Re: Generic factory method/abstract factory: Forwarding function signatures?

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <>
Tue, 21 Jun 2011 18:18:18 CST
Am 20.06.2011 21:15, schrieb Arne Mertz:

On 17 Jun., 09:50, Daniel Kr?gler<>


I guess I was not very clear in my question. Of course I know that the
constructor is too late. My idea was something like the following:

template<class ManagementPolicy, class DefaultCreator, class
DefaultDestructor> //Note: no explicit Signature parameter
class GenericFactory
    typedef SINGATURE_OF(DefaultCreator) Signature; //here it is
    /* ... */

So the question was: is there some way to implement the "SIGNATURE_OF"

Let me first start with the remark that your description misses some
relevant details, e.g. you never explain what kind of types
"CreationSignature" or "DefaultDestructor" are supposed to be. I can
only assume, that these are assumed to be a function type. In the
following I assume that they are function types because you are feeding
boost::function with it.

The problem is, that there does not exists a *single* type that
represents a function signature. Of course you can use some
"signature-compression" form, e.g. by using a single function type as
the representation of the parameter types, but you cannot "unpack" them
within a primary template like GenericFactory to declare a function from

It is possible to realize this with a large amount of work in C++03 by
partially specializing GenericFactory, e.g.

template<class ManagementPolicy, class DefaultCreator, class
struct GenericFactory; // Not defined

template<class ManagementPolicy, class DefaultCreator, class R>
struct GenericFactory<ManagementPolicy, DefaultCreator, R()> {

   R Create();

template<class ManagementPolicy, class DefaultCreator, class R, class Arg>
struct GenericFactory<R(Arg)> {

   R Create(Arg);

Whether this a reasonable approach depends on the number of parameters
you need to support. This is the reason, why most C++03 implementations
instead add a series of overloads within the class.

In C++0x this is solvable by defining a single partial specialization
due to variadics, but as you say, you cannot use this approach for the
moment due to compiler limitations.

Alternatively you could impose some constraints on the caller, e.g. you
could require that the argument is expressed as a single tuple (this
works with tr1::tuple and boost::tuple), in this case you would just write:

template<class FunctionType>
struct Signature; // Not defined

template<class R>
struct Signature<R()> {
   typedef R return_type;
   typedef tuple<> arguments;

template<class R, class Arg>
struct Signature<R(Arg)> {
   typedef R return_type;
   typedef tuple<Arg> arguments;

// etc...

template<class ManagementPolicy, class DefaultCreator, class
struct GenericFactory {
    typedef Signature<DefaultDestructor> sig;
    typename sig::return_type Create(typename sig::arguments);

This approach is similar to what the /piecewise construction/ idiom in
the C++0x std::pair template provides but as mentioned above it means
that callers have to obey this special protocol.

HTH & Greetings from Bremen,

Daniel Kr?gler

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