GH-48740: [C++] Add missing CTypeTraits for decimal types

[shubhamkoti]

Rationale for this change

As reported in #48740, the CTypeTraits specializations were missing for Decimal128Type and Decimal256Type. This prevented generic code from correctly identifying the C++ types associated with these Arrow types.

What changes are included in this PR?

• Added CTypeTraits<Decimal128Type> mapping to Decimal128.
• Added CTypeTraits<Decimal256Type> mapping to Decimal256.

Are these changes tested?

Yes. I verified the changes locally by running the existing type traits tests.
Command used: ctest -R arrow-type-traits

Are there any user-facing changes?

No.

Closes #48740

• GitHub Issue: [C++] Some CTypeTraits are missing #48740

[github-actions]

⚠️ GitHub issue #48740 has been automatically assigned in GitHub to PR creator.

[HuaHuaY]

I think that what you need is TypeTraits but not CTypeTraits

[pitrou]

cc @zanmato1984

[zanmato1984]

We might also want to do for Decimal32/64 as well.

[zanmato1984]

I think that what you need is TypeTraits but not CTypeTraits

Hi @HuaHuaY , this issue is specifically for CTypeTraits which are now missing for certain types (mostly Decimals).

[HuaHuaY]

I think that what you need is TypeTraits but not CTypeTraits

Hi @HuaHuaY , this issue is specifically for CTypeTraits which are now missing for certain types (mostly Decimals).

But I think that this PR is implementing types' TypeTraits instead of CTypeTraits. You can check the code around line 354 which is similar to this PR.

// line 354
template <>
struct TypeTraits<Decimal128Type> {
  ...
  using CType = Decimal128;
  ...
};

template <>
struct TypeTraits<Decimal256Type> {
  ...
  using CType = Decimal256;
  ...
};

// this PR
template <>
struct CTypeTraits<Decimal128Type> {
  using ArrowType = Decimal128Type;
  using CType = Decimal128;
};

template <>
struct CTypeTraits<Decimal256Type> {
  using ArrowType = Decimal256Type;
  using CType = Decimal256;
};

I guess that there should not be CType in CTypeTraits and arrow type should not be the template parameter. We may need these code instead, or you may use some other CType template parameter but I guess it should not be Decimal128Type itself:

template <>
struct CTypeTraits<Decimal128Type::CType> {
  using ArrowType = Decimal128Type;
};

template <>
struct CTypeTraits<Decimal256Type::CType> {
  using ArrowType = Decimal256Type;
};

[HuaHuaY]

template <>
struct CTypeTraits<Decimal128Type::CType> {
  using ArrowType = Decimal128Type;
};

template <>
struct CTypeTraits<Decimal256Type::CType> {
  using ArrowType = Decimal256Type;
};

I found that my understanding of this code isn't complete either. Referring to other implementations, CTypeTraits should inherit from TypeTraits.