* Include flattests_cpp17 in unit tests when C++17 build is enabled.
* [C++17] Generate generic table factory function.
1. For each table, generate a convenient free-standing factory
function that allows creating the table in a generic way by
specifying only the type. This is the first change in a series
of changes to make Flatbuffers generated C++ code more friendly
to code bases that make use of C++ template metaprogramming
techniques to manage the serialization process. Example:
Before :(
// The name of the Flatbuffers type (and namespace) must
// be hard-coded when writing the factory function.
auto monster = MyGame::Example::CreateMonster(fbb, ...);
After :)
using type_to_create = MyGame::Example::Monster;
// No namespace needed on CreateByTagType.
auto monster = CreateByTagType((type_to_create*)nullptr,
fbb, ...);
This feature requires building with C++14 or greater, and thus
it is guarded behind --cpp-std >= c++17 in the flatbuffers C++
generator.
2. Fix a CMake bug to include C++17 unit tests in test suite.
* [C++17] Replace standalone variadic factory function with type_traits.
Add a `type_traits` to each table class. This `type_traits` can be
populated with various compile-time info about the table. Initially,
we have the Create* function and type, but is extensible in the future.
* Remove empty line and fix stale comments.
* Rename type_traits to Traits and move fwd declaration.
* Fix parameter evaluation order issue and use lambda for scope.
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