為什麼派生模板類別無法存取基底模板類別標識符？

Accessibility of Base Template Class Identifiers in Derived Template Classes

In C++, when a derived template class inherits from a base template class, it's natural to expect the derived class to have access to the base class's identifiers. However, in certain scenarios, you may encounter a situation where this access is restricted.

Consider the following code:

template <typename T>
class Base
{
public:
    static const bool ZEROFILL = true;
    static const bool NO_ZEROFILL = false;
};

template <typename T>
class Derived : public Base<T>
{
public:
    Derived( bool initZero = NO_ZEROFILL );    // NO_ZEROFILL is not visible
    ~Derived();
};

In this example, the Derived class cannot access the NO_ZEROFILL identifier defined in the Base class. This behavior is caused by the two-phase lookup mechanism in C++.

During template expansion, the base class template is instantiated with a specific type for T. In this case, the compiler does not know the actual type of T until the template is used. Therefore, it cannot resolve identifiers in the base class that depend on T, such as NO_ZEROFILL.

To address this issue, you must explicitly specify the base class template when accessing its identifiers. For example, you would need to write Derived::NO_ZEROFILL instead of simply NO_ZEROFILL.

This explicit base class template specification instructs the compiler to search for the identifier NO_ZEROFILL within the context of the Derived class. This ensures that the correct identifier is found even though the base class template is not fully instantiated at the time of template expansion.

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