Re: Templates vs factories

Imre <>
Thu, 10 May 2007 22:07:28 CST
(Already tried to post this once, but it didn't appear on the ng, so
I'll just give it another try.)

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On May 7, 4:19 pm, ""
<> wrote:

On May 3, 7:40 pm, Imre <> wrote:

(I have a half-solution, but in some cases it violates the One
Definition Rule, so it's not really usable.)

Could you post this solution? It can be a good starting point on the
way of implementing what you want.

First the code, then some explanation.

// --- InstDep.h ---

#ifndef InstDep_H
#define InstDep_H

#include <boost/mpl/if.hpp>
namespace mpl = boost::mpl;

static int const nDependentIdBits = 4;

template <typename B, int id>
struct InstantiateDependent
    enum { Exists = 0 };

template <typename B, int id>
struct InstantiateAllDependentsI
    typedef typename mpl::if_c<InstantiateDependent<B, id>::Exists,
InstantiateAllDependentsI<B, id>,
        typename InstantiateAllDependentsI<B, id - 1>::ThisOrNext>::type
    static void Do()
        InstantiateDependent<B, id>::Do();
        typedef InstantiateAllDependentsI<B, id - 1>::ThisOrNext Next;

template <typename B>
struct InstantiateAllDependentsI<B, -1>
    typedef InstantiateAllDependentsI<B, -1> ThisOrNext;
    static void Do() {}

template <typename B>
struct InstantiateAllDependentsI<B, 1 << nDependentIdBits>
    static void Do()
        typedef InstantiateAllDependentsI<B, (1 << nDependentIdBits) -
1>::ThisOrNext Next;

template <typename B>
struct InstantiateAllDependents:
    public InstantiateAllDependentsI<B, 1 << nDependentIdBits>


// --- Factory.h ---

#ifndef Factory_H
#define Factory_H

#include <string>
#include <map>
#include <assert.h>
#include "InstDep.h"

template <typename T>
class Base
    virtual ~Base() {}
    // interface goes here

template <typename T>
class Factory
    typedef Base<T>*(*Creator)();

    static void Initialize() { InstantiateAllDependents<Factory>::Do(); }
    static Factory& Instance()
        static Factory* instance = NULL;
        if (instance == NULL)
            instance = new Factory;
        return *instance;

    void Register(std::string const& subClassId, Creator creator)
{ creators[subClassId] = creator; }
    Base<T>* Create(std::string const& subClassId)
        Creator creator = creators[subClassId];
        assert(creator != NULL);
        return creator();

    std::map<std::string, Creator> creators;

template <class Derived>
class Registrar
    void Dummy() {} // Needed to force instantiation
    Registrar() { Derived::Register(); }


// --- Derived1.h ---

#ifndef Derived1_H
#define Derived1_H

#include "InstDep.h"
#include "factory.h"

template <typename T>
class Derived1:
    public Base<T>
    static void Dummy() { registrar.Dummy(); } // Needed to force
    static void Register() { Factory<T>::Instance().Register("Derived1",
&Derived1::Create); }
    static Base<T>* Create() { return new Derived1; }

    static Registrar<Derived1> registrar;

template <typename T>
Registrar<Derived1<T> > Derived1<T>::registrar;

template <typename T>
struct InstantiateDependent<Factory<T>, 1>
    enum { Exists = 1 };
    static void Do() { Derived1<T>::Dummy(); }


// --- Derived2.h ---

#ifndef Derived2_H
#define Derived2_H

#include "InstDep.h"
#include "factory.h"

template <typename T>
class Derived2:
    public Base<T>
    static void Dummy() { registrar.Dummy(); } // Needed to force
    static void Register() { Factory<T>::Instance().Register("Derived2",
&Derived2::Create); }
    static Base<T>* Create() { return new Derived2; }

    static Registrar<Derived2> registrar;

template <typename T>
Registrar<Derived2<T> > Derived2<T>::registrar;

template <typename T>
struct InstantiateDependent<Factory<T>, 2>
    enum { Exists = 1 };
    static void Do() { Derived2<T>::Dummy(); }


// --- main.cpp ---

#include "factory.h"
#include "Derived1.h"
#include "Derived2.h"

int main(int argc, char* argv[])
    Base<int>* pb1 = Factory<int>::Instance().Create("Derived1");
    Base<int>* pb2 = Factory<int>::Instance().Create("Derived2");
    return 0;

// --- end of code ---

(Tested only on VC80.)

