid dynamic_cast.
ConcreteFoo* foo_cast( Base* d )
{
if( d->id() == the_foo_id )
{
return static_cast<ConcreteFoo*>(d);
}
throw std::runtime_error("you're screwed");
}
This requires one more method in the Base class which is completely
acceptable, but it requires the id's be managed. That gets difficult
when users can register their own Builders with the dynamic factory.
I'm not too fond of any of the casting solutions as it requires the
user classes to be defined where the specialized methods are used.
But maybe I'm just being a scope nazi.
The cstdarg solution.
Bjarn Stroustrup said:
A well defined program needs at most few functions for which the
argument types are not completely specified. Overloaded functions and
functions using default arguments can be used to take care of type
checking in most cases when one would otherwise consider leaving
argument types unspecified. Only when both the number of arguments and
the type of arguments vary is the ellipsis necessary
class Base
{
...
/// \return true if 'kind' supported
virtual bool concrete_specific( int kind, ... ) =0;
};
The disadvantages here are:
almost no one knows how to use cstdarg correctly
it doesn't feel very c++-y
it's not typesafe.
Could you create other non-concrete subclasses of Base and then use multiple factory methods in DynamicFactory?
Your goal seems to be to subvert the point of subclassing. I'm really curious to know what you're doing that requires this approach.
If the concrete object has a class-specific method then it implies that you'd only be calling that method specifically when you're dealing with an instance of that class and not when you're dealing with the generic base class. Is this coming about b/c you're running a switch statement which is checking for object type?
I'd approach this from a different angle, using the "unacceptable" first solution but with no parameters, with the concrete objects having member variables that would store its state. Though i guess this would force you have a member associative array as part of the base class to avoid casting to set the state in the first place.
You might also want to try out the Decorator pattern.
You could do something akin to the CrazyMetaType or the cstdarg argument but simple and C++-ish. (Maybe this could be SaneMetaType.) Just define a base class for arguments to concrete_specific, and make people derive specific argument types from that. Something like
class ConcreteSpecificArgumentBase;
class Base
{
...
virtual void concrete_specific( ConcreteSpecificArgumentBase &argument ) =0;
};
Of course, you're going to need RTTI to sort things out inside each version of concrete_specific. But if ConcreteSpecificArgumentBase is well-designed, at least it will make calling concrete_specific fairly straightforward.
The weird part is that the users of your DynamicFactory receive a Base type, but needs to do specific stuff when it is a ConcreteFoo.
Maybe a factory should not be used.
Try to look at other dependency injection mechanisms like creating the ConcreteFoo yourself, pass a ConcreteFoo type pointer to those who need it, and a Base type pointer to the others.
The context seems to assume that the user will be working with your ConcreteType and know it is doing so.
In that case, it seems that you could have another method in your factory that returns ConcreteType*, if clients know they're dealing with concrete type and need to work at that level of abstraction.
Related
I'm in a situation where I have a class, let's call it Generic. This class has members and attributes, and I plan to use it in a std::vector<Generic> or similar, processing several instances of this class.
Also, I want to specialize this class, the only difference between the generic and specialized objects would be a private method, which does not access any member of the class (but is called by other methods). My first idea was to simply declare it virtual and overload it in specialized classes like this:
class Generic
{
// all other members and attributes
private:
virtual float specialFunc(float x) const =0;
};
class Specialized_one : public Generic
{
private:
virtual float specialFunc(float x) const{ return x;}
};
class Specialized_two : public Generic
{
private:
virtual float specialFunc(float x) const{ return 2*x; }
}
And thus I guess I would have to use a std::vector<Generic*>, and create and destroy the objects dynamically.
A friend suggested me using a std::function<> attribute for my Generic class, and give the specialFunc as an argument to the constructor but I am not sure how to do it properly.
What would be the advantages and drawbacks of these two approaches, and are there other (better ?) ways to do the same thing ? I'm quite curious about it.
For the details, the specialization of each object I instantiate would be determined at runtime, depending on user input. And I might end up with a lot of these objects (not yet sure how many), so I would like to avoid any unnecessary overhead.
virtual functions and overloading model an is-a relationship while std::function models a has-a relationship.
Which one to use depends on your specific use case.
Using std::function is perhaps more flexible as you can easily modify the functionality without introducing new types.
Performance should not be the main decision point here unless this code is provably (i.e. you measured it) the tight loop bottleneck in your program.
First of all, let's throw performance out the window.
