problem with classes and pointers

Akkernight · 02-20-2009, 02:37 PM

WARNING: Not very clean code xP

Well, I have this class called player.

Code:

class player : public character{

public:
	player();
	~player();

	float speed;

private:
	//hgeQuad

};

And then I have this class called character.

Code:

class character {

public:
	character();
	~character();

	float x, y;
	static const int gravityPull = 1;

	void gravity();
	bool gravEffected();

};

And in the main.cpp file I call player* P1, this crashes the game, and I've had laods of problems, when I call it as a pointer, but not when I call it like; player P1, why is this?
I still havn't figured out the differences between the two calls...
The thing I'm having problem with is the x and y from character, by the way.
Thanks in advance

Meldreth · 02-20-2009, 03:00 PM

when you use a pointer, are you allocating memory to it or pointing it to an object that already exists? you have to do one of those before you can use the pointer.

Elysia · 02-20-2009, 03:05 PM

I assume you are doing this?

Code:

player* p1;
p1->whatever();

If yes, then Meldreth has already told you the answer.
A pointer is a variable that contains a memory address. It does not create storage; it must be pointed to a valid storage before use.

Akkernight · 02-20-2009, 03:21 PM

Well, I was doing this

Code:

if(hge->Input_GetKeyState(HGEK_LEFT)) P1->x -= P1->speed * dt;
	if(hge->Input_GetKeyState(HGEK_RIGHT)) P1->x += P1->speed * dt;

	if(P1->gravEffected()) P1->gravity();

Now I'm doing this

Code:

if(hge->Input_GetKeyState(HGEK_LEFT)) P1.x -= P1.speed * dt;
	if(hge->Input_GetKeyState(HGEK_RIGHT)) P1.x += P1.speed * dt;

	if(P1.gravEffected()) P1.gravity();

the first one didn't work

Elysia · 02-20-2009, 03:22 PM

Does P1 point to a valid instance?

Akkernight · 02-20-2009, 03:30 PM

huh? o.O
P1 is declared like... player * P1, or was... Now it's: player P1
You see the player class on the thread start

Elysia · 02-20-2009, 03:31 PM

http://apps.sourceforge.net/mediawik...llocating_them

Akkernight · 02-20-2009, 04:23 PM

That really doesn't tell me anything at all xP
I've figured out what my real question is now xD
When should I use pointers to classes? Like, if I have a class that I'll use often, like player (there'll be atleast 2 players) should it then be pointed to or not? like what is more right:

player* P1;
player* P2;

Or:

player P1;
player P2;

Elysia · 02-20-2009, 04:26 PM

Use pointers to point to the same object. Like when you need to keep references to objects.
When you do not need pointers, do not use them.

tabstop · 02-20-2009, 04:55 PM

Quote:

Originally Posted by Akkernight

That really doesn't tell me anything at all xP
I've figured out what my real question is now xD
When should I use pointers to classes? Like, if I have a class that I'll use often, like player (there'll be atleast 2 players) should it then be pointed to or not? like what is more right:

player* P1;
player* P2;

Or:

player P1;
player P2;

The first example has 0 players; the second example has 2 players. Which do you want?

Akkernight · 02-20-2009, 05:12 PM

Awsome! Thanks

Pointers are really simple when you think of it, but still they're a pain in the ass sometimes xD

Elysia · 02-20-2009, 05:14 PM

But as said, pointers do not actually create objects. They will point to them, and it's your job to thus create objects and make sure those pointers actually point to those objects later.
There's the difference.

matsp · 02-20-2009, 06:03 PM

Also, in a case where inheritance is involved, pointers are important to manage objects of different types.

Let's say we have:

Code:

class character {

public:
	character();
	virtual~character();

	float x, y;
	static const int gravityPull = 1;

	void gravity();
	bool gravEffected();

};

class player: public character
{
  ... stuff here. 
};

class monster: public character
{
   .... other stuff here. 
};

// Helper to the player - a "robot" that helps the player. 
class helper: public character
{
    ... stuff to keep track of a player and "help" the player. 
};

Now, in some cases, you will certainly KNOW which type you are dealing with, but in other places, you may well have a situation where you do not know what sort of derived character you are dealing with. In these situations you MUST use a pointer or reference that refers to the actual character.

Also note that teh destructor of any type that inherits should be virtual. Otherwise, any case where you GENERICALLY destroy an object, it will call the wrong destructor.

[And by the way, I'm by no means saying that your monsters should be based on character - I was just lacking in imagination as to what type of classes could derive from character, and monster was the first one to come to mind].

--
Mats

Akkernight · 02-20-2009, 06:10 PM

Well, this game won't have monsters exactly, it'll have players, then I'll make computer AI players too, if I can figure how

But I'll be putting them under character too, shouldn't I be doing this?
And I should add virtual to the ~character() ?

matsp · 02-20-2009, 06:14 PM

Quote:

Originally Posted by Akkernight

Well, this game won't have monsters exactly, it'll have players, then I'll make computer AI players too, if I can figure how

But I'll be putting them under character too, shouldn't I be doing this?
And I should add virtual to the ~character() ?

Yes, if you inherit from a class, the destructor should be virtual. That's a rule.

