Protected Members and Templated Inheritance

Hi all,

I am a bit confused about the following piece of code (as a reduced part of an other problem, also related to templates):

Code:

template< const int A >
class Base
{
public:
  inline float getP() { return p; }
protected:
  float p;
};

template< const int B >
class Child : public Base< B >
{
  void member() {
    //this->p = 5.f;
    p = 5.f;
  }
};



int main () {
  Child<4> myChild;
  myChild.getP();
  
  return 0;  
}

When I use the inheritance

Code:

template< const int B >
class Child : public Base< B > {/* ... */};

I am not allowed^[1] to enter p without "this->":

template.cpp: In member function ‘void Child<B>::member()’:
template.cpp:15:5: error: ‘p’ was not declared in this scope

If I inherit via a specialization like

Code:

 class Child : public Base< 4 > {/* ... */};

everything works out as normal.


Is there any explanation for this behaviour?
Do other compilers create the same error?


More Details:

• [1] compiled with:
  Code:

  $ g++ -Wall template.cpp
  $ g++ --version
     g++ (Ubuntu/Linaro 4.5.2-8ubuntu4) 4.5.2

Chers and thanks a lot,
Axl

Originally Posted by Axl

Is there any explanation for this behaviour?
Do other compilers create the same error?

Read the answer to this FAQ: Why am I getting errors when my template-derived-class uses a member it inherits from its template-base-class?

Thank you!

Ok, I expanded my problem a bit and found a solution for it.
The next problem was calling inherited member functions with templates.

That's my solution (found in the library "loki"):

Code:

template< const int A >
class Base
{
public:
  template< class C>
  C getP() { return static_cast<C>(p); }
protected:
  float p;
};

template< const int B >
class Child : public Base< B >
{
public:
  void member() {
    Base<B>::p = 5.f;
    // this crashes with:
    //    error: expected primary-expression before ‘double’
    //Base<B>::getP<double>();
    
    // this crashes with:
    //    error: no matching function for call to ‘Child<4>::getP()’
    //Base<B>::getP();
    
    // thats how to overcome the issue:
    Base<B>::template getP<double>();
  }
};



int main () {
  Child<4> myChild;
  myChild.member();
  
  return 0;  
}

Axl

I believe this->getP<double>() should work as well.
Similarly, this->p should work as well.

Originally Posted by Elysia

I believe this->getP<double>() should work as well.
Similarly, this->p should work as well.

Sure, this->p works without any harm.

But you get in trouble with this->getP<double>() Just try it

Code:

g++ -Wall template.cpp
template.cpp: In member function ‘void Child<B>::member()’:
template.cpp:29:16: error: expected primary-expression before ‘double’
template.cpp:29:16: error: expected ‘;’ before ‘double’

Yes, it is another issue with dependent names. The compiler doesn't know what getP is. It trusts you that it is defined in the base class. However, since it doesn't know that method is a template as well, it must assume that the < symbol is less-than operator. Again, you need to explicitly tell it that it is not the case.

Code:

this->template getP<double>();