If you use virtual functions, as you stated, you may end up with a lot of classes with the same interface:
class generic {
virtual f(float x);
};
class spec1 : public generic {
virtual f(float x);
};
class spec2 : public generic {
virtual f(float x);
};
Using std::function<void(float)> as a member would allow you to avoid all the specializations:
class meaningful_class_name {
std::function<void(float)> f;
public:
meaningful_class_name(std::function<void(float)> const& p_f) : f(p_f) {}
};
In fact, if this is the ONLY thing you're using the class for, you might as well just remove it, and use a std::function<void(float)> at the level of the caller.
Advantages of std::function:
1) Less code (1 class for N functions, whereas the virtual method requires N classes for N functions. I'm making the assumption that this function is the only thing that's going to differ between classes).
2) Much more flexibility (You can pass in capturing lambdas that hold state if you want to).
3) If you write the class as a template, you could use it for all kinds of function signatures if needed.
Using std::function solves whatever problem you're attempting to tackle with virtual functions, and it seems to do it better. However, I'm not going to assert that std::function will always be better than a bunch of virtual functions in several classes. Sometimes, these functions have to be private and virtual because their implementation has nothing to do with any outside callers, so flexibility is NOT an advantage.
Disadvantages of std::function:
1) I was about to write that you can't access the private members of the generic class, but then I realized that you can modify the std::function in the class itself with a capturing lambda that holds this. Given the way you outlined the class however, this shouldn't be a problem since it seems to be oblivious to any sort of internal state.
What would be the advantages and drawbacks of these two approaches, and are there other (better ?) ways to do the same thing ?
The issue I can see is "how do you want your class defined?" (as in, what is the public interface?)
Consider creating an API like this:
class Generic
{
// all other members and attributes
explicit Generic(std::function<float(float)> specialFunc);
};
Now, you can create any instance of Generic, without care. If you have no idea what you will place in specialFunc, this is the best alternative ("you have no idea" means that clients of your code may decide in one month to place a function from another library there, an identical function ("receive x, return x"), accessing some database for the value, passing a stateful functor into your function, or whatever else).
Also, if the specialFunc can change for an existing instance (i.e. create instance with specialFunc, use it, change specialFunc, use it again, etc) you should use this variant.
This variant may be imposed on your code base by other constraints. (for example, if want to avoid making Generic virtual, or if you need it to be final for other reasons).
If (on the other hand) your specialFunc can only be a choice from a limited number of implementations, and client code cannot decide later they want something else - i.e. you only have identical function and doubling the value - like in your example - then you should rely on specializations, like in the code in your question.
TLDR: Decide based on the usage scenarios of your class.
Edit: regarding beter (or at least alternative) ways to do this ... You could inject the specialFunc in your class on an "per needed" basis:
That is, instead of this:
class Generic
{
public:
Generic(std::function<float(float> f) : specialFunc{f} {}
void fancy_computation2() { 2 * specialFunc(2.); }
void fancy_computation4() { 4 * specialFunc(4.); }
private:
std::function<float(float> specialFunc;
};
You could write this:
class Generic
{
public:
Generic() {}
void fancy_computation2(std::function<float(float> f) { 2 * f(2.); }
void fancy_computation4(std::function<float(float> f) { 4 * f(4.); }
private:
};
This offers you more flexibility (you can use different special functions with single instance), at the cost of more complicated client code. This may also be a level of flexibility that you do not want (too much).
In C# I can define this:
public interface BaseObject
{
int GetValue();
}
public class Test<T> where T : BaseClass
{
T BaseObject;
}
which means I know that I can alwaysa call BaseObject.GetValue() / BaseObject->GetValue(); because I know that the baseobject has this method.
Is there a similiar way to do this in C++? So that I can define an interface that multiple classes can inherit and a class that can take advantage of this.
Templates, which are even more powerful than C# generics (not to say they are necessarily better, just different).
template<class T>
class foo
{
public:
int whatever()
{
return obj.GetValue();
}
private:
T obj;
};
A separate class is created for each template argument you use. If you provide a template type which would result in an error you will know at compile time.
You're asking about C++ concepts, a way to specify requirements for template parameters. They were proposed during the work on C++11, but proved complicated enough that they weren't done in time. But they've just been delayed, not forgotten.
In the meantime, duck typing remains very powerful, and it will catch when you pass a template parameter that doesn't have the required interface. It just won't report the problem as neatly.
