Single-header C++ object-factory library
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ce1c6950e9d5 — Chris Cannam 2 months ago
Add references to other bqlibs; formatting
7686116dcdd5 — Chris Cannam 3 years ago
Remove some debug by default

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#bqthingfactory

A single-header C++ library providing a factory for objects from classes that share a common base class, have identical single-argument constructors, can be identified by URI, and that can register their existence with the factory, so that the factory does not have to know in advance about all buildable classes.

C++ standard required: C++98 (does not use C++11)

Copyright 2007-2020 Particular Programs Ltd. Under a permissive BSD/MIT-style licence: see the file COPYING for details.

#How to use these classes

Given a base class A with many subclasses B, C, D, etc, all of which need to be passed parameters class P in their constructor:

  • in a header associated with A,

    • Create a template class ABuilder<T> which inherits from ConcreteThingBuilder<T, A, P>. This is your class which will be specialised to provide a builder for each subclass of A. Its constructor must accept a std::string containing the URI that identifies the class of object being built, which is passed to the parent class's constructor. Optionally, it may also accept and pass to the parent class a vector<string> of "tags", which are strings used to identify the sorts of facility this builder's object supports -- for example, file extensions or MIME types that the object can parse. If two builders register support for the same tag, only the first to register will be used (note that which one this is may depend on static object construction ordering, so it's generally better if tags are unique to a builder).

    • You may also wish to typedef ThingFactory<A, P> to something like AFactory, for convenience.

  • in a .cpp file associated with each of B, C, D etc,

    • Define a static variable of class ABuilder<B>, ABuilder<C>, ABuilder<D>, etc, passing the class's identifying URI and optional supported tag list to its constructor. (If you like, this could be a static member of some class.)

You can then do the following:

  • call AFactory::getInstance()->getURIs() to retrieve a list of all registered URIs for this factory.

  • call AFactory::getInstance()->create(uri, parameters), where parameters is an object of type P, to construct a new object whose class is that associated with the URI uri.

  • call AFactory::getInstance()->getTags() to retrieve a list of all tags known to be supported by some builder. Remember that builders do not have to support any tags and many builders could support the same tag, so you cannot retrieve all builders by starting from the tags list: use getURIs for that.

  • call AFactory::getInstance()->getURIFor(tag), where tag is a std::string corresponding to one of the tags supported by some builder, to obtain the URI of the first builder to have registered its support for the given tag.

  • call AFactory::getInstance()->createFor(tag, parameters), where tag is a std::string corresponding to one of the tags supported by some builder and parameters is an object of type P, to construct a new object whose class is that built by the first builder to have registered its support for the given tag.