Create subclass of class template

Question

The following working code displays the smaller of two given numbers.

#include "stdafx.h"
#include <iostream>
using namespace std;

template <class T>
class Bucky {
public:
  Bucky(T a, T b) {
    first  = a;
    second = b;
  }
  T smaller() {
    return (first<second?first:second);
  }
private:
  T first, second;
};


int main() {
  Bucky <int>obj(69, 105);
  cout << obj.smaller() << endl;
  return 0;
}

I would like to derive from class 'Bucky' and create a subclass having a member function that displays "Hi There!"

Please help.

P.S. Here is my best failed attempt to create my subclass:

template <class T>
class mySubclass : public Bucky<T>
{
public:
  mySubclass(T a, T b) {  // error C2512: 'Bucky<T>' : no appropriate default constructor available
    //first  = a;
    //second = b;
  }
  void greet() {
    cout << "Hi there!";
  }
};

Please include your best attempt and the errors that it gives you. On Stack Overflow, we enjoy helping you solve your problems, rather than just solving them. — Joseph Mansfield, Jul 02 '14 at 19:05
@JosephM Thank you, I will add my best failed attempt in a minute. — Caroline Beltran, Jul 02 '14 at 19:10
You should read about the *member initializer list*. For example, [here](http://stackoverflow.com/q/6923722/420683) and [here](http://stackoverflow.com/q/926752/420683) — dyp, Jul 02 '14 at 19:13
If you thought it was related to templates, you should have tried without them to test that hypothesis. — juanchopanza, Jul 02 '14 at 19:18
@juanchopanza, that is a very good suggestion and now I see how important that is. — Caroline Beltran, Jul 03 '14 at 01:48

score 4 · Accepted Answer · answered Jul 02 '14 at 19:19

When you construct a derived class, it will also construct its base class. If you don't explicitly specify this, it will attempt to construct the base class using its default constructor (with no arguments). However, your base class doesn't have a default constructor. Instead, you want to delegate to the constructor of Bucky that takes two T arguments:

mySubclass(T a, T b)
  : Bucky<T>(a, b) { }

As you can see, this uses a strange syntax beginning with : and leaves the constructor body empty. This syntax is the member initialization list. It says that we're initialising the Bucky<T> base class by passing a and b to its constructor. In fact, you should also use this for your Bucky constructor:

Bucky(T a, T b)
  : first(a), second(b) { }

Here we are initialising the first and second members with a and b respectively.

I carefully looked at all of the answers which were all correct (tested them all). Although all answers were top notch, I chose @JosephM because he explained everything exceptionally well. — Caroline Beltran, Jul 02 '14 at 20:25

score 3 · Answer 2 · answered Jul 02 '14 at 19:17

The mySubclass constructor does not explicitly initialize the base class object Bucky. So the compiler will attempt to do so implicitly by calling the default constructor for Bucky. As the error says, Bucky does not have a default constructor. To fix the error, initialize Bucky explicitly in the contructor's member initializer list.

mySubclass(T a, T b)
: Bucky<T>(a, b)
{}

You should also use the member initializer list to initialize Bucky's data members

Bucky(T a, T b) 
: first(a)
, second(b)
{}

Will Custode · Answer 3 · 2014-07-02T19:18:12.093

You can call the base constructor from the derived constructor as such:

mySubClass(T a, T b)
    : Bucky(a, b)
{
}

As the error states, by implementing a constructor (one that accepts arguments) you have removed the default constructor. And when you create a constructor for a subclass it must initialize its base class. The way to do this is to call the base constructor in the initializer list, as someone in the comments mentioned.

score 1 · Answer 4 · answered Jul 02 '14 at 19:17

As the compiler points out, it cannot find an appropriate default constructor for your base class. So, you need to call one explicitely in the member initializer list.

template <class T>
class mySubclass : public Bucky<T>
{
public:
  mySubclass(T a, T b) : Bucky<T>(a, b) { }
  void greet() {
    cout << "Hi there!";
  }
};

score 0 · Answer 5 · answered Jul 02 '14 at 19:18

template <class T>
class a:public Bucky<T> {
public:
  a(T r, T l): Bucky<T>(r, l) {}
  void Hello(void) { std::cout << "Hello there!" << std::endl; }
};

int main() {
  Bucky <int>obj(69, 105);
  a<int> obj2(69, 105);

  obj2.Hello();
  cout << obj.smaller() << endl;
  return 0;
}

I think this get's you what you're looking for.

Create subclass of class template

5 Answers5