As a workaround, a simple way to check the constraint you showed takes advantage of the fact that pointer conversions are implicit only when upcasting:
public class Test<T> where T : BaseClass
{
static T* enforcement_helper = 0;
static BaseClass* enforce_inheritance_constraint = enforcement_helper;
};
Depending on how new your compiler is, you may need to put those lines inside a special member function (destructor is good, because it's almost always processed).
But you should only check constraints in order to improve error messages (by causing the failure in a clearly commented section of code). C++ templates are duck typed, and they will work with any template parameters that provide the required operations. No formal "interface" is required.
My reason for asking the question:
I am using a large framework not of my own design. I need to use several "user information" classes which are unrelated as far as the code is concerned. They do not derive from any common base class, and I do not have access to the source code to recompile.
These information classes work like this: there are classes A, B, C, etc. These classes each have an information class, Ainfo, Binfo, etc. associated with them. Because the user (i.e. me) needs to attach different informations to a given object of a given class (meaning I might have two different classes deriving from Ainfo that I want to attach to an object of A), and there is only one information slot, I want to make an information object that can old other various information objects. That way, I can just add my information into this fake information-object-which-is-a-container-for-other-information-objects.
The problem arises in that I would like to do this for Ainfo, Binfo, Cinfo, Dinfo etc. So I would like to write a mixin or something that just adds the container functionality to any of the plain old info classes.
The problem is that the information classes Ainfo, Binfo, etc. require different constructor arguments.
So the question:
Is it possible to pass a vector of types into the constructor of the mixin? That way I could have a variable list of appropriate constructor parameters passed in? Can you assign a type to a variable outside of a template argument? Can you cast with this variable?
or
Is it possible to inherit from a specific object? Could I for example create a new Ainfo object using the correct constructor, then do the mixin on that specific object. This would be like the usage of the decorator pattern, except I have no common interface. (the object being decorated is the interface)
or
am I just going to have to bite the bullet and write 15,000 (exaggeration :) ) classes which are exactly the same, but inherit from a different base class and contain a different type of object?
Summary:
I need to add a container feature to several different classes while maintaining the interface of each class and utilizing their argument-taking constructors. I would like to not duplicate code.
Thanks in advance. Sorry for totally butchering terminology.
It sounds to me like what you want is a Boost.Variant. It's like a C++-style union. It is strongly typed (so that you always know what you actually stored in it), and it has a powerful visitation mechanism that makes it easy to map many different types to a single operation.
For example, you can do this:
typedef boost::variant<Ainfo, Binfo, Cinfo> CommonInfo;
//In a function.
CommonInfo someInfo = Ainfo();
You can then write visitor functors that can be used to call members of the info objects.
class DoThingInfoVisitor : boost::static_visitor<>
{
void operator()(Ainfo &info) {info.DoThing()}
void operator()(Binfo &info) {info.DoThing2()}
void operator()(Cinfo &info) {info.StepA(); info.StepB();}
};
Armed with this object, if you want to do whatever this DoThing means for any CommonInfo type:
CommonInfo someInfo = Ainfo();
boost::apply_visitor( times_two_visitor(), someInfo );
This will call the Ainfo version, since that's what happens to be stored in someInfo. If it had stored Binfo, then you could use that. You can build a suite of these visitors; they can return values, take parameters (though you'll need to store them in the functor), and various other tricks you can learn from the docs.
If it's not doable in templates, and you can't hack it with the preprocessor, then you're gonna have to do it by hand. C++ doesn't contain any type manipulation at run-time, typeid() and dynamic_cast is all you've got.
This may be a bit of an oversimplification, but if nothing else it should help to clarify your question. Using templates, you can easily generate classes that derive from your info classes. The following classes illustrate this concept.
class Ainfo {
std::string _a;
public:
void setContent(const std::string& A);
const char * print() const; // prints _a
};
class Binfo {
std::string _b;
public:
void setContent(const std::string& B);
const char * print() const; // prints _b
};
template<class Tinfo>
class Info : public Tinfo {
};
You could then use this template as follows.
Info<Ainfo> my_info;
my_info.setContent("test");
std::cout << my_info.print();
UPDATE: If you also want to override the template's constructor, try using a member template.
template<class Tinfo>
class Info : public Tinfo {
public:
template<typename arg>
Info(arg rhs) : Tinfo(rhs) { }
};
Using this, you can compile and run the following.
Info<Ainfo> my_info("Testing...");
std::cout << my_info.print();
I could be wrong, but I have a feeling that we're getting pretty close now...
I've been programming in Java way too long, and finding my way back to some C++. I want to write some code that given a class (either a type_info, or its name in a string) can create an instance of that class. For simplicity, let's assume it only needs to call the default constructor. Is this even possible in C++, and if not is it coming in a future TR?
