Blueprints store a minimal ruleset for instantiating or configuring specialised representations of a class such as a uniformly filled array, or the unit vectors of each dimension. This can of course, be done inside a class, but via the blueprints, we introduce an extra layer of indirection that helps get around two issues.
Overloading the constructor can become difficult when you cannot find a unique signature for one of many constructor calls. For example,
Traditionally, the way around it would be to define a static function within the class. However, this runs into difficulty when incorporating a 3rd party library - you cannot add static functions to pre-existing classes. This is often very desirable if you want to create your own pre-defined instantiations of things like matrices. So in these cases, an externally applied method is necessary.
Even when there are static methods available to generate specialised instances, operator assignment will usually result in a construction followed by a full copy, which is expensive.
By generating a minimal blueprint and then passing to a class's assignment operator, or forcing the blueprint to generate an instance externally, we resolve the two problems above. In some cases we also get a speedup if we need to repeatedly use a blueprint, in sofar as the blueprint can save and store the state required to generate the class from the blueprint.
Include the following at the top of any translation unit for which you are running compile time checks on your blueprint classes.
Since it only uses macros and templatised objects, no linking is required for just this feature.
Creating a blueprint class can be done in any namespace, it just has to satisfy the three conditions outlined below:
A simple example for a blueprint related to the Foo class:
That, of course, was complicating a redundantly trivial situation. A better, practicial example is the ConstantArray blueprint for array containers in ecl_containers.
While its possible to simply use the blueprint class in the following manner (in this example, ConstrantArray is a blueprint):
It's not very convenient. I usually wrap them in a factory class which possesses static methods that will either result in instantiations or blueprints. A direct implementation of such a factory might like like this:
This is about as far as you can go with a factory class working for a 3rd party class that you cannot modify. If however, you can modify the base_type class, then it is possible to insert a constructor and assignment operator to directly handle blueprints, e.g. in the Array container class (ecl_containers), we have the following declarations:
Then we can handle both construction and assignment equivalently:
Furthermore, if our base class inherits the factory, then we can simplify this further:
These are the relevant components for array type blueprints which can be found in ecl_containers.