Question

This is what I want to achieve. The leaf component will inherit Component<ParentT>, others will inherit Component<ParentT, ChildT>

template <typename T>
class Component{
  protected:
    typedef Component<T> ParentComponentT;
  ...
};

template <typename ParentT, typename ChildT>
class Component: public Component<ParentT>{
  protected:
    typedef std::vector<ChildT*> CollectionT;
  ...
};

But the problem is Template parameters getting redeclared. and I cannot move the second one above first one because second one inherits first one.

error: redeclared with 2 template parameter(s)
note: previous declaration ‘template class Component’ used 1 template parameter(s)

Answer 1

This compiles and as far as I understand does what you like:

#include <vector>

class NoneT {};

template <typename ParentT,typename ChildT=NoneT>
class Component: public Component<ParentT>{
  protected:
    typedef std::vector<ChildT*> CollectionT;
};

Specialization for NoneT:

template<>
template<typename T>
class Component<T,NoneT>{
protected:
   typedef Component<T> ParentComponentT;
};

int main(){
   typedef Component<double> someT;
   typedef Component<double,int> someT2;
   typedef Component<double,void> someT3;
}

someT will have ParentComponentT and someT2 will have CollectionT.

EDIT:

Answer to comment/question below: typename ChildT=noneT means that the default ChildT will be noneT. So, if no second template argument is given the noneT type will be used.

The specialization then defines the class content for the that one-argument version.

EDIT2:

Since I know from the chat that you use Component as a base class, I suggest that instead of something like

class myclass: public Component<Section, Line>

you could use multiple inheritance

class myclass: public ParentComponent<Section>, CollectionComponent<Line>

with

template <typename T>
class ParentComponent{
  protected:
    typedef Component<T> ParentComponentT;
};

template <typename ChildT>
class CollectionComponent {
  protected:
    typedef std::vector<ChildT*> CollectionT;
};