ENH: add unary functor for Foam::zero

- acts somewhat like an identity op that 'swallows' its parameter
  and maps back to zero.

applications/test/predicates/Test-predicates.C

@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2017-2021 OpenCFD Ltd.
+    Copyright (C) 2017-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -81,32 +81,35 @@ int main(int argc, char *argv[])
 
     labelList values(identity(40, -10));
 
-    Info<<"words:  " << flatOutput(words) << endl;
-    Info<<"values: " << flatOutput(values)  << endl;
+    Info<< "words:  " << flatOutput(words) << endl;
+    Info<< "values: " << flatOutput(values)  << endl;
 
     regExp matcher(".*_.*");
 
-    Info<<"With '_': ";
+    Info<< "With '_': ";
     printMatching(words, matcher);
 
-    Info<<"All: ";
+    Info<< "All: ";
     printMatching(words, predicates::always());
 
-    Info<<"None: ";
+    Info<< "None: ";
     printMatching(words, predicates::never());
 
-    Info<<"Neg values: ";
+    Info<< "Neg values: ";
     printMatching(values, [](const label v) { return v < 0; });
 
-    Info<<"Even values: ";
+    Info<< "Even values: ";
     printMatching(values, [](const label v) { return !(v % 2); });
 
-    Info<<"All: ";
+    Info<< "All: ";
     printMatching(values, predicates::always());
 
-    Info<<"None: ";
+    Info<< "None: ";
     printMatching(values, predicates::never());
 
+    Info<< "zero:";
+    Info<< static_cast<label>(Foam::zero{}(values)) << nl;
+
     return 0;
 }
 

src/OpenFOAM/primitives/traits/one.H

@@ -73,28 +73,16 @@ public:
 
 
     //- Return 1 for label
-    constexpr operator label() const noexcept
-    {
-        return 1;
-    }
+    constexpr operator label() const noexcept { return 1; }
 
     //- Return 1 for float
-    constexpr operator float() const noexcept
-    {
-        return 1;
-    }
+    constexpr operator float() const noexcept { return 1; }
 
     //- Return 1 for double
-    constexpr operator double() const noexcept
-    {
-        return 1;
-    }
+    constexpr operator double() const noexcept { return 1; }
 
     //- Component-wise or element-wise access returns one
-    one operator[](const label) const noexcept
-    {
-        return one{};
-    }
+    one operator[](const label) const noexcept { return one{}; }
 };
 
 

src/OpenFOAM/primitives/traits/zero.H

@@ -77,38 +77,30 @@ public:
 
 
     //- Return false (0) for bool
-    constexpr operator bool() const noexcept
-    {
-        return false;
-    }
+    constexpr operator bool() const noexcept { return false; }
 
     //- Return 0 for label
-    constexpr operator label() const noexcept
-    {
-        return 0;
-    }
+    constexpr operator label() const noexcept { return 0; }
 
     //- Return 0 for float
-    constexpr operator float() const noexcept
-    {
-        return 0;
-    }
+    constexpr operator float() const noexcept { return 0; }
 
     //- Return 0 for double
-    constexpr operator double() const noexcept
-    {
-        return 0;
-    }
+    constexpr operator double() const noexcept { return 0; }
 
     //- Component-wise or element-wise access returns zero
-    zero operator[](const label) const noexcept
-    {
-        return zero{};
-    }
+    zero operator[](const label) const noexcept { return zero{}; }
 
     // FUTURE?: Swallow assignment (as per std::ignore)
     // template<class T>
     // constexpr const zero& operator=(const T&) const { return *this; }
+
+    //- Unary functor returns zero
+    template<class T>
+    constexpr zero operator()(const T&) const noexcept
+    {
+        return zero{};
+    }
 };