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Function Template Specialization

Started by Tang of the Mountain Mar 21, 2009 at 12:04 AM 4 replies 1.1k views

Original Post

Mar 21, 2009 at 12:04 AM

(It's late, I thought I clicked in general programming, my apologies) Ok, I have this cluster of inherited objects. Base A derives from base B,C,D derive from A Classes B, C, D's ctors have the same params. A is the outcast with one extra param. I am developing a templated object manager which will create any of the given types. The problem is the specialization needed for A. If I do the following inside the manager:


...
template <typename T>
T* Create(const st::string & name)...}

template <>
A* Create(const std::string name, int id){...} 
//fails but ideally what im trying to accomplish

template <>
A* Create(const std::string name)
{...} //would work but useless for my purpose
...

Well, I can't. Because its not really a specialization of a template. Its a new function. But I want the functionality of this function to work solely for A. I don't want to have to overload Create and always return NULL then create a specialization of the overloaded Create for type A to work normally. However, that's what I'm thinking. Any ideas? I could change the ctor of A but I'm trying to avoid that for now.

Deyja

125

Deyja

Mar 21, 2009 at 4:04 AM

The first attempt is not a specialization of the template, it is an entirely new set of templates/specializations. You'll need to create a template for it to specialize first, perhaps one that fails to compile so it will only work with A.

Granted that extra parameter entirely defeats the purpose of using templates at all..

Metsan

Mar 21, 2009 at 4:18 AM

Also, this thread may be of interest to you, as several approaches for the design and implementation of generic factories are referenced and/or discussed in it.

[Edited by - Metsan on March 21, 2009 6:18:34 AM]

loufoque

Mar 21, 2009 at 7:27 AM

If I understand what you're saying, you want to overload a function, but make that overload only visible if the template argument is A.

This is exactly what SFINAE does.

Antheus

216

Antheus

Mar 21, 2009 at 9:07 AM

Would this work:

Base* Create(const std::string & name, int id = 0)...} {}

Quote:
Any ideas? I could change the ctor of A but I'm trying to avoid that for now.

This is generally a better idea.

But instead of changing constructor, consider changing the way they are used. If there's one exception, there's likely to be more later on.

When constructing objects from a factory, I prefer to pass configuration parameters to it. Something like:

Base * Create( std::map<std::string, std::string> &params);

Then, each instance queries the map for parameters it needs. This supports future extensions and customizations without breaking the API. Gotcha: parameter pairs need to be documented in a visible place, or using such code becomes cumbersome. It's also better to derive parameter class for each class to provide named functions for setting the values.

Zahlman

257

Zahlman

Mar 22, 2009 at 8:25 PM

1) Is it really appropriate for client code to Create A instances directly instead of a derived instance?

2) Instead of specializing the template, you might just override it?

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