How Can CRTP Achieve Statically Changing Return Types Using Typedefs from Derived Classes?

Statically Changing Return Types in CRTP with Typedefs from Derived Classes

The concept of Curiously Recurring Template Pattern (CRTP) in C allows for static polymorphism, enabling the implementation of derived class behavior within the base class. However, there can be situations where one desires to alter the return types of functions in the base class based on the derived type.

Unfortunately, this modification is not immediately possible in the given code sample due to the incompleteness of the derived class when used as a template parameter for the base class. To address this, the code employs a common workaround: creating a traits class template.

Utilizing Traits Classes

A traits class template, such as the base_traits template shown in the revised code, acts as a bridge between the base and derived classes. It defines the necessary members, including typedefs and function calls, that the base class needs to access from the derived class.

Specializing Traits Classes

The revised code specializes base_traits for derived to provide the specific value_type and call_foo function implementation required by base. This specialization ensures that base has access to the correct return types and function behavior from the derived class.

Implementation Details

The derived class now contains a derived_foo function that returns a value of type value_type. The base_foo function in base calls the call_foo function from the specialized base_traits to invoke derived_foo and retrieve the appropriate return value.

By using a traits class template and specializing it for the desired derived types, it becomes possible to dynamically change return types in CRTP based on the derived classes.

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