[C++ Qt Client] Add OneOf/discriminator support with std::variant implementation

[BigMichi1]

Add OneOf/discriminator support for C++ Qt client generator

Summary

Adds support for OpenAPI oneOf schemas with discriminators in the C++ Qt client generator. This implementation uses C++17 std::variant to represent polymorphic types in a type-safe manner.

Changes Made

New Template Files

• model-oneof.mustache - Header template for oneOf models with discriminator

  • Declares variant type containing all mapped model types
  • Provides type checking methods (isModelName()) and conversion methods (asModelName())
  • Uses std::variant with std::monostate for uninitialized state

• model-oneof-body.mustache - Implementation template for oneOf models

  • Implements discriminator-based deserialization from JSON
  • Type-safe variant handling using std::visit
  • Automatic discriminator value setting when storing instances
  • Validation that delegates to the actual instance

Modified Template Files

• model-body.mustache

  • Added conditional logic to use oneOf template for models with discriminators
  • Added #include <QJsonValue> for JSON handling

• model-header.mustache

  • Added conditional logic to use oneOf header template for models with discriminators
  • Ensures proper template routing based on discriminator presence

Key Features

• ✅ Type-safe polymorphic model representation using std::variant
• ✅ Automatic discriminator field handling during serialization/deserialization
• ✅ Runtime type checking with isModelName() methods
• ✅ Safe type conversion with asModelName() methods returning shared_ptr
• ✅ Proper validation delegation to actual instance types
• ✅ C++17 compliant implementation

Technical Details

• Uses std::variant<std::monostate, shared_ptr<Type1>, shared_ptr<Type2>, ...> for type safety
• Discriminator property is automatically included in JSON output
• Utilizes std::visit for variant pattern matching
• Falls back to warning log for unknown discriminator values

Related Issue

• [BUG][C++][QT5] OneOf, AnyOf and AllOf Handling Erroneous #6378

@aminya

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Summary by cubic

Adds oneOf/discriminator support to the C++ Qt client generator using C++17 std::variant. Enables type-safe polymorphic models with automatic discriminator handling during JSON (de)serialization. Relates to #6378.

• New Features

  • Generate oneOf model templates (model-oneof.mustache, model-oneof-body.mustache).
  • Use std::variant<std::monostate, std::shared_ptr<...>> to store mapped models.
  • Deserialize by discriminator value; serialize with discriminator included.
  • Add isModelName() and asModelName() helpers for runtime type checks.
  • Delegate validation to the active instance.

• Refactors

  • Route models with discriminators to oneOf templates in model-header.mustache and model-body.mustache.
  • Add QJsonValue include for JSON handling.

^{Written for commit bfce96b. Summary will update on new commits.}

[aminya]

The tests need to be updated @BigMichi1