I have found a way to do this, but I'm hoping there is something more "dynamic". For the classes I expect to wish to instantiate (this is a problem in itself, as I want to leave that decision up to configuration), I have created a singleton factory with a statically-created instance that registers itself with another class. eg. for the class Foo, there is also a FooFactory that has a static FooFactory instance, so that at program startup the FooFactory constructor gets called, which registers itself with another class. Then, when I wish to create a Foo at runtime, I find the FooFactory and call it to create the Foo instance. Is there anything better for doing this in C++? I'm guessing I've just been spoiled by rich reflection in Java/C#.
For context, I'm trying to apply some of the IOC container concepts I've become so used to in the Java world to C++, and hoping I can make it as dynamic as possible, without needing to add a Factory class for every other class in my application.
You could always use templates, though I'm not sure that this is what your looking for:
template <typename T>
T
instantiate ()
{
return T ();
}
Or on a class:
template <typename T>
class MyClass
{
...
};
Welcome in C++ :)
You are correct that you will need a Factory to create those objects, however you might not need one Factory per file.
The typical way of going at it is having all instanciable classes derive from a common base class, that we will call Base, so that you'll need a single Factory which will serve a std::unique_ptr<Base> to you each time.
There are 2 ways to implement the Factory:
You can use the Prototype pattern, and register an instance of the class to create, on which a clone function will be called.
You can register a pointer to function or a functor (or std::function<Base*()> in C++0x)
Of course the difficulty is to register those entries dynamically. This is typically done at start-up during static initialization.
// OO-way
class Derived: public Base
{
public:
virtual Derived* clone() const { return new Derived(*this); }
private:
};
// start-up...
namespace { Base* derived = GetFactory().register("Derived", new Derived); }
// ...or in main
int main(int argc, char* argv[])
{
GetFactory().register("Derived", new Derived(argv[1]));
}
// Pointer to function
class Derived: public Base {};
// C++03
namespace {
Base* makeDerived() { return new Derived; }
Base* derived = GetFactory().register("Derived", makeDerived);
}
// C++0x
namespace {
Base* derived = GetFactory().register("Derived", []() { return new Derived; });
}
The main advantage of the start-up way is that you can perfectly define your Derived class in its own file, tuck the registration there, and no other file is impacted by your changes. This is great for handling dependencies.
On the other hand, if the prototype you wish to create requires some external information / parameters, then you are forced to use an initialization method, the simplest of which being to register your instance in main (or equivalent) once you have the necessary parameters.
Quick note: the pointer to function method is the most economic (in memory) and the fastest (in execution), but the syntax is weird...
Regarding the follow-up questions.
Yes it is possible to pass a type to a function, though perhaps not directly:
if the type in question is known at compile time, you can use the templates, though you'll need some time to get acquainted with the syntax
if not, then you'll need to pass some kind of ID and use the factory approach
If you need to pass something akin to object.class then it seems to me that you are approaching the double dispatch use case and it would be worth looking at the Visitor pattern.
No. There is no way to get from a type's name to the actual type; rich reflection is pretty cool, but there's almost always a better way.
no such thing as "var" or "dynamic" in C++ last time I've checked(although that was a WHILE ago). You could use a (void*) pointer and then try casting accordingly. Also, if memory serves me right, C++ does have RTTI which is not reflection but can help with identifying types at runtime.
I am considering a factory function to create different classes in the same hierarchy. I understand that normally a factory is normally implemented as follows:
Person* Person::Create(string type, ...)
{
// Student, Secretary and Professor are all derived classes of Person
if ( type == "student" ) return new Student(...);
if ( type == "secretary" ) return new Secretary(...);
if ( type == "professor" ) return new Professor(...);
return NULL;
}
I am trying to think of a way so that the process can be automated so that the various conditions do not need to be hard-coded.
So far the only way I can think of is using a map and the prototype pattern:
The map will hold the type string in the first element and a class instance (prototype) in the second:
std::map<string, Person> PersonClassMap;
// This may be do-able from a configuration file, I am not sure
PersonClassMap.insert(make_pair("student", Student(...)));
PersonClassMap.insert(make_pair("secondary", Secretary(...)));
PersonClassMap.insert(make_pair("professor", Professor(...)));
The function may look something like this:
Person* Person::Create(string type)
{
map<string, Person>::iterator it = PersonClassMap.find(type) ;
if( it != PersonClassMap.end() )
{
return new Person(it->second); // Use copy constructor to create a new class instance from the prototype.
