ENH: add low-level polymorphic/boxed exprValue

- primarily for handling expression results,
  but can also be used as a universal value holder.

  Has some characteristics suitable for type-less IO:
  eg, is_integral(), nComponents()

ENH: add is_pointer() check for expression scanToken

applications/test/exprValue/Test-exprValue.C

@@ -0,0 +1,141 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2021-2023 OpenCFD Ltd.
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM, distributed under GPL-3.0-or-later.
+
+Application
+    Test-exprValue
+
+Description
+    Test low-level polymorphic value container (exprValue)
+
+\*---------------------------------------------------------------------------*/
+
+#include "argList.H"
+#include "IOstreams.H"
+#include "ITstream.H"
+#include "exprValue.H"
+
+using namespace Foam;
+
+void printInfo(const expressions::exprValue& val)
+{
+    Info<< "Boxed type:" << int(val.typeCode())
+        << " (" << val.valueTypeName() << ") good:"
+        << val.good() << " => " << val << nl;
+}
+
+
+expressions::exprValue tryParse(const std::string& str)
+{
+    expressions::exprValue val, val2;
+
+    ITstream is(str);
+
+    const bool ok = val.read(is);
+
+    Info<< "read " << Foam::name(val.typeCode()) << " from " << str;
+
+    if (ok)
+    {
+        Info<< " trailing tokens:" << is.nRemainingTokens() << nl
+            << "value: " << val << nl;
+    }
+    else
+    {
+        Info<< " FAILED" << nl;
+    }
+
+    if (ok)
+    {
+        Info<< "Direct from string: ";
+        if (expressions::exprValue::read(str, val2))
+        {
+            Info<< "good" << nl;
+        }
+        else
+        {
+            Info<< "bad" << nl;
+        }
+    }
+    return val;
+}
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+int main(int argc, char *argv[])
+{
+    argList::noBanner();
+    argList::noParallel();
+
+    #include "setRootCase.H"
+
+    // Aborts
+    // expressions::exprValue value(std::string(""));
+
+    {
+        expressions::exprValue value;
+
+        // Nothing
+        printInfo(value);
+
+        value.set(scalar(100));
+        printInfo(value);
+
+        value.set(vector(1,2,3));
+        printInfo(value);
+
+        value = vector(4,5,6);
+        printInfo(value);
+
+        value = Zero;
+        printInfo(value);
+
+        value.clear();
+        printInfo(value);
+
+        value = 100 * vector(1,0,0);
+        printInfo(value);
+    }
+
+
+    {
+        Info<< nl << "Test parsing" << nl << nl;
+
+        for
+        (
+            const auto& input :
+            stringList
+            ({
+                "()",  // bad
+                "( 1 2  ",  // also bad
+                "(  ",  // really bad
+                "(1 16 12)",
+                "(1 bad)",
+                "(5)",
+                "1.2345",
+                "5.678 trailing",
+                "true",
+                "false",
+                " 1 ",
+                "  yes no "
+            })
+        )
+        {
+            (void) tryParse(input);
+        }
+    }
+
+    return 0;
+}
+
+
+// ************************************************************************* //