c++ oop question...

void OnCollision(CEntity *pEnt)
{
	mPosition = mOldPos;
	CPlayer *player;
	if(pEnt->mEntType == ePlayer)
	{
		player = (CPlayer*)pEnt;
		Say("Dragon: You dare to approach me human!!");
		player->mHealth -= 4;
	}

	pEnt = player;			
}

quote:

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Original post by no one
Im actually already using an rtti system. However, accessing members within a derived class from the base class isn't possible?

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#include <iostream>
#include <string>

using namespace std;

enum rtti
{
	edefault,
	ederived1,
	ederived2,
};

class CBase
{
public:

	CBase()
	{
		SetType(edefault);
		mpInt = 1;
	}

	~CBase(){}

	int mpInt;
	rtti mpType;

	virtual void SetType(rtti type)
	{
		mpType=type;
	}

	virtual void Disp(void)=0;
};

class CDerived1 : public CBase
{
public:

	CDerived1()
	{
		SetType(ederived1);
	}

	~CDerived1(){}

	float mpFloat;
	
	void Disp()
	{
		cout<<"derived attribute "<<mpInt<<endl
		<<"derived1 class''s attribute "<<mpFloat<<endl;
	}
};

class CDerived2 : public CBase
{
public:
		
	CDerived2()
	{
		SetType(ederived2);
		mpChar = new char;
	}

	~CDerived2(){delete mpChar;}

	char *mpChar;

	
	void Disp()
	{
		cout<<"derived attribute "<<mpInt<<endl
		<<"derived2 class''s attribute "<<mpChar<<endl;
	}
};

CBase *g_list[3];


void Init()
{
	CDerived1 *d1 = new CDerived1;
	CDerived2 *d2 = new CDerived2;
	CDerived2 *d3 = new CDerived2;


	d1->mpFloat = (float)rand();
	d2->mpChar = "whats up";
	d3->mpChar = "test";

	g_list[0] = d1;
	g_list[1] = d2;
	g_list[2] = d3;
}

int main(void)
{
	Init();

	g_list[0]->Disp();
	g_list[1]->Disp();

	CDerived1 *d;
	if(g_list[0]->mpType == ederived1)
	{
		d = (CDerived1*) g_list[0];
		d->mpFloat = 10.16;
	}

	g_list[0] = d;

	g_list[0]->Disp();



	float newvalue;
	CDerived1 *d2;
	if(g_list[0]->mpType == ederived1)
	{
		d2 = (CDerived1*) g_list[0];
		newvalue = d2->mpFloat;
	}

	g_list[0] = d2;
	
	cout<<"new value "<<newvalue<<endl<<endl;

	char *name;
	CDerived2 *d3;
	if(g_list[2]->mpType == ederived2)
	{
		d3 = (CDerived2*)g_list[2];
		name = d3->mpChar;
	}
	g_list[2] = d3;

	cout<<name<<endl;
	
	for(int i = 0; i < 2; i++)
		delete g_list[i];

	return 0;
}

quote:

---

Original post by no one
I just did a test on this and it seems that by casting a entity pointer to some derived class, you can access the members and get their correct values...

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class IFuelSupply // Interface for entities that supply Fuel

{
public:
   // Pure abstract functions

   virtual int FuelStore() = 0;        // How much fuel on board

   virtual int ReFuel(int amount) = 0; // Take 'amount' of fuel from store, unless

                                       // there is not enough, then take as much as

                                       // you can. Returns total fuel taken.

};

class IAmmoSupply // Interface for entities that supply Ammunition

{
public:
   // Pure abstract functions

   virtual int AmmoStore() = 0;	       // How much Ammo on board

   virtual int ReAmmo(int amount) = 0; // Take 'amount' of ammo from store, unless

                                       // there is not enough, then take as much as

                                       // you can. Returns total ammo taken.

