Question

Is the following permissible when using templates.

template <typename PAR1 = X,
          typename PAR2 = Y,
          typename PAR3 = Z>
class Base {
   //some stuff inside
};

template <typename PAR1
          typename PAR2 = Z>
class Derived : public Base <PAR1,Y,PAR2> {
   //some stuff here
};

what i want here is that Derived class should always have the PAR2 type as Y.

Can we fix the parameter value when others parameters after it as optional while inheriting ?

Answer 1

I guess you are confused by template default value and partial specialization. Because Base takes 3 parameters with default value, you could instantiate Base with from 0 to 3 parameters.

Base<> b1;              #1, PAR1=X,   PAR2=Y,   PAR3=Z
Base<int> b2;           #2  Par1=int, PAR2=Y,   PAR3=Z
Base<int, int> b3;      #3  PAR1=int, PAR2=int, PAR3=Z
Base<int, int, int> b4; #4  PAR1=int, PAR2=int, PAR3=int 

In Derived class, you are inheriting from situation #4 which is of course valid.

So you can inherit from any form of Base above.

Base template parameter can be independent type(value) or passed from template <typename PAR1, typename PAR2 = Y>

template <typename PAR1,
          typename PAR2 = Y>
class Derived : public Base <> {
};

OR

template <typename PAR1,
          typename PAR2 = Y>
class Derived : public Base <PAR1> {
};

OR

template <typename PAR1,
          typename PAR2 = Y>
class Derived : public Base <PAR1, PAR2> {
};