problem compilng c++ code with a template child class by gcc

[Korosov]

Hi! GCC (4.1.2 20070502 (Red Hat 4.1.2-12)) gives an error:

test.cpp: In constructor <Dclass<T>::Dclass()>:
test.cpp:27: error: <a> was not declared in this scope


when compiling the following code:

#include <stdio.h>
 
//=== declare parent class
template <class T>
class Rclass {
   public:
      T a;         //parent class variable
      Rclass();    //parent class constructor
};
 
//=== parent class constructor
template <class T>
Rclass<T> :: Rclass(){
   a = 1;
};
 
//=== declare child class
template <class T>
class Dclass : public Rclass<T>{
   public:
      Dclass();   //child class constructor
};
 
//=== child class constructor
template <class T>
Dclass<T> :: Dclass(){
    a = 2;  // <===  this parent class variable 'WAS NOT DECLARED IN THIS SCOPE'
};
 
//=== test
int main(){
 
    Dclass<float> b;
    printf("r=%f\n", b.a);
 
return 0;
};

It seems that the compiler doesn't see the variable 'a' in the child class, becasue it was declared in the template parent class. When the type is fixed and templates are not used it runs OK.

However no errors ocur when I try to compile with Borland C++ or older version of GCC.
What can be the problem and the solution? Should I use some options to gcc or declare (use) the variable 'a' in another way ?
Thank you!!

[weaknessforcats]

test.cpp: In constructor <Dclass<T>::Dclass()>:
test.cpp:27: error: <a> was not declared in this scope

The base object has not been created so there is no public data member.

You need to add:

template <class T>
class Rclass {
   public:
      T a;         //parent class variable
      Rclass();    //parent class constructor
      Rclass(T);                <<<<<
};
template <class T>                              <<<<
Rclass<T> :: Rclass(T arg){                 <<<<
   a = arg;                                           <<<<
};                                                       <<<<
template <class T>
class Dclass : public Rclass<T>{
   public:
      Dclass();   //child class constructor
      Dclass(T);                                     <<<<
};
template <class T>                                             <<<<
Dclass<T> :: Dclass(T arg): Rclass<T>(arg){       <<<<
                                                                         <<<<
};                                                                       <<<<
 

Remember, these are templates. There is no Rclass::a until an Rclass object is created and you create one (and use the Rclass template) by using the initialize list in the Dclass constructor.

Had you not used a template, the you would have had no problem since you would have already declared Rclass to contain a float.

BTW: Don't use public data members.

[Korosov]

The base object has not been created so there is no public data member.

Remember, these are templates. There is no Rclass::a until an Rclass object is created and you create one (and use the Rclass template) by using the initialize list in the Dclass constructor.

Had you not used a template, the you would have had no problem since you would have already declared Rclass to contain a float.

BTW: Don't use public data members.

Thank you weaknessforcats!!

Another solution can be:

 
template <class T>
class Dclass : public Rclass<T>{
   public:
      Dclass();   //child class constructor
};
template <class T>
Dclass<T> :: Dclass(T arg)){ 
 
Rclass<T> :: a = 2;
 
};                                        
 

In this case Rclass object is also created. However your solution seems to be more beautiful. If I have more than one variable in parent class used by child class, I would have to place Rclass<T> :: in front of each of them.