};

// ... other interfaces.


class CEntity
{
   ...
};

class CFuelTanker : public CEntity, IFuelSupply
{
   ...

public:
   // Must be defined, since they are abstract in interface

   int FuelStore() { /* do whats necessary */ };
   int ReFuel(int amount) { /* etc */ };

   ...
};

class CCar : public CEntity
{
   ...

public:
   void OnCollision(CEntity* ent)
   {
      IFuelSupply* fs;
      int fuel_rcvd;

      fs = dynamic_cast<IFuelSupply*>(ent);

      if(fs != NULL)
      {
         // Entity has the IFuelSupply interface

         // Attempt to get 50 units of fuel

         fuel_rcvd = fs->ReFuel(50);

         // fuel_rcvd now hold the amount of fuel you managed

         // to get from the FuelSupply


         // add fuel_recd to fuel tank or whatever

         // etc

      }
   };
};

#include <cstdlib>
#include <iostream>

template<class T> //T must support the draw operation

void render(const T& obj) {
     obj.draw();
}

class a {
public:
   void draw() const {
	   std::cout << "CLASS A\n";
   }
};

class b {
public:
   void draw() const {
	   std::cout << "CLASS B\n";
   }
};

int main() {
	a _a;
	b _b;

	render(_a);
	render(_b);

	std::system("pause");

	return EXIT_SUCCESS;
}

quote:

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Original post by snk_kid
Which is the exact same as doing it by ABC classes but no runtime overhead.

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quote:

---

Original post by Archimage

quote:

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Original post by snk_kid
Which is the exact same as doing it by ABC classes but no runtime overhead.

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Although I see where you are coming from with the static polymorphism, it does put one major restriction on the system - that you (or more precisely the compiler) know exactly what derived-class each entity is at compile time.

The situation we''ve been discussing above, especially when applied to dynamically creating the entities via a run-time script, voids this possibility.

Archimage
(Ann-Marie Ratcliffe)


[edited by - Archimage on May 12, 2004 6:14:27 AM]

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quote:

---

Original post by snk_kid
I should have read the discussion properly, i just thought i would mention it because people tend to go overboard with dynamic polymorphism when most of the time it isn''t needed and most things tend to be known.

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quote:

---

Original post by no one
I just did a test on this and it seems that by casting a entity pointer to some derived class, you can access the memebers and get there correct values...

Heres a small program I just made to test this...

#include <iostream>
#include <string>

using namespace std;

enum rtti
{
	edefault,
	ederived1,
	ederived2,
};

class CBase
{
public:

	CBase()
	{
		SetType(edefault);
		mpInt = 1;
	}

	~CBase(){}

	int mpInt;
	rtti mpType;

	virtual void SetType(rtti type)
	{
		mpType=type;
	}

	virtual void Disp(void)=0;
};

class CDerived1 : public CBase
{
public:

	CDerived1()
	{
		SetType(ederived1);
	}

	~CDerived1(){}

	float mpFloat;
	
	void Disp()
	{
		cout<<"derived attribute "<<mpInt<<endl
		<<"derived1 class's attribute "<<mpFloat<<endl;
	}
};

class CDerived2 : public CBase
{
public:
		
	CDerived2()
	{
		SetType(ederived2);
		mpChar = new char;
	}

	~CDerived2(){delete mpChar;}

	char *mpChar;

	
	void Disp()
	{
		cout<<"derived attribute "<<mpInt<<endl
		<<"derived2 class's attribute "<<mpChar<<endl;
	}
};

CBase *g_list[3];


void Init()
{
	CDerived1 *d1 = new CDerived1;
	CDerived2 *d2 = new CDerived2;
	CDerived2 *d3 = new CDerived2;


	d1->mpFloat = (float)rand();
	d2->mpChar = "whats up";
	d3->mpChar = "test";

	g_list[0] = d1;
	g_list[1] = d2;
	g_list[2] = d3;
}

int main(void)
{
	Init();

	g_list[0]->Disp();
	g_list[1]->Disp();

	CDerived1 *d;
	if(g_list[0]->mpType == ederived1)
	{
		d = (CDerived1*) g_list[0];
		d->mpFloat = 10.16;
	}

	g_list[0] = d;

	g_list[0]->Disp();



	float newvalue;
	CDerived1 *d2;
	if(g_list[0]->mpType == ederived1)
	{
		d2 = (CDerived1*) g_list[0];
		newvalue = d2->mpFloat;
	}

	g_list[0] = d2;
	
	cout<<"new value "<<newvalue<<endl<<endl;

	char *name;
	CDerived2 *d3;
	if(g_list[2]->mpType == ederived2)
	{
		d3 = (CDerived2*)g_list[2];
		name = d3->mpChar;
	}
	g_list[2] = d3;

	cout<<name<<endl;
	
	for(int i = 0; i < 2; i++)
		delete g_list[i];

	return 0;
}

I feel thats the answer. Now, ill just have to come up with a clean way of doing all of that nasty code.

Thanks.

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quote:

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Original post by snk_kid
Now i had better look at this discussion ;-) wouldn't it better to have type ID stuff as class members instead of instance members if you get what i mean

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