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error when using template base class members

Phuong Ngo answered on July 21, 2022 Popularity 3/10 Helpfulness 1/10

answer error when using template base class members

error when using template base class members

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template<typename T>
class B {
public:
  void f() { }  // Member of class B<T>
};
template<typename T>
class D : public B<T> {
public:
  void g()
  {
    f();  // Bad (even though some compilers erroneously (temporarily?) accept it)
  }
};

/*************
Q:
Why am I getting errors 
when my template-derived-class uses a member it inherits from its template-base-class?

A:
Within D<T>::g(), the name f does not depend on template parameter T, 
so f is known as a nondependent name. 
On the other hand, B<T> is dependent on template parameter T 
so B<T> is called a dependent name.

Here’s the rule: 
the compiler does not look in dependent base classes (like B<T>) 
when looking up nondependent names (like f).

This doesn’t mean that inheritance doesn’t work. 
Class D<int> is still derived from class B<int>, 
the compiler still lets you implicitly do the is-a conversions (e.g., D<int>* to B<int>*), 
dynamic binding still works when virtual functions are invoked, etc. 
But there is an issue about how names are looked up.

Workarounds:
>>
Change the call from f() to this->f(). 
Since this is always implicitly dependent in a template, 
this->f is dependent and the lookup is therefore deferred 
until the template is actually instantiated, 
at which point all base classes are considered.
>>
Insert using B<T>::f; just prior to calling f().
>>
Change the call from f() to B<T>::f(). 
Note however that this might not give you what you want if f() is virtual, 
since it inhibits the virtual dispatch mechanism.

*************/

Popularity 3/10 Helpfulness 1/10 Language cpp

Source: isocpp.org

Tags: c++ using when

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Contributed on Jul 21 2022

Phuong Ngo

0 Answers Avg Quality 2/10

error when using template base class members

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