coding/patterns/registry.md

+<!-- --- title: OpenFOAM C++ Coding Patterns (registry) -->
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+
+## Coding Patterns - objectRegistry
+
+The `objectRegistry` in OpenFOAM behaves much like a global collection
+of `regIOobject` items (and things derived from `regIOobject` such as
+IODictionary, IOField, volScalarField, etc). The items within the
+collection are the so-called *registered* entries.
+
+The items known by the `objectRegistry` can either have their storage
+within the registry or outside of the registry.
+
+
+[[_TOC_]]
+
+### Maintaining a cached value
+
+This is a common use. A derived quantity is calculated within a
+function object or within a local scope with the following requirements:
+
+1. item should be persistent (available) for other functions
+2. reuse or create the item as required.
+
+```
+// Retrieve existing (if any)
+auto* fieldPtr = mesh.getObjectPtr<volScalarField>(resultName_);
+
+if (!fieldPtr)
+{
+    // It does not exist, create a new one. Note use of raw pointer.
+    fieldPtr = new volScalarField
+    (
+        IOobject
+        (
+            resultName_,
+            mesh.time().timeName(),
+            mesh.thisDb(),
+            IOobject::NO_READ,
+            IOobject::NO_WRITE
+            // implicit:  IOobject::REGISTER
+        ),
+        mesh,
+        dimensionedScalar(dimless, Zero)
+        // implicit:  calculatedType()
+    );
+
+    // Pass pointer ownership to the registry
+    mesh.objectRegistry::store(fieldPtr);
+}
+
+// The field
+auto* result = *fieldPtr;
+
+// Perform some calculations...
+```
+
+----
+
+Copyright (C) 2023 OpenCFD Ltd.
+
+[code-patterns]: /coding/patterns/patterns