Allow assigning values or virtual records to a One on construction

examples/alpaka/asyncblur/asyncblur.cpp

@@ -128,8 +128,7 @@ struct BlurKernel
             LLAMA_INDEPENDENT_DATA
         for (auto x = start[1]; x < end[1]; ++x)
         {
-            llama::One<PixelOnAcc> sum;
-            sum = 0;
+            llama::One<PixelOnAcc> sum = 0;
 
             using ItType = std::int64_t;
             const ItType iBStart = SHARED ? ItType(y) - ItType(bi[0] * ElemsPerBlock) : y;

examples/cuda/nbody/nbody.cu

@@ -110,8 +110,7 @@ __global__ void updateSM(View particles)
     const auto ti = threadIdx.x + blockIdx.x * blockDim.x;
     const auto tbi = blockIdx.x;
 
-    llama::One<Particle> pi;
-    pi = particles(ti);
+    llama::One<Particle> pi = particles(ti);
     for (std::size_t blockOffset = 0; blockOffset < PROBLEM_SIZE; blockOffset += SHARED_ELEMENTS_PER_BLOCK)
     {
         LLAMA_INDEPENDENT_DATA
@@ -132,8 +131,7 @@ __global__ void update(View particles)
 {
     const auto ti = threadIdx.x + blockIdx.x * blockDim.x;
 
-    llama::One<Particle> pi;
-    pi = particles(ti);
+    llama::One<Particle> pi = particles(ti);
     LLAMA_INDEPENDENT_DATA
     for (auto j = std::size_t{0}; j < PROBLEM_SIZE; ++j)
         pPInteraction(pi, particles(j));

examples/nbody/nbody.cpp

@@ -82,8 +82,7 @@ namespace usellama
         LLAMA_INDEPENDENT_DATA
         for (std::size_t i = 0; i < PROBLEM_SIZE; i++)
         {
-            llama::One<Particle> pi;
-            pi = particles(i);
+            llama::One<Particle> pi = particles(i);
             for (std::size_t j = 0; j < PROBLEM_SIZE; ++j)
                 pPInteraction(pi, particles(j));
             particles(i)(tag::Vel{}) = pi(tag::Vel{});

include/llama/VirtualRecord.hpp

@@ -309,12 +309,13 @@ namespace llama
         /// AccessibleRecordDim is the same as `Mapping::RecordDim`.
         using AccessibleRecordDim = GetType<RecordDim, BoundRecordCoord>;
 
+        /// Creates an empty VirtualRecord. Only available for if the view is owned. Used by llama::One.
         LLAMA_FN_HOST_ACC_INLINE VirtualRecord()
             /* requires(OwnView) */
             : arrayDimsCoord({})
             , view{allocViewStack<1, RecordDim>()}
         {
-            static_assert(OwnView, "The default constructor of VirtualRecord is only available if the ");
+            static_assert(OwnView, "The default constructor of VirtualRecord is only available if it owns the view.");
         }
 
         LLAMA_FN_HOST_ACC_INLINE
@@ -327,6 +328,20 @@ namespace llama
         VirtualRecord(const VirtualRecord&) = default;
         VirtualRecord(VirtualRecord&&) = default;
 
+        /// Create a VirtuaRecord from a single value or a different VirtualRecord. Only available for if the view is
+        /// owned. Used by llama::One.
+        template <typename T>
+        LLAMA_FN_HOST_ACC_INLINE VirtualRecord(const T& other)
+            /* requires(OwnView) */
+            : VirtualRecord()
+        {
+            static_assert(
+                OwnView,
+                "The copy constructor of VirtualRecord from a different VirtualRecord is only available if it owns the "
+                "view.");
+            *this = other;
+        }
+
         /// Access a record in the record dimension underneath the current virtual
         /// record using a \ref RecordCoord. If the access resolves to a leaf, a
         /// reference to a variable inside the \ref View storage is returned,

tests/virtualrecord.cpp

@@ -890,3 +890,22 @@ TEST_CASE("VirtualRecord.loadAs.constref")
         CHECK(pos.y == 1);
     }
 }
+
+TEST_CASE("VirtualRecord.One_ctor_from_view")
+{
+    auto view = llama::allocView(llama::mapping::AoS{llama::ArrayDims{5}, Name{}});
+    view(1u) = 1;
+
+    llama::One<Name> vr2 = view(1u);
+    llama::One<Name> vr3(view(1u));
+    llama::One<Name> vr4{view(1u)};
+
+    vr2 = 2;
+    vr3 = 3;
+    vr4 = 4;
+
+    CHECK(vr2 == 2);
+    CHECK(vr3 == 3);
+    CHECK(vr4 == 4);
+    CHECK(view(1u) == 1);
+}