Fix Variant issue with certain Cuda versions

include/openPMD/auxiliary/Memory.hpp

@@ -22,15 +22,13 @@
 
 #include "openPMD/Dataset.hpp"
 #include "openPMD/Datatype.hpp"
+#include "openPMD/Error.hpp"
 #include "openPMD/auxiliary/UniquePtr.hpp"
 
-#include <complex>
+#include <any>
 #include <functional>
-#include <iostream>
 #include <memory>
-#include <type_traits>
 #include <utility>
-#include <variant>
 
 namespace openPMD
 {
@@ -48,30 +46,58 @@ namespace auxiliary
      */
     struct WriteBuffer
     {
-        using EligibleTypes = std::
-            variant<std::shared_ptr<void const>, UniquePtrWithLambda<void>>;
-        EligibleTypes m_buffer;
+        /*
+         * Sic. Have to put the unique_ptr behind a shared_ptr because
+         * std::variant does not want non-copyable types.
+         * Use a separate class to avoid mistakes in double dereference.
+         */
+        struct CopyableUniquePtr
+            : private std::shared_ptr<UniquePtrWithLambda<void>>
+        {
+        private:
+            using parent_t = std::shared_ptr<UniquePtrWithLambda<void>>;
 
-        WriteBuffer();
+        public:
+            CopyableUniquePtr();
+            CopyableUniquePtr(UniquePtrWithLambda<void> ptr_in);
+            auto get() -> void *;
+            [[nodiscard]] auto get() const -> void const *;
+            [[nodiscard]] auto release() -> UniquePtrWithLambda<void>;
+        };
+        using SharedPtr = std::shared_ptr<void const>;
+        /*
+         * Use std::any publically since some compilers have trouble with
+         * certain uses of std::variant, so hide it from them.
+         * Look into Memory_internal.hpp for the variant type.
+         * https://github.com/openPMD/openPMD-api/issues/1720
+         */
+        std::any m_buffer;
 
-        template <typename... Args>
-        explicit WriteBuffer(Args &&...args)
-            : m_buffer(std::forward<Args>(args)...)
-        {}
+        WriteBuffer();
+        WriteBuffer(std::shared_ptr<void const> ptr);
+        WriteBuffer(UniquePtrWithLambda<void> ptr);
 
-        WriteBuffer(WriteBuffer &&) noexcept(
-            noexcept(EligibleTypes(std::declval<EligibleTypes &&>())));
+        WriteBuffer(WriteBuffer &&) noexcept;
         WriteBuffer(WriteBuffer const &) = delete;
-        WriteBuffer &operator=(WriteBuffer &&) noexcept(noexcept(
-            std::declval<EligibleTypes &>() =
-                std::declval<EligibleTypes &&>()));
+        WriteBuffer &operator=(WriteBuffer &&) noexcept;
         WriteBuffer &operator=(WriteBuffer const &) = delete;
 
         WriteBuffer const &operator=(std::shared_ptr<void const> ptr);
-
-        WriteBuffer const &operator=(UniquePtrWithLambda<void const> ptr);
+        WriteBuffer const &operator=(UniquePtrWithLambda<void> ptr);
 
         void const *get() const;
+
+        template <typename variant_t>
+        auto as_variant() -> variant_t &
+        {
+            return *std::any_cast<variant_t>(&m_buffer);
+        }
+
+        template <typename variant_t>
+        auto as_variant() const -> variant_t const &
+        {
+            return *std::any_cast<variant_t>(&m_buffer);
+        }
     };
 } // namespace auxiliary
 } // namespace openPMD

include/openPMD/auxiliary/Memory_internal.hpp

@@ -0,0 +1,30 @@
+/* Copyright 2025 Franz Poeschel
+ *
+ * This file is part of openPMD-api.
+ *
+ * openPMD-api is free software: you can redistribute it and/or modify
+ * it under the terms of of either the GNU General Public License or
+ * the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * openPMD-api is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License and the GNU Lesser General Public License
+ * for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * and the GNU Lesser General Public License along with openPMD-api.
+ * If not, see <http://www.gnu.org/licenses/>.
+ */
+#pragma once
+
+#include "openPMD/auxiliary/Memory.hpp"
+
+namespace openPMD::auxiliary
+{
+// cannot use a unique_ptr inside a std::variant, so we represent it with this
+using WriteBufferTypes =
+    std::variant<WriteBuffer::CopyableUniquePtr, WriteBuffer::SharedPtr>;
+} // namespace openPMD::auxiliary

