How Can I Achieve Flexible Return Types with C Static Polymorphism (CRTP) and Avoid Compilation Errors?

C Static Polymorphism (CRTP) Using Derived Class Typedefs

The curiously recurring template pattern (CRTP) allows for static polymorphism in C , but it becomes challenging when aiming to modify function return types based on the derived type. While CRTP can typically infer the derived type, the following code encounters compilation issues in MSVC 2010:

template <typename derived_t>
class base {
public:
    typedef typename derived_t::value_type value_type;
};

template <typename T>
class derived : public base<derived<T>> {
public:
    typedef T value_type;
};

This error arises because the derived class is incomplete when used as a template parameter for the base class in the base class list. To address this issue, a common workaround involves employing a traits class template:

// Declare the base_traits class template
template <typename derived_t>
struct base_traits;

// Define the base class
template <typename derived_t>
struct base {
    typedef typename base_traits<derived_t>::value_type value_type;
};

// Define the derived class
template <typename T>
struct derived : public base<derived<T>> {
    typedef typename base_traits<derived<T>>::value_type value_type;
};

// Specialization of base_traits for derived
template <typename T>
struct base_traits<derived<T>> {
    typedef T value_type;
};

By specializing the base_traits template for the desired derived types, you can define the required members, including custom return types for functions like foo(). This technique enables static polymorphism with more flexibility in return types while avoiding compilation errors.

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