Notice that main.cpp only includes Derived1.h and Derived2.h, but
doesn't explicitly reference Derived1<int> or Derived2<int>, and still
can create instances of them through the Factory.

That ugly template metaprogramming stuff in InstDep.h is used to
instantiate dependent templates. By dependent class template, I mean
that B<> is dependent on A<> if for each A<T>, B<T> should also be
instantiated. In this example, Derived1 and Derived2 are dependent on
Factory.Basically we iterate through a range of possible integer ids
(16 through 0 in this example) for dependent class templates, and for
each id, we check if there's a dependent template with that id.
The check is done through InstantiateDependent<B, id>::Exists (B is
Factory in this case). According to the primary template, this is 0
for all ids, so we do nothing. When we declare a class template
dependent on another one (for example, we declare Derived1 dependent
on Factory), we do so by specializing this template so that Exists
will be 1, and we also write a Do() static function that references
the dependent template.
During the iteration, if we find an id for which
InstantiateDependent<B, id>::Exists = 1, then we call
InstantiateDependent<B, id>::Do(), which instantiates the dependent
The iteration starts from Factory<T>::Initialize(), which is called by
the client (main.cpp in this example). (Maybe this could be done in
Factory::Instance(), but it's not trivial.)

Now if the client includes the headers of the dependent templates
before calling Factory<T>::Initialize(), then all this means that at
the point where the template metaprogram is instantiated, the
InstantiateDependent<Factory, id> specializations are visible.
Therefore, their Do() will be called, and the appropriate dependent
templates initialized.

The problem with this solution is that the client might think that
it's enough to include the headers of those dependent templates that
they might actually use. Say, they may think that in a certain
translation unit, Derived3 is totally impossible to be created, only
Derived1 or Derived2 (based on some runtime info, maybe read from a
file). This leads to a violation of the One Definition Rule.
Suppose we have two translation units, one of them only includes
Derived1.h, the other only Derived2.h (in addition to Factory.h). Now
the InstantiateAllDependentsI<Factory, 1 << nDependentIdBits>::Do()
function will be different in the two translation unit. In one of
them, the Next typedef will be an alias for
InstantiateAllDependentsI<Factory<int>, 1>, while in the other, it
will be an alias for InstantiateAllDependentsI<Factory<int>, 2>.
We are in undefined behavior land now. Probably most compilers will
happily compile this, but the resulting program won't do what we
expect. I played around a bit with different variations of the above
code with VC8, and while in some cases things actually worked
(somewhat surprisingly), in other cases it seemed that the compiler
correctly created the different versions of the offending function(s),
but then the linker merged them, and really just kept one of them
(practically randomly), calling that one from all places.

If the client always includes the same list of headers containing
dependent templates, then this should work. The easy way to do this
would be of course to include them from factory.h, but that's what I'd
like to avoid. Factory.h is in a library, and the client should be
able to add new class that can be created through the factory, without
modifying the library's source. One problem is that this kind of
client mistake can't be detected. Another is that it may force the
client into #include'ing headers that they might be sure not to need;
but probably there's no way around this.

(Another problem is the need to manually assign a unique int id to all
dependent templates, but that can be made somewhat better by
calculating compile-time hash values from some sort-of string id. The
best I could come up with was something like HASH(8, (D, e, r, i, v,
e, d, 1)). Still ugly, but better than having to know the ids of all
preexisting dependents.)

Uh, I may just hope that at least some of this was comprehensible.
Sorry, english is not my native language.

Honestly my guess is also that it's impossible to correctly implement,
but I'm not exactly a C++ guru, so...
I still think it's interesting, so any comments are welcome.


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