include/openPMD/openPMD.hpp

@@ -25,21 +25,6 @@
 namespace openPMD
 {}
 
-#if defined(CUDA_VERSION) && !defined(OPENPMD_SKIP_CHECK_ISSUE_1720)
-static_assert(__cplusplus < 202002L || CUDA_VERSION >= 12040, R"(
-Cannot use the openPMD-api in C++20 projects under a Cuda version lower
-than 12.4.0 due to a bug in the implementation of std::variant.
-Further information at:
-https://github.com/openPMD/openPMD-api/issues/1720
-https://forums.developer.nvidia.com/t/nvcc-c-20-std-variant-complie-failed/270162/5
-This cannot be fixed on our side, please either upgrade to CUDA >= 12.4.0
-or use C++17.
-If you think that this assertion is shown wrongly, please apply
-'#define OPENPMD_SKIP_CHECK_ISSUE_1720' before including
-'<openPMD/openPMD.hpp>'.
-)");
-#endif
-
 // IWYU pragma: begin_exports
 #include "openPMD/Dataset.hpp"
 #include "openPMD/Datatype.hpp"

src/IO/ADIOS/ADIOS2File.cpp

@@ -26,6 +26,8 @@
 #include "openPMD/IO/AbstractIOHandler.hpp"
 #include "openPMD/IterationEncoding.hpp"
 #include "openPMD/auxiliary/Environment.hpp"
+#include "openPMD/auxiliary/Memory.hpp"
+#include "openPMD/auxiliary/Memory_internal.hpp"
 #include "openPMD/auxiliary/StringManip.hpp"
 
 #include <cstdint>
@@ -114,22 +116,13 @@ void WriteDataset::call(ADIOS2File &ba, detail::BufferedPut &bp)
             }
             else if constexpr (std::is_same_v<
                                    ptr_type,
-                                   UniquePtrWithLambda<void>>)
+                                   auxiliary::WriteBuffer::CopyableUniquePtr>)
             {
                 BufferedUniquePtrPut bput;
                 bput.name = std::move(bp.name);
                 bput.offset = std::move(bp.param.offset);
                 bput.extent = std::move(bp.param.extent);
-                /*
-                 * Note: Moving is required here since it's a unique_ptr.
-                 * std::forward<>() would theoretically work, but it
-                 * requires the type parameter and we don't have that
-                 * inside the lambda.
-                 * (ptr_type does not work for this case).
-                 */
-                // clang-format off
-                    bput.data = std::move(arg); // NOLINT(bugprone-move-forwarding-reference)
-                // clang-format on
+                bput.data = arg.release();
                 bput.dtype = bp.param.dtype;
                 ba.m_uniquePtrPuts.push_back(std::move(bput));
             }
@@ -139,7 +132,7 @@ void WriteDataset::call(ADIOS2File &ba, detail::BufferedPut &bp)
                     always_false_v<ptr_type>, "Unhandled std::variant branch");
             }
         },
-        bp.param.data.m_buffer);
+        bp.param.data.as_variant<auxiliary::WriteBufferTypes>());
 }
 
 template <int n, typename... Params>

src/auxiliary/Memory.cpp

@@ -20,7 +20,11 @@
  */
 
 #include "openPMD/auxiliary/Memory.hpp"
+#include "openPMD/ChunkInfo.hpp"
+#include "openPMD/auxiliary/Memory_internal.hpp"
+#include "openPMD/auxiliary/UniquePtr.hpp"
 
+#include <any>
 #include <complex>
 #include <functional>
 #include <iostream>
@@ -157,24 +161,58 @@ allocatePtr(Datatype dtype, Extent const &e)
     return allocatePtr(dtype, numPoints);
 }
 
