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Why are Static Members of Base Template Classes Inaccessible in Derived Classes?
Why are Static Members of Base Template Classes Inaccessible in Derived Classes?
Nov 18, 2024 am 12:34 AM
Visibility of Base Template Class Identifiers in Derived Template Class
Consider the following code snippet:
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();
};
When compiled with GCC g 3.4.4 (cygwin), this code compilation fails because NO_ZEROFILL is not visible to the Derived template class. This behavior can be attributed to two-phase lookup in C .
Two-Phase Lookup in C
When the compiler encounters a template declaration, it only performs a preliminary lookup for identifiers used within that template. Since the actual type for T is not determined at this stage, the compiler cannot resolve identifiers that depend on this type, such as Base
In the two-phase lookup process:
- Preliminary lookup: Identifiers are searched for within the current scope and in previously declared namespaces.
- Template instantiation: Once a template is instantiated with specific type parameters, a second lookup is performed to find the instantiated members and identifiers.
In this case, NO_ZEROFILL is not visible during the preliminary lookup because it depends on the unknown type T. As a result, you must explicitly specify Base
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