serialize std::variant

[leekillough]

This adds serialization of std::variant.

[github-actions]

CMAKE-FORMAT TEST - PASSED

[github-actions]

CLANG-FORMAT TEST - FAILED (on last commit):
Run > ./scripts/clang-format-test.sh using clang-format v20 to check formatting

[github-actions]

CLANG-FORMAT TEST - PASSED

[github-actions]

CMAKE-FORMAT TEST - PASSED

[sst-autotester]

Status Flag 'Pre-Test Inspection' - - This Pull Request Requires Inspection... The code must be inspected by a member of the Team before Testing/Merging
NO INSPECTION HAS BEEN PERFORMED ON THIS PULL REQUEST! - This PR must be inspected by setting label 'AT: PRE-TEST INSPECTED'.

[leekillough]

template <typename... Types>
template <size_t... INDEX>
constexpr void (*serialize_impl<std::variant<Types...>>::set_index<std::index_sequence<INDEX...>>[])(
    std::variant<Types...>&) { [](std::variant<Types...>& obj) { obj.template emplace<INDEX>(); }... };

In case the code is too hard to understand, here's a dissection:

template <typename... Types>

The outer template parameter list.

template <size_t... INDEX>

The inner template parameter list.

constexpr

The constexpr applies to the set_index variable template being defined here, which also means that, as a constexpr static member variable of serialize_impl<std::variant<Types...>>, set_index is inline, and so it can be defined in a header file without causing ODR violations (multiply-defined symbols).

 void (*                                                                                [])( std::variant<Types...>&)
        serialize_impl<std::variant<Types...>>::set_index<std::index_sequence<INDEX...>> 

A partial specialization of the set_index member variable template of serialize_impl<std::variant<Types...> which matches std::index_sequence<INDEX...>, which is a list of 0, 1, 2, ..., sizeof...(Types)-1.

This specialization is declared as an unspecified-size array of pointers to functions taking std::variant<Types...>& and returning void.

 { 
   [](std::variant<Types...>& obj) { obj.template emplace<INDEX>(); }
   ... 
 };

An initialization list for the unspecified-size array of function pointers, which gets expanded for each element of the INDEX list of subscripts.

Each element of the list is a lambda expression with no captures which takes a std::variant<Types...>& and (implicitly auto) returns void. Because there are no captures, the lambda expression is implicitly convertible to the function pointer type of the array elements.

The lambda function calls the emplace() method on the obj argument with the template argument INDEX, which is the index corresponding to the current function pointer array element in the initialization list.

Passing the emplace() method a template argument of INDEX causes the Types... type corresponding to INDEX to become the variant's new type.

The emplace() argument list is empty, so it default-initializes the new variant type.

[feldergast]

Thanks for the comments. I mostly understand what's going on and am willing to take the rest on faith. Can you add those comments to the appropriate places in the code so that we have it in the future when someone needs to understand the code.

[github-actions]

CLANG-FORMAT TEST - PASSED

[github-actions]

CMAKE-FORMAT TEST - PASSED

[leekillough]

I have added a long comment at the end of the file explaining what the table set_index[] does and the motivation for it, e.g.

Why not just change the active variant by setting it to an index which changes at runtime?

Because changing the active variant requires that the new variant index be known at compile time, so a table of each possible new variant is needed, and this table can be dispatched with a runtime new value of index.

I have also reformatted the code in a // clang-format off ... // clang-format on region so that the syntactic elements are clearer.

[sst-autotester]

Status Flag 'Pre-Test Inspection' - SUCCESS: The last commit to this Pull Request has been INSPECTED by label AT: PRE-TEST INSPECTED! Autotester is Removing Label; this inspection will remain valid until a new commit to source branch is performed.

[sst-autotester]

Status Flag 'Pull Request AutoTester' - Testing Jenkins Projects:

Pull Request Auto Testing STARTING (click to expand)

Build Information

Test Name: SST__AutotestGen2_NewFW_sst-test_OMPI-4.1.4_PY3.6_sst-elements

• Build Num: 2068
• Status: STARTED

Build Information

Test Name: SST__AutotestGen2_NewFW_sst-test_OMPI-4.1.4_PY3.6_sst-elements_MR-2

• Build Num: 2024
• Status: STARTED

Build Information

Test Name: SST__AutotestGen2_NewFW_sst-test_OMPI-4.1.4_PY3.6_sst-elements_MT-2

• Build Num: 2023
• Status: STARTED

Build Information

Test Name: SST__AutotestGen2_NewFW_sst-test_OMPI-4.1.4_PY3.6_sst-core_Make-Dist

• Build Num: 801
• Status: STARTED

Build Information

Test Name: SST__AutotestGen2_NewFW_OSX-14-XC15-ARM2_OMPI-4.1.6_PY3.10_sst-elements

• Build Num: 579
• Status: STARTED

Using Repos:

Repo: CORE (leekillough/sst-core)

• Branch: serialize_variant
• SHA: f668eeb
• Mode: TEST_REPO

Repo: SQE (sstsimulator/sst-sqe)

• Branch: devel
• SHA: bb4b04da3cbca0dbf36fff9d79593f5f1db96034
• Mode: SUPPORT_REPO

Repo: ELEMENTS (sstsimulator/sst-elements)

• Branch: devel
• SHA: 9213397d54ddaad188280f5f62430b45b30665e7
• Mode: SUPPORT_REPO

Repo: MACRO (sstsimulator/sst-macro)

• Branch: devel
• SHA: 31e2c16aa4d07b502bcd9c97b1872a0329ed0b38
• Mode: SUPPORT_REPO

Pull Request Author: leekillough

[sst-autotester]

Status Flag 'Pull Request AutoTester' - Jenkins Testing: all Jobs PASSED

Pull Request Auto Testing has PASSED (click to expand)

Build Information

Test Name: SST__AutotestGen2_NewFW_sst-test_OMPI-4.1.4_PY3.6_sst-elements

• Build Num: 2068
• Status: PASSED

Build Information

Test Name: SST__AutotestGen2_NewFW_sst-test_OMPI-4.1.4_PY3.6_sst-elements_MR-2

• Build Num: 2024
• Status: PASSED

Build Information

Test Name: SST__AutotestGen2_NewFW_sst-test_OMPI-4.1.4_PY3.6_sst-elements_MT-2

• Build Num: 2023
• Status: PASSED

Build Information

Test Name: SST__AutotestGen2_NewFW_sst-test_OMPI-4.1.4_PY3.6_sst-core_Make-Dist

• Build Num: 801
• Status: PASSED

Build Information

Test Name: SST__AutotestGen2_NewFW_OSX-14-XC15-ARM2_OMPI-4.1.6_PY3.10_sst-elements

• Build Num: 579
• Status: PASSED

[sst-autotester]

Status Flag 'Pre-Merge Inspection' - SUCCESS: The last commit to this Pull Request has been INSPECTED AND APPROVED by [ feldergast ]!

[sst-autotester]

Status Flag 'Pull Request AutoTester' - Pull Request MUST BE MERGED MANUALLY BY Project Team - This Repo does not support Automerge