-WriteBuffer::WriteBuffer() : m_buffer(UniquePtrWithLambda<void>())
+WriteBuffer::CopyableUniquePtr::CopyableUniquePtr() = default;
+
+WriteBuffer::CopyableUniquePtr::CopyableUniquePtr(
+    UniquePtrWithLambda<void> ptr_in)
+    : parent_t{std::make_shared<UniquePtrWithLambda<void>>(std::move(ptr_in))}
 {}
 
-WriteBuffer::WriteBuffer(WriteBuffer &&) noexcept(
-    noexcept(EligibleTypes(std::declval<EligibleTypes &&>()))) = default;
-WriteBuffer &WriteBuffer::operator=(WriteBuffer &&) noexcept(noexcept(
-    std::declval<EligibleTypes &>() = std::declval<EligibleTypes &&>())) =
-    default;
+auto WriteBuffer::CopyableUniquePtr::get() -> void *
+{
+    return (**this).get();
+}
+
+auto WriteBuffer::CopyableUniquePtr::get() const -> void const *
+{
+    return (**this).get();
+}
+
+auto WriteBuffer::CopyableUniquePtr::release() -> UniquePtrWithLambda<void>
+{
+    if (parent_t::use_count() > 1)
+    {
+        throw error::Internal(
+            "Control flow error: UniquePtr variant of WriteBuffer "
+            "has been copied.");
+    }
+    UniquePtrWithLambda<void> res = std::move(**this);
+    this->reset();
+    return res;
+}
+
+WriteBuffer::WriteBuffer() : m_buffer(std::make_any<CopyableUniquePtr>())
+{}
+WriteBuffer::WriteBuffer(std::shared_ptr<void const> ptr)
+    : m_buffer(std::make_any<WriteBufferTypes>(std::move(ptr)))
+{}
+WriteBuffer::WriteBuffer(UniquePtrWithLambda<void> ptr)
+    : m_buffer(
+          std::make_any<WriteBufferTypes>(CopyableUniquePtr(std::move(ptr))))
+{}
+
+WriteBuffer::WriteBuffer(WriteBuffer &&) noexcept = default;
+WriteBuffer &WriteBuffer::operator=(WriteBuffer &&) noexcept = default;
 
 WriteBuffer const &WriteBuffer::operator=(std::shared_ptr<void const> ptr)
 {
-    m_buffer = std::move(ptr);
+    m_buffer = std::make_any<WriteBufferTypes>(std::move(ptr));
     return *this;
 }
-
-WriteBuffer const &WriteBuffer::operator=(UniquePtrWithLambda<void const> ptr)
+WriteBuffer const &WriteBuffer::operator=(UniquePtrWithLambda<void> ptr)
 {
-    m_buffer = std::move(ptr);
+    m_buffer =
+        std::make_any<WriteBufferTypes>(CopyableUniquePtr(std::move(ptr)));
     return *this;
 }
 
@@ -186,6 +224,6 @@ void const *WriteBuffer::get() const
             // we're being sneaky and don't distinguish the types here
             return static_cast<void const *>(arg.get());
         },
-        m_buffer);
+        as_variant<WriteBufferTypes>());
 }
 } // namespace openPMD::auxiliary

test/CoreTest.cpp

@@ -11,7 +11,7 @@
 
 #include "openPMD/IO/ADIOS/macros.hpp"
 #include "openPMD/auxiliary/Filesystem.hpp"
-#include "openPMD/auxiliary/JSON.hpp"
+#include "openPMD/auxiliary/Memory_internal.hpp"
 #include "openPMD/auxiliary/UniquePtr.hpp"
 
 #include <catch2/catch.hpp>
@@ -1252,7 +1252,7 @@ TEST_CASE("use_count_test", "[core]")
         std::get<std::shared_ptr<void const>>(
             static_cast<Parameter<Operation::WRITE_DATASET> *>(
                 pprc.get().m_chunks.front().parameter.get())
-                ->data.m_buffer)
+                ->data.as_variant<auxiliary::WriteBufferTypes>())
             .use_count() == 1);
 #endif
 }