Inhertiance

[ZKM]

I am learning how to use derived classes using inheritance but am so far unable to declare the derived class without error. I have boiled down my code to demonstrate the error I am receiving in the program I am working on

test.h

#ifndef TEST_H
#define TEST_H

#include <string>
#include <iostream>

using namespace std;

template < typename T >
class Aaa
{
public:
  T var1;
  void out(){cout << var1;};
  Aaa(T a): var1(a){};

};

template < typename T >
class Bbb : public Aaa  
{                       //Error: expected class-name before '{' token

public:
  T var2;
  void out(){cout << var2;}
  Bbb(T b): var2(b) {};

};

#endif 

tester.cpp

#include "test.h"

int main()
{
  Aaa<string> Test1("hello");
  Bbb<string> Test2("goodbye");
  Test1.out();
  Test2.out();

  return 0;
}

[George P]

The error I get in Visual Studio 2019:

'Aaa': use of class template requires template argument list

Easy to fix in your header file (you need to have a base class default ctor):

#ifndef TEST_H
#define TEST_H

#include <string>
#include <iostream>

template <typename T>
class Aaa
{
public:
   T var1 { };
   void out() { std::cout << var1 << '\n'; };
   Aaa(T a) : var1(a) { }

   Aaa() { } // <---- default ctor needed!
};

template <typename T>
class Bbb : public Aaa<T>
{
public:
   T var2 { };
   void out() { std::cout << var2 << '\n'; }
   Bbb(T b) : var2(b) { }
};

#endif 

main source:

#include <iostream>
#include <string>
#include "test.h"

int main()
{
   Aaa<std::string> Test1("hello");
   Bbb<std::string> Test2("goodbye");

   Test1.out();
   Test2.out();
}

hello
goodbye

Learning inheritance using templated classes IMO over-complicates things.

[ZKM]

It looks like my problem was this:

class Bbb : public Aaa<T>

I did not have the <T> on my code, including that fixed my problem.

Having a default constructor was the problem my compiler would have showed next, so thank you for solving my future problem and helping me recognize my current problem!

[doug4]

I will take a slightly different view from @Furry Guy. You may have actually been trying to do this:

#ifndef TEST_H
#define TEST_H

#include <string>
#include <iostream>

template <typename T>
class Aaa
{
public:
   T var1 { };
   void out() { std::cout << var1 << '\n'; };
   Aaa(T a) : var1(a) { }
};

template <typename T>
class Bbb : public Aaa<T>
{
public:
   Bbb(T b) : Aaa<T>(b) { }
};

#endif  

Either way, you need to derive Bbb<T> from Aaa<T>, not Aaa. In your original code (and @Furry Guy's solution) you have duplicate member data (var1 and var2) and you need a default constructor. In my solution, you only have 1 copy of the member data, and you don't need a default constructor.

[George P]

I retained the member data in the two classes since they might not be duplicates. The derived class might be storing different data from the base class.