Basic C++ class how to implement with Objective C

Question

I have some problems with implementing class which written in C++, because I not familiar with c++ .. if someone could help with implementing it in Objective C

#ifndef ENTITY_H
#define ENTITY_H

class BaseGameEntity
{

private:

  int          m_ID;
  static int  m_iNextValidID;

  void SetID(int val);

public:

  BaseGameEntity(int id)
  {
    SetID(id);
  }

  virtual ~BaseGameEntity(){}

  virtual void  Update()=0;

  int           ID()const{return m_ID;}  
};



#endif


#include "BaseGameEntity.h"
#include <cassert>

int BaseGameEntity::m_iNextValidID = 0;

void BaseGameEntity::SetID(int val)
{
  assert ( (val >= m_iNextValidID) && "<BaseGameEntity::SetID>: invalid ID");

  m_ID = val;

  m_iNextValidID = m_ID + 1;
}

Are you receiving a specific error?

drewh
– drewh

2010-05-17 17:24:29 +00:00
Commented May 17, 2010 at 17:24 — drewh
– drewh, Commented May 17, 2010 at 17:24

Chuck · Accepted Answer · 2010-05-17 20:43:17Z

1

This is rougly equivalent to that C++ class:

// In BaseGameEntity.h
#import <Cocoa/Cocoa.h>

@interface BaseGameEntity : NSObject {
    NSInteger m_ID;
}

- (id)initWithID:(NSInteger)ID;
- (void)update; // must be defined by subclasses
- (NSInteger)ID;
@end


// In BaseGameEntity.m
#import "BaseGameEntity.m"

@implementation BaseGameEntity
static NSInteger m_iNextValidID;

- (id)initWithID:(NSInteger)ID {
    self = [super init];
    if (!self)
        return nil;
    NSAssert2(ID >= m_iNextValidID, @"%s invalid id: %d", __PRETTY_FUNCTION__, ID);
    m_ID = ID;
    m_iNextValidID = m_ID + 1;
    return self;
}

- (NSInteger)ID {
    return m_ID;
}

- (void)setID:(NSInteger)newID {
    NSAssert2(newID >= m_iNextValidID, @"%s invalid id: %d", __PRETTY_FUNCTION__, newID);
    m_ID = newID;
    m_iNextValidID = m_ID + 1;
}
@end

A few differences to note:

All methods are virtual in Objective-C, so there's no need to declare them as such
Objective-C doesn't have pure virtual functions
Objective-C doesn't have class-static variables — it only has function-static and file-static variables like C does — so we declare the static int in the implementation file
I included the setID: method for equivalence, but it's considered bad form to use setters in Objective-C initializers, so it's never actually called — I just copied the body of the method into the init method

edited May 17, 2010 at 20:43

answered May 17, 2010 at 18:01

Chuck

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3 Comments

Yesman Over a year ago

Thank you so much, especially for the comments :))

Yesman Over a year ago

What will be better? For the update() leave it as it is or to adopt by the protocol? @protocol Instantiating - (void) update; @end

Chuck Over a year ago

The normal way to force a subclass to provide its own implementation is to have the superclass declare it as part of its interface but raise an exception when the method is called.

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