allow to use reflection in constant time evaluation

[fdr400]

It is useful for constexpr switch cases, for example, using magic_enum::enum_switch

Example from our code base:

::parquet::schema::NodePtr makeNode(const ::reflection::Field& field) {
  const auto name = field.name()->str();

  return magic_enum::enum_switch(
      [&name](auto val) -> ::parquet::schema::NodePtr {
        constexpr auto base_type = val;
        if constexpr (::reflection::IsScalar(base_type)) { // <- must be constexpr to be evaluated
          return makeScalarSchema<ToCppNativeType<base_type>>(name);
        } else if constexpr (base_type == ::reflection::BaseType::String){
          return makeStringSchema<std::string>(name);
        } else {
          static_assert(false);
        }
      },
      field.type()->base_type());
}

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[jtdavis777]

should we look at any other functions in this file to mark constexpr? We just need to keep C++11 compatible.

[fdr400]

@jtdavis777 thanks for review! Yes, it is possible. I made two more functions constexpr. I believe no more functions in the file can be converted to constexpr because of C++11 limitations

[jtdavis777]

is it required to place the constexpr array outside the function for c++11? it's probably fine to expose that, but I'd prefer if we didn't

[fdr400]

@jtdavis777 unfortunately, yes(
In C++11 constexpr function may only have a single return statement

Alternative solution is

constexpr size_t GetTypeSize(reflection::BaseType base_type) {
  return
      base_type == reflection::None ? 0 :
      base_type == reflection::UType ? 1 :
      base_type == reflection::Bool ? 1 :
      base_type == reflection::Byte ? 1 :
      base_type == reflection::UByte ? 1 :
      base_type == reflection::Short ? 2 :
      base_type == reflection::UShort ? 2 :
      base_type == reflection::Int ? 4 :
      base_type == reflection::UInt ? 4 :
      base_type == reflection::Long ? 8 :
      base_type == reflection::ULong ? 8 :
      base_type == reflection::Float ? 4 :
      base_type == reflection::Double ? 8 :
      base_type == reflection::String ? 4 :
      base_type == reflection::Vector ? 4 :
      base_type == reflection::Obj ? 4 :
      base_type == reflection::Union ? 4 :
      base_type == reflection::Array ? 0 :
      base_type == reflection::Vector64 ? 8 :
      0; // MaxBaseType / fallback
}

but it looks a bit awkward, I guess..

[jtdavis777]

Yeah that is t worth it. I thinks it is good as is.