Dynamic Size for a private member

Dev Shed Forums Sponsor:

March 18th, 2002, 04:36 PM

syin

Member

Join Date: Oct 2000

Posts: 13

Time spent in forums: < 1 sec
Reputation Power: 0

Dynamic Size for a private member

Hi,

Let's say I have a class called "CDlibrary", and inside this class, I have a class called "CD" as a private member. An instance of CDlibrary can contain multiple CDs. I don't know how many CDs I have before I initialize the CDlibrary instance. How do I design the CDlibrary class so that different instances can contain different # of CDs?

eg. If every library has 1 CD then:

class CDlibrary {
private:
CD disc1;
};

Thanks for your help in advance.

March 18th, 2002, 06:10 PM

Scorpions4ever

Banned ;)

Dev Shed God 9th Plane (9000 - 9499 posts)

Join Date: Nov 2001

Location: Woodland Hills, Los Angeles County, California, USA

Posts: 9,383

Time spent in forums: 1 Month 4 Weeks 1 Day 20 h 49 m 30 sec
Reputation Power: 4080

You could try declaring a pointer to an array of class CD. In the class constructor for CDLibrary, you can initialize the pointer to null. Then, when you know how many CDs you have, you can allocate the memory as needed. Also, in your destructor, you can check if you allocated memory to hold the CDs or not and destroy the array appropriately E.g.

Code:

class CDlibrary { 
  private: 
      CD *discs; 
      int nDiscs;
  public:
      CDLibrary();
      ~CDLibrary();
      void CreateCDs(int num);
}; 

CDLibrary::CDLibrary() {
    discs = NULL;
}

CDLibrary::~CDLibrary() {
   if (discs) {
        delete discs;
        discs = NULL;
   }
}

void CDLibrary::CreateCDs(int num) {
    nDiscs = discs;
    discs = new CD[num];
}

March 18th, 2002, 06:18 PM

syin

Member

Join Date: Oct 2000

Posts: 13

Time spent in forums: < 1 sec
Reputation Power: 0

Ah stupid me. Somehow I thought the CD instances in the array would be deleted after the CreateCDs() function exits...

Thanks for your help =)

March 18th, 2002, 11:19 PM

The Ostrich

Contributing User

Join Date: Mar 2002

Location: Oxford, UK

Posts: 28

Time spent in forums: < 1 sec
Reputation Power: 0

Um....

When you delete an array, you MUST use

delete [] discs;

The reason is that delete discs will only delete the first element, so you will leave most of your allocated memory hanging.

March 19th, 2002, 12:28 AM

Scorpions4ever

Banned ;)

Join Date: Nov 2001

Location: Woodland Hills, Los Angeles County, California, USA

Posts: 9,383

Time spent in forums: 1 Month 4 Weeks 1 Day 20 h 49 m 30 sec
Reputation Power: 4080

Oops, my bad... Nice catch!

Dynamic Size for a private member

Developer Shed Advertisers and Affiliates