Inheriting a template base class - scope resolution of base class members

Question

I have written a template base class as below:

template<typename T>
class Base {
public:
    Base(T x);
    void print_base();
    virtual ~Base();
protected:
    T data;
};

template<typename T>
Base<T>::Base(T x) : data(x) {}

template<typename T>
void Base<T>::print_base()
{
    std::cout << data << std::endl;
}

template<typename T>
Base<T>::~Base() {}

Now, I am trying to write a derived class as the below:

template<typename T>
class Derived : public Base<T>
{
public:
    Derived(T x);
    void print_derived();
private:
};

template<typename T>
Derived<T>::Derived(T x) : this->Base(x) {}

template<typename T>
void Derived<T>::print_derived()
{
    this->print_base();
    std::cout << " data " << this->data << std::endl;
}

But the compiler reports error on the below line:

template<typename T>
Derived<T>::Derived(T x) : this->Base(x) {}

The error is as below:

template_derived_class_this.h:12:28: error: expected identifier before 'this'
 Derived<T>::Derived(T x) : this->Base(x) {}

If I change it to the below the error goes away:

 Derived<T>::Derived(T x) : Base<T>::Base(x) {}

My question is what is the reason for the error in the this-> prefix format? Is it because before the constructor has finished running the this pointer is not yet ready or am I making any syntactical error here? I have seen this-> being used inside the constructor elsewhere. So not sure what is the reason for the error in my case. I am compiling with the below command:

g++ template_inheritance_main_this.cpp -o template_inheritance_main_this  -Wall
  -Wextra -Wpedantic -Werror

Answer 1

You can use this inside constructor body, during initialisation of base class this is unavailable and you just use base class name to initialise it as you did in changed code snippet of yours. Constructor of your derived class should look like this:

template<typename T>
Derived<T>::Derived(T x) : Base<T>(x) {}