# Container class help

Staff Emeritus
Gold Member
C++ container class help

Hi, I'm working on a templated container class called "bag" and running into some trouble. This my header file, bagTemplate.h
Code:
#ifndef BAGTEMPLATE_H
#define BAGTEMPLATE_H

template <typename T>
class bag {
public:
bag(int num=10); // construct a bag to hold num elements
~bag( );                // destructor
int size(); //returns current size
int count(const bag<T> & target) const;//counts # items of certain value in bag
void insert (const bag<T> & new_item); //add one item to the bag, double cap if cap exceeded

private:
int used; // size, or number of elements currently stored in the bag
int capacity; // the maximum capacity of the bag
bag <T> * container;   // internal storage array of pointers to items
};

#include "Bag.template"
#endif
And this is my template file, bag.template:
Code:
#include <iostream>
#include <stdlib.h>

using namespace std;

// constructor builds a bag with the ability to hold
// capacity items; the bag is initially empty
template <typename T>
bag<T> :: bag(int num)
{
capacity = num;
used = 0;
container = new bag<T> [num];
}

// destructor destroys the bag by recycling the memory it used
template <typename T>
bag<T> :: ~bag( )
{
delete [ ] container;
}

template <typename T>
int bag<T>::size()
{
return used;
}

template <typename T>
int bag<T>::count(const bag<T> & target) const
{
int ct = 0;
for (int i = 0; i < used; i++)
{
if (target == container[i])
c++;
}
return ct;
}

template <typename T>
void bag<T>::insert (const bag<T> & new_item)
{
if (used == capacity)
capacity = capacity * 2;
container[used ++] = new_item;

}
Here is my driver:
Code:
#include "bagTemplate.h"
#include <iostream>
using namespace std;

int main()
{
cout << "test 1";
bag<int> b;
cout << "test 2";
return 0;
}
When I run this, "test 1" prints, but "test 2" does not. It must be something simple I'm missing, as usual. I am not getting any error messages.
Any help is appreciated. I'm using VS C++ 6 at the moment. Will be trying it in VS C++ .NET 2003 when I get home.
tanx!

Last edited:

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Are you sure this is right?

Code:
bag <T> * container;   // internal storage array of pointers to items
You want bag to hold an array of pointers to bags? Or do you want bag to hold an array of type Ts? If you want to create an array of type T then you should use

Code:
T * container;
If you want an array of pointers to other bags, then you should use this (although I'm not entirely sure this is legitimate):

Code:
bag <T> ** container;   // internal storage array of pointers to items
I'm assuming what you want is the former case, so you have to go through your code and change things like this..

Code:
template <typename T>
void bag<T>::insert (const bag<T> & new_item)
{
if (used == capacity)
capacity = capacity * 2;
container[used ++] = new_item;

}
to

Code:
template <typename T>
void bag<T>::insert (const T & new_item)
{
if (used == capacity)
capacity = capacity * 2;
container[used ++] = new_item;

}
I might have completely misunderstood what you're trying to do, but this is what I think the problem is. By the way, if you're using VS, then just debug your code with F10, until you get to this line in main() :

Code:
bag<int> b;
You'll see that on this line an exception is thrown here. If you use F11 you can step into the calls made at this point, and you'll see that the offending bit of code is what I mentioned before.

You also might want to change this:
Code:
template <typename T>
int bag<T>::count(const bag<T> & target) const
{
int ct = 0;
for (int i = 0; i < used; i++)
{
if (target == container[i])
c++;
}
return ct;
}

Staff Emeritus
Gold Member
thanks! good catches! I appreciate the help!
I'm trying to sort of kluge things together based on a templated link list class example we had in class so it's been a confusing process.
And yes, I'm trying to hold an array of type Ts, but we have to expand it dynamically as needed.

Last edited:
Well if you're getting confused about whether you should be putting bag<T> or T then the best thing to do is to just write your class for integers only (ignoring templates for the time being), make sure all the methods work and so on. When you're satisfied it's working, then go through your code and generalise it to use a template. So where you would have "class bag{}" and "int *container" and so on, change it to read "template<typename T> class bad{};" and "T *container" etc. This might help.

Staff Emeritus
Gold Member
Thank you. You're absolutely right. I'll see if I can get everything going for one data type. The templating has been driving me nuts because it's been an extra layer of abstraction while I'm still trying to get a grasp of the fundamentals of classes.