Why Doesn't Unqualified Name Lookup Work with Template Parameter-Dependent Base Classes in C ?

Template Parameter-Dependent Base Class and Unqualified Name Lookup

In C , when defining a class template, the base class scope is not examined during unqualified name lookup if the base class depends on a template parameter. This behavior is outlined in 14.6.2/3 of the C Standard.

Consider the following code:

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

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

In this example, the B class inherits from the A class template, which contains a type alias Vec_t. However, within the B class, attempting to use Vec_t without fully qualifying it (i.e., A::Vec_t) results in a compiler error, indicating that Vec_t is not recognized.

To resolve this issue, the name of the type alias must be fully qualified within the B class:

template<typename T>
class B : public A<T>
{
private:
    typename A<T>::Vec_t v;  // correct - fully qualified name
};

This is because, during unqualified name lookup in the definition of a class template or its member, the base class scope is not examined if the base class depends on a template parameter. Therefore, it is necessary to explicitly qualify the name of the type alias to avoid compiler errors.

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