[SOLVED] Dynamic list with different type

Lobinho · 06-26-2010, 09:56 AM

Hi,

I'm quite new to c++ programming, so my concepts are not so increased yet...

I wanna to create a list to store my objects. I'll resume them here:

Code:

class Event { //I tried to built this class like an "interface" on java
public:
  Event(int timeout); //on .cpp I'll do: this->timeout = timeout
  virtual ~Event();
 
private:
  int timeOut; //event timeout - all Events will have one

public:
  int getTimeout();

  //The message is particular to each Event
  virtual int getMessage(char* message) = 0; //pure virtual method
}

class EventA : public Event {
public:
  EventA(int timeout);
  ~EventA();

private:
  int attributeOne; 
  int attributeTwo;

public:
  int getMessage(char* message); 
  //this is implemented on cpp:
  /* int EventA::getMessage(char* message) { 
   *   strcpy(message, "EventA msg");
   * }
   */
}


class EventB : public Event {
public:
  EventB(int timeout);
  ~EventB();

private:
  int attributeOne;
  bool attributeTwo; 
  char attributeThree[128];

public:
  int getMessage(char* message); 
  //this is implemented on cpp:
  /* int EventB::getMessage(char* message) { 
   *   strcpy(message, "EventB msg");
   * }
   */
}

...

And what I want to do is an event list:

Code:

Event** eventList = new Event*[5];
eventList[0] = new EventA(4);
eventList[1] = new EventB(6);
eventList[2] = new EventC(8);
eventList[3] = new EventD(1);
eventList[4] = new EventE(2);

char temp[128] = "";
eventList[0]->getMessage(temp);
printf("%s\n", temp);
eventList[1]->getMessage(temp);
printf("%s\n", temp);
eventList[2]->getMessage(temp);
printf("%s\n", temp);
...

Sometimes this works and sometimes I got "segmentation fault".

I think I'm doing it wrong, cause these objects have different sizes, so I don't know if I could have all of them in a single array.

Someone have any tip or tutorial or how to to share with me?

Thanks in advance!

dugan · 06-26-2010, 11:01 AM

Replace strcpy with something that can't segfault the program. The best practice would be to use std::string and its = operator.

Also, the class is Events in the first code sample and Event in the second code sample

Lobinho · 06-26-2010, 11:27 AM

Thanks for the tip dugan.

I never used the std::string before (I don't know why...), now I'll implement the modifications on my system to use std::string.
But the doubt about an array list with different objects inside still remains.

Lobinho · 06-26-2010, 11:49 AM

When I do:

I have the method:

Code:

void setMessage(std::string message) {
  this->message = message;
}

When I do this... I'm copying the content of message to this->message or copying the pointer of message to this->message?

dugan · 06-26-2010, 02:15 PM

Quote:

Originally Posted by Lobinho

Code:

this->message = message;

I'm copying the content of message to this->message or copying the pointer of message to this->message?

The former.

Lobinho · 06-26-2010, 03:11 PM

Thanks dugan... I changed my code and now is better and more readable.
The array for objects is working fine.

dugan · 06-26-2010, 03:32 PM

Glad to hear it. BTW, this would be more in line with current C++ best practices:

Code:

#include <vector>
#include <string>
#include <iostream>

using namespace std;

class event
{
public:
    virtual string get_message() = 0;
};

class event_a: public event
{
public:
    string get_message();
};

class event_b: public event
{
public:
    string get_message();
};

string event_a::get_message()
{
    return "Event A Message";
}

string event_b::get_message()
{
    return "Event B Message";
}

int main()
{
    vector<event *> event_list;
    event_list.push_back(new event_a);
    event_list.push_back(new event_b);

    event *a = new event_a;
    a->get_message();
  
    for (vector<event *>::iterator itr = event_list.begin(); itr != event_list.end(); ++itr)
    {
        cout << (*itr)->get_message() << endl;
        delete *itr;
    }
    return 0;
}

Or if you want to use smart pointers:

Code:

#include <vector>
#include <string>
#include <iostream>
#include <boost/shared_ptr.hpp>

using namespace std;
using namespace boost;

class event
{
public:
    virtual string get_message() = 0;
};

class event_a: public event
{
public:
    string get_message();
};

class event_b: public event
{
public:
    string get_message();
};

string event_a::get_message()
{
    return "Event A Message";
}

string event_b::get_message()
{
    return "Event B Message";
}

int main()
{
    typedef shared_ptr<event> event_ptr;
    vector<event_ptr> event_list;
    event_list.push_back(event_ptr(new event_a));
    event_list.push_back(event_ptr(new event_b));

    for (vector<event_ptr>::iterator itr = event_list.begin(); itr != event_list.end(); ++itr)
    {
        cout << (*itr)->get_message() << endl;
    }
     
    return 0;
}

Lobinho · 06-26-2010, 04:16 PM

I don't know vectors, but they look like arraylist on java...
I'll study them too!

I'm build some applications for an embedded system, then I was trying to use light resources. Do you know if these classes (vectors, strings...) will make too much difference (running on 400mhz processor and 64mb ram)?
I'll try to find out how much memory they are occupying with sizeof() and then try to build some examples with vectors.

Thanks again dugan!

dugan · 06-26-2010, 04:35 PM

They don't have enough overhead to make any difference at all.