constructing a class with a struct inside of it c++

Question

So I have a class which holds a struct inside its private variables and inside this struct I have an array where the size of the array is only determined after the construction of the class.

template <typename T> 
class btree {

  public:
    btree(size_t maxNodeElems);
    ~btree() {}

  private:
    // The details of your implementation go here
    size_t maxNodeElems;
    struct node {

      list <T> elements;
      node lvl[];

    };

};

Firstly, do I have to make it so its node * lvl and how do I call the variables inside this struct? Is it the same as a private variable, so whenever I use it inside one of the functions in btree class I can call it be btree.lvl or is it btree->node->lvl or is there a special way to do this?

Also, my array has to be of maxNodeElems+1 if someone can help me, that'd be much appreciated!

Luchian Grigore · Accepted Answer · 2011-10-19T08:58:21.713

3

You are just declaring the type, not an actual object of that type. You need to make your struct declaration public and the object private:

template <typename T> 
class btree {

  public:
    btree(size_t maxNodeElems);
    ~btree() {}

    struct node {   // <- this is just a declaration of a private inner-class
      list <T> elements;
      node lvl[];
    };

  private:
    size_t maxNodeElems;
    node*  memberNode;   // <- this is the actual private member

};

You can create objects of that type from outside:

btree<A>::node* n = new btree<A>::node;

For accessing members, you can have public getters & setters in your btree class:

class btree {
public:
   node* getNode()
   {
      return memberNode;
   }
   //...........
   //...........
};

EDIT:

The following works for me (initializing the member):

template <typename T> 
class btree {

  public:
    btree()
    {
       memberNode = new btree<T>::node;
    }
    ~btree() {}

    void init()
    {
       memberNode->lvl = new node[10];
    }

    struct node {   // <- this is just a declaration of a private inner-class
      list <T> elements;
      node* lvl;
    };

  private:
    size_t maxNodeElems;
    node*  memberNode;   // <- this is the actual private member

};

int _tmain(int argc, _TCHAR* argv[])
{
   btree<char> b;
   b.init();
}

edited Oct 19 '11 at 08:58

answered Oct 19 '11 at 08:43

Luchian Grigore

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so would this mean, I can do `n->lvl = new node[maxNodeElems+1];` if I change `node lvl[]` to `node *lvl` ? – SNpn Oct 19 '11 at 08:45
Yes. You can do this directly inside the class (I named my variable memberNode). Struct members are public by default so this should work. – Luchian Grigore Oct 19 '11 at 08:46
its throwing a bunch of errors when I try `memberNode = new btree::node;` mainly `btree.tem:8: error: cannot convert 'int*' to 'btree::node*' in assignment` – SNpn Oct 19 '11 at 08:53
how did you get that code to compile? When I run it, it spits out a tonne of errors – SNpn Oct 19 '11 at 10:55
Note that this code contains memory leaks - it needs a destructor, copy constructor and copy assignment operator to be usable. It would be easier to use `std::vector` for `lvl` rather than than to manage a dynamic array yourself. – Mike Seymour Oct 19 '11 at 10:57
@MikeSeymour agreed, you should free the memory in the destructor. – Luchian Grigore Oct 19 '11 at 11:04

constructing a class with a struct inside of it c++

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