Injection

Automatic Component Toolkit

Example: Component injection and shared ownership

This example demonstrates the usage of the importcomponent-element in the reference counting mechanism provided by ACT.

The Numbers-component provides a class Variable that can hold a numerical value. The Calculation-component provides a class Calculator that can sum or multiply instances of class Variable.

How to run the example:

1. Run ACT for Calculation.xml (This repo already contains autogenerated sources and a working implementation). ACT automatically also generates the Numbers-component from Numbers.xml.
2. Build one of the implementations:
   1. C++
   2. Pascal
3. Build and run one of the following examples
   1. CppDynamic
   2. Cpp
   3. Pascal
   4. Python

How does component-injection work:

1. To be injected, the Numbers-component must specify a symbollookupmethod in its component definition-file. The symbollookupmethod provides the addresses of all functions exported by the Numbers-component and is generated and implemented automatically by ACT.

2. The definition of the Numbers-component is injected into the component Calculator via the importcomponent in the Calculation.xml-file. To allow injecting any of the imported components into the Calculation-component, it must provide a injectionmethod. The injectionmethod takes the symbollookupmethod of the injected componenent (Numbers) to create a language binding of that component inside the outer component (Calculation) at runtime.

3. The Numbers-component is injected into the Calculation-component at runtime via an explicit call of calculationWrapper->InjectComponent("Numbers", numbersWrapper->GetSymbolLookupMethod()); (analogously in other languages).

How does reference-counting/shared ownership work in ACT:

1. Reference-counting for all classes is now (v1.6.0) the default in ACT. It is implemented in the base-class by

   • an integer member m_nReferenceCount within the base-class of a component and methods
   • IncRefCount which increases the reference count of an instance.
   • DecRefCount which decreases the reference count and deletes the instance, if it has reached zero.

   In addition:

   • The releasemethod now (v1.6.0) does not simple delete/free an instance of a class, but instead calls DecRefCount.
   • All components must define an acquiremethod in the global section of their xml. Its autogenerated implementation calls IncRefCount.
   • In order to use shared-pointers of instances of ACT-classes (C++ only, not Pascal), only use the autogenerated shared_ptr<>-classes $NameSpace::PI$ClassName. They have a custom deletor that correctly calls DecRefCount.

2. A Calculator instance can enlist a Variable-instance. The implementation of the enlist-method increases the reference-count of the enlisted Variable-instance, i.e. it acquires shared ownership of it. Thus, one can release the ownership of this Variable-instance on the user-code side.

3. One can retrieve a Variable-instance that is enlisted in a Calculator. This also increases the reference-count of this instance.

4. Reference-counting can be disabled by modifing the implemetation of a component's base class:

   • Remove the m_nReferenceCount-member of the base class.
   • Modify the IncRefCount-method to do nothing.
   • Mpdify the DecRefCount-method such that it always deletes the instance.