How Can We Address Compile-Time Errors When Using CRTP and Typedef in C Derived Classes?

Static Polymorphism and Typedef Usage from Derived Classes in C

One of the powerful features of C , Curiously Recurring Template Pattern (CRTP), enables static polymorphism. This allows for compile-time flexibility in determining the type of objects created based on the template arguments. However, extending CRTP to allow modification of function return types based on derived types poses a challenge.

Consider the following code that attempts to generalize CRTP:

template <typename derived_t>
class base {
public:
    typedef typename derived_t::value_type value_type;
    value_type foo() {
        return static_cast<derived_t*>(this)->foo();
    }
};

template <typename T>
class derived : public base<derived<T>> {
public:
    typedef T value_type;
    value_type foo() {
        return T();
    }
};

This code fails to compile in Microsoft Visual Studio 2010 due to the error: 'value_type' is not a member of 'derived'.

The issue stems from the fact that derived is incomplete when used as a template argument for base in its base class list. To address this, a common workaround is to use a traits class template.

Introducing the base_traits template:

template <typename derived_t> 
struct base_traits;

Redefining the base class using the traits:

template <typename derived_t> 
struct base { 
    typedef typename base_traits<derived_t>::value_type value_type;
    value_type base_foo() {
        return base_traits<derived_t>::call_foo(static_cast<derived_t*>(this));
    }
};

And specializing base_traits for derived:

template <typename T> 
struct base_traits<derived<T> > {
    typedef T value_type;

    static value_type call_foo(derived<T>* x) { 
        return x->derived_foo(); 
    }
};

This approach allows for the use of both types and functions from the derived class through the traits. By specializing base_traits for each template argument, you can provide the required members for each derived class.

The above is the detailed content of How Can We Address Compile-Time Errors When Using CRTP and Typedef in C Derived Classes?. For more information, please follow other related articles on the PHP Chinese website!