}
}
Unfortunately, the prototype method only works if you only want the class created by the factory to be identical every time, since it does not support arguments.
Does anybody know if it is possible to do it in a nice way, or am I stuck with the factory function?
I usually build a factory method (or a factory object) when the clients will be providing some information about the object to be created, but they don't know what concrete class the result will be. The determination about how to express the interface to the factory depends completely on what information the clients have. It could be that they provide a string (program text to be parsed, for example), or a set of parameter values (number of dimensions and sizes if we're creating geometric objects in n-space). The factory method then examines the information and decides what kind of object to create or which more specific factory to call.
So the decision about what to build shouldn't be made by the caller; if she knows, then there's no reason for a factory. If the list of things to be built is open-ended, you might even have a registration protocol that allows specific implementations to provide both their construction method and a discriminator function that would allow the factory method to decide which method to call.
It very much depends on what information is necessary and sufficient to decide which kind of object to build.
You can register a factory method (instead of the prebuilt element to copy). This will allow you to call the abstract factory with parameters that are passed to the concrete factory. The limitation here is that the set of parameters of all concrete factories must be the same.
typedef std::string discriminator;
typedef Base* (*creator)( type1, type2, type3 ); // concrete factory, in this case a free function
typedef std::map< discriminator, creator > concrete_map;
class Factory // abstract
{
public:
void register_factory( discriminator d, creator c ) {
factories_[ d ] = c;
}
Base* create( discriminator d, type1 a1, type2 a2, type3 a3 )
{
return (*(factories_[ d ]))( a1, a2, a3 );
}
private:
concrete_map factories_;
};
I have used free function creators to reduce the sample code, but you can define a concrete_factory type and use it instead of the 'creator' element above. Again, as you can see, you are limited to a fixed set of arguments in the factory 'create' method.
Each concrete factory can pass the arguments to the constructor of the given type:
Base* createDerived1( type1 a1, type2 a2, type3 a3 )
{
return new Derived1( a1, a2, a3 );
}
This is more flexible than your approach as you can create instances that hold references to external objects (those can only be initialized during construction) or constant members, or objects that cannot be reset to a different state after construction in a more general wording.
I would add a pure abstract clone method to class Person (which definitely looks like it should be an abstract class, existing mainly for the sake of being subclassed -- if you need a concrete "none of the above" kind of Person it's best done via a separate concrete subclass OtherKindOfPerson, rather than as the base class itself):
virtual Person* clone() const = 0;
and override it in every concrete subclass, e.g. in Student, with a new that invokes the specific concrete subclass's copy ctor:
Person* clone() const { return new Student(*this); }
You also need to change the registry map to:
std::map<string, Person*> PersonClassMap;
[[You could use some smarter pointer than a plain old Person *, but as the map and all of its entries probably needs to survive as long as the process does, this is definitely not a big deal -- the main added value you might get from smarter pointers being smarter behavior upon destruction of the "pointer"!-)]]
Now, your factory function can simply end with:
return it->second->clone();
The changes are needed to avoid the "slicing" effect of using the base class's copy ctor on a subclass that has extra attributes, as well as preserve any virtual method's resolution.
Subclassing a concrete class to yield other concrete classes is a bad idea exactly because these effects can be tricky and a source of bugs (see Haahr's recommendation against it: he writes about Java, but the advice is also good for C++ and other languages [indeed I find his recommendation even more crucial in C++!].
I am not familar with c++ but in many langugaes there concepts of delegates or closures. Means that instead of mapping to instance, you map to the function(delegate, closure) that is responsible to create object.
You could make an enum of each type of person:
enum PersonType { student, secretary, professor };
Well if you want a faster way to do it then using an enum and a switch statement will be many time faster than processing sequential if/else if statements ...
If you look at your two implementations, logically they are identical.
The first implementation is the same as your second if the recursive loop was unrolled. So really there is no advantage of your second implementation.
Regardless what you do you will need to list some where your types mapped to your constructors.
One way of doing it that can be useful is to have this map in a seperate xml file
<person>
<type> student </type>
<constructor> Student </type>
</person>
....
You can then read this xml file in to memory and use reflection to get back your constructor. For the given type. Given that you are using C++ however this will not be that simple as C++ does not have reflection as standard. You will have to look for an extension to provide you with reflection in C++.
But regardless, all these alternatives can not escape what you did in your original implementioan, ie: list the mapping from type to constructor and search the map.