CODEWITHSUNDEEP
c++classc++11templatesclass-template
Matt
Matt

Reputation: 1166

Initialization of a struct defined in a templated parent class

I have defined a struct within the protected section of a parent class, which I would like to use in an inherited class.

This works as expected if the parent/child classes aren't templated classes. But does not compile as-is below.

Specifically, the compiler (clang 8.0.1) reports:

inheritance_example.cpp:33:26: error: unknown type name 'Node'
        this->head = new Node(toAdd);

From what I have read, I am guessing that the template type specification isn't being assigned to Node, and is thus not being found by the inherited class, but trying the fixes I have found in that vein (i.e. add something along the lines of using Parent<T>::Node, or add a type specifier to the call to the Node constructor), have not worked for me.

Any ideas on how to fix this issue?

#include<iostream>

template <class T>
class Parent 
{
protected:
   struct Node
   {
      Node(int value) 
      {
         this->data = value;
         this->next = nullptr;
      };

      ~Node() {};
      Node* next;
      int data;
   };

   Node* head;

public:
   Parent() {};
   ~Parent() {};
};

template <class T>
class Child : Parent<T>
{
public:

   Child() 
   {
      this->head = nullptr;
   };

   ~Child()
   {
      delete this->head;
      this->head = nullptr;
   };

   void dummyAdd(T toAdd) {
      this->head = new Node(toAdd);
   };

   void dummyPrint() 
   {
      std::cout << this->head->data << std::endl;
   };
};

int main() 
{
   Child<int> t;
   t.dummyAdd(5);
   t.dummyPrint();
   return 0;
}

Upvotes: 1

Views: 273

Answers (1)

JeJo
JeJo

Reputation: 32847

In order to pack the comment into an answer!

Node is a depended name and hence you need to use keyword typename there. Meaning in the dummyAdd function, you need

void dummyAdd(T toAdd) 
{
   this->head = new typename Parent<T>::Node(toAdd);
   //               ^^^^^^^^^^^^^^^^^^^^ 
};

That is, however, a bit verbose/ more typing. Therefore providing a type alias for Node in the Child will be a good idea.

template <class T>
class Child : Parent<T> 
{
   using Node = typename Parent<T>::Node;  // template type alias

public:
   void dummyAdd(T toAdd) 
   {
      this->head = new Node(toAdd);       // now you can this
   };

   // other code...
};

Upvotes: 2

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