How Can I Properly Inherit Type Aliases from a Base Class Template to a Derived Class Template in C ?

Propagating Type Aliases from Base to Derived Class for Templates

In C , it is common to define type aliases to enhance code readability and maintainability. However, when working with class templates and inheritance, propagating typedefs from a base class to a derived class requires a specific approach.

Suppose we have a base class template A that contains typedefs only:

template<typename T>
class A {
public:
    typedef std::vector<T> Vec_t;
};

Now, let's create a derived class template B that inherits from A:

template<typename T>
class B : public A<T> {
private:
    Vec_t v;  // fails - Vec_t is not recognized
};

In this code, trying to use Vec_t in the derived class B results in an error. This is because qualified name lookup for Vec_t is not performed during the definition of the derived class template or during an instantiation of the template.

To resolve this issue, the fully qualified name of the typedef must be used in the derived class:

typename A<T>::Vec_t v;

This notation fully specifies the scope of the typedef and allows it to be propagated to the derived class.

In essence, the C Standard requires that base class scopes that depend on template parameters are not examined during unqualified name lookup. This restriction ensures that the resolution of typedefs is consistent and predictable across different instantiations of class templates.

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