How Can a Templated Class Inherit and Access Protected Members from its Dependent Base Class?

Dependent Name Resolution in Templated Inheritance

In C , the ability to access inherited variables depends on the type of inheritance and the context. Consider the following code, where Bar inherits from a templated class Foo:

template<class T> class Foo { ... };
template<class T> class Bar : public Foo<T> { ... };

In this scenario, the question arises: can Bar access the protected variable a of Foo?

Standard Compliance and Rationale

New versions of the GNU C compiler adhere to the C standard, which states that unqualified names in templates are non-dependent. This means that these names must be resolved when the template is defined. However, since the definition of a dependent base class (Foo) is unknown at that time, unqualified names in Bar cannot be resolved.

Solution: Explicit Name Resolution

To access a, Bar must explicitly specify its qualified name: Foo::a. Alternatively, a "using" declaration can be employed within Bar:

using Foo<T>::a;
int c = a * 4; // Accesses 'a' through namespace resolution

In conclusion, while it may seem intuitive for Bar to inherit the protected variable a from Foo, the C standard dictates that this is only possible through explicit name resolution or "using" declarations. This ensures consistent and well-defined behavior across different template instantiations.

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