ENH: extend VectorSpace traits to include pTraits_cmptType

- The pTraits_cmptType returns the data type of 'cmptType' (for
  arithmetic and VectorSpace types) or is simply a pass-through.

  This can be combined with the pTraits_nComponents for casting.
  For example,

  function
  (
      reinterpret_cast<pTraits_cmptType<Type>::type*>(buf.data()),
      (buf.size()/pTraits_nComponents<Type>::value)
  );

ENH: extend Foam::identityOp so support array indexing (pass-through)

applications/test/UList/Test-UList.C

@@ -115,7 +115,22 @@ int main(int argc, char *argv[])
         Info<<"UList: "; print(ulist);
     }
 
+    {
+        Foam::identityOp ident;
+
+        Info<< nl << "identityOp as functor or array/map" << nl;
+
+        for (const label val : labelRange(5))
+        {
+            Info<< "value:" << val
+                << " () = " << ident(val)
+                << " [] = " << ident[val] << nl;
+        }
+    }
+
+    Info<< "\nEnd\n" << nl;
     return 0;
 }
 
+
 // ************************************************************************* //

applications/test/pTraits/Test-pTraits.C

@@ -34,6 +34,7 @@ Description
 #include "boolVector.H"  // A FixedList pretending to be a vector
 #include "vector.H"
 #include "tensor.H"
+#include "complex.H"
 #include "uLabel.H"
 #include "Switch.H"
 
@@ -110,6 +111,7 @@ void printTraits()
         << " vector-space=" << Switch::name(is_vectorspace<T>::value)
         << " is_label=" << Switch::name(is_contiguous_label<T>::value)
         << " is_scalar=" << Switch::name(is_contiguous_scalar<T>::value)
+        << " cmptType=" << typeid(typename pTraits_cmptType<T>::type).name()
         << endl;
 }
 
@@ -120,6 +122,12 @@ void printTraits(const pTraits<T>& p)
     Info<< p.typeName << " == " << p << endl;
 }
 
+template<class T>
+void printDecltype()
+{
+    Info<< "cmptType : " << typeid(T).name() << nl;
+}
+
 
 #pragma GCC diagnostic warning "-Wmaybe-uninitialized"
 #pragma GCC diagnostic warning "-Wuninitialized"
@@ -129,12 +137,16 @@ int main()
     printTraits<bool>();
     printTraits<label>();
     printTraits<scalar>();
-    printTraits<vector>();
+    printTraits<complex>();     // Uses specialized pTraits_...
+    printTraits<floatVector>();
+    printTraits<doubleVector>();
     printTraits<tensor>();
-    printTraits<boolVector>();
+    printTraits<boolVector>();  // Uses specialized pTraits_...
     printTraits<word>();
     printTraits<std::string>();
 
+    Info<< nl;
+
     {
         pTraits<bool> b(true);
         printTraits(b);
@@ -147,6 +159,8 @@ int main()
 
     printTraits(pTraits<scalar>(3.14159));
 
+    Info<< nl;
+
     label abc;
     Info<< "uninitialized primitive:"<< abc << endl;
 

src/OpenFOAM/include/stdFoam.H

@@ -94,6 +94,13 @@ struct identityOp
     {
         return std::forward<T>(val);
     }
+
+    // Allow use as an identity array/map
+    template<class T>
+    constexpr T&& operator[](T&& val) const noexcept
+    {
+        return std::forward<T>(val);
+    }
 };
 
 
@@ -129,10 +136,10 @@ void Swap(T (&a)[N], T (&b)[N])
 //- or that are in a state of change.
 //
 // SeeAlso
-//     - http://en.cppreference.com/w/cpp/iterator/begin
-//     - http://en.cppreference.com/w/cpp/iterator/end
-//     - http://en.cppreference.com/w/cpp/iterator/rbegin
-//     - http://en.cppreference.com/w/cpp/iterator/rend
+//     - https://en.cppreference.com/w/cpp/iterator/begin
+//     - https://en.cppreference.com/w/cpp/iterator/end
+//     - https://en.cppreference.com/w/cpp/iterator/rbegin
+//     - https://en.cppreference.com/w/cpp/iterator/rend
 
 namespace stdFoam
 {

src/OpenFOAM/primitives/Vector/bools/boolVector.H

@@ -60,6 +60,12 @@ class boolVector
 {
 public:
 
+    // Typedefs
+
+        //- The component type is bool
+        typedef bool cmptType;
+
+
     // Member Constants
 
         //- Rank of a vector is 1
@@ -176,6 +182,22 @@ public:
 template<> struct is_contiguous<boolVector> : std::true_type {};
 
 
+/*---------------------------------------------------------------------------*\
+                      Specialization pTraits<boolVector>
+\*---------------------------------------------------------------------------*/
+
+//- A boolVector has bool data components.
+template<>
+struct pTraits_cmptType<boolVector> { typedef bool type; };
+
+//- A boolVector has three data components
+template<>
+struct pTraits_nComponents<boolVector>
+:
+    std::integral_constant<Foam::direction, 3>
+{};
+
+
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
 } // End namespace Foam

src/OpenFOAM/primitives/complex/complex.H

@@ -226,6 +226,20 @@ public:
                         Specialization pTraits<complex>
 \*---------------------------------------------------------------------------*/
 
+//- The underlying component data type for complex is scalar.
+//  The regular pTraits<T>:cmptType as complex is currently (2023-11)
+//  likely not quite correct (issue #3018)
+template<>
+struct pTraits_cmptType<complex> { typedef scalar type; };
+
+//- A complex has two scalar components
+template<>
+struct pTraits_nComponents<complex>
+:
+    std::integral_constant<Foam::direction, 2>
+{};
+
+
 //- Template specialisation for pTraits<complex>
 template<>
 class pTraits<complex>
@@ -274,7 +288,7 @@ public:
     // Constructors
 
         //- Copy construct from primitive
-        explicit pTraits(const complex& val)
+        explicit pTraits(const complex& val) noexcept
         :
             p_(val)
         {}

src/OpenFOAM/primitives/traits/pTraits.H

@@ -52,7 +52,7 @@ namespace stdFoam
 template<class... >
 using void_t = void;
 
-} // End namespace Foam
+} // End namespace stdFoam
 
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
@@ -131,6 +131,21 @@ struct pTraits_rank
 {};
 
 
+//- The underlying component data type: default is pass-through.
+template<class T, class = void>
+struct pTraits_cmptType { typedef T type; };
+
+//- The underlying component data type for vector-space (or complex).
+//  Enabled when pTraits<T>:zero has a defined type (ie, exists),
+//  such as for arithmetic primitives, vector-space etc. where the concept
+//  of a component type also makes sense.
+template<class T>
+struct pTraits_cmptType<T, stdFoam::void_t<decltype(pTraits<T>::zero)>>
+{
+    typedef typename pTraits<T>::cmptType type;
+};
+
+
 //- The vector-space number of components: default is 1.
 template<class T, class = void>
 struct pTraits_nComponents : std::integral_constant<Foam::direction, 1> {};