Whether you should or should not inherit from the same base for the computer controlled and the human-controlled players, I can't answer, as I don't know if a "AI player" is-a character or not. If it is, then, yes, it should inherit. It isn't, then you shouldn't...

--
Mats

02-20-2009, 02:37 PM	#1
Akkernight Hail to the king, baby. Join Date: Oct 2008 Location: Faroe Islands Posts: 713	problem with classes and pointers WARNING: Not very clean code xP Well, I have this class called player. Code: class player : public character{ public: player(); ~player(); float speed; private: //hgeQuad }; And then I have this class called character. Code: class character { public: character(); ~character(); float x, y; static const int gravityPull = 1; void gravity(); bool gravEffected(); }; And in the main.cpp file I call player* P1, this crashes the game, and I've had laods of problems, when I call it as a pointer, but not when I call it like; player P1, why is this? I still havn't figured out the differences between the two calls... The thing I'm having problem with is the x and y from character, by the way. Thanks in advance __________________ I deny the Holy Spirit; burn me. http://blasphemychallenge.com/ <- Do you dare? Last edited by Akkernight; 02-20-2009 at 02:52 PM.
Akkernight is offline

02-20-2009, 03:00 PM	#2
Meldreth Registered User Join Date: Feb 2009 Posts: 138	when you use a pointer, are you allocating memory to it or pointing it to an object that already exists? you have to do one of those before you can use the pointer.
Meldreth is offline

02-20-2009, 03:21 PM	#4
Akkernight Hail to the king, baby. Join Date: Oct 2008 Location: Faroe Islands Posts: 713	Well, I was doing this Code: if(hge->Input_GetKeyState(HGEK_LEFT)) P1->x -= P1->speed * dt; if(hge->Input_GetKeyState(HGEK_RIGHT)) P1->x += P1->speed * dt; if(P1->gravEffected()) P1->gravity(); Now I'm doing this Code: if(hge->Input_GetKeyState(HGEK_LEFT)) P1.x -= P1.speed * dt; if(hge->Input_GetKeyState(HGEK_RIGHT)) P1.x += P1.speed * dt; if(P1.gravEffected()) P1.gravity(); the first one didn't work __________________ I deny the Holy Spirit; burn me. http://blasphemychallenge.com/ <- Do you dare?
Akkernight is offline

02-20-2009, 03:30 PM	#6
Akkernight Hail to the king, baby. Join Date: Oct 2008 Location: Faroe Islands Posts: 713	huh? o.O P1 is declared like... player * P1, or was... Now it's: player P1 You see the player class on the thread start __________________ I deny the Holy Spirit; burn me. http://blasphemychallenge.com/ <- Do you dare?
Akkernight is offline

02-20-2009, 04:23 PM	#8
Akkernight Hail to the king, baby. Join Date: Oct 2008 Location: Faroe Islands Posts: 713	That really doesn't tell me anything at all xP I've figured out what my real question is now xD When should I use pointers to classes? Like, if I have a class that I'll use often, like player (there'll be atleast 2 players) should it then be pointed to or not? like what is more right: player* P1; player* P2; Or: player P1; player P2; __________________ I deny the Holy Spirit; burn me. http://blasphemychallenge.com/ <- Do you dare?
Akkernight is offline

02-20-2009, 05:12 PM	#11
Akkernight Hail to the king, baby. Join Date: Oct 2008 Location: Faroe Islands Posts: 713	Awsome! Thanks Pointers are really simple when you think of it, but still they're a pain in the ass sometimes xD __________________ I deny the Holy Spirit; burn me. http://blasphemychallenge.com/ <- Do you dare?
Akkernight is offline

02-20-2009, 06:03 PM	#13
matsp Kernel hacker Join Date: Jul 2007 Location: Farncombe, Surrey, England Posts: 15,686	Also, in a case where inheritance is involved, pointers are important to manage objects of different types. Let's say we have: Code: class character { public: character(); virtual~character(); float x, y; static const int gravityPull = 1; void gravity(); bool gravEffected(); }; class player: public character { ... stuff here. }; class monster: public character { .... other stuff here. }; // Helper to the player - a "robot" that helps the player. class helper: public character { ... stuff to keep track of a player and "help" the player. }; Now, in some cases, you will certainly KNOW which type you are dealing with, but in other places, you may well have a situation where you do not know what sort of derived character you are dealing with. In these situations you MUST use a pointer or reference that refers to the actual character. Also note that teh destructor of any type that inherits should be virtual. Otherwise, any case where you GENERICALLY destroy an object, it will call the wrong destructor. [And by the way, I'm by no means saying that your monsters should be based on character - I was just lacking in imagination as to what type of classes could derive from character, and monster was the first one to come to mind]. -- Mats __________________ Compilers can produce warnings - make the compiler programmers happy: Use them! Please don't PM me for help - and no, I don't do help over instant messengers.
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02-20-2009, 06:10 PM	#14
Akkernight Hail to the king, baby. Join Date: Oct 2008 Location: Faroe Islands Posts: 713	Well, this game won't have monsters exactly, it'll have players, then I'll make computer AI players too, if I can figure how But I'll be putting them under character too, shouldn't I be doing this? And I should add virtual to the ~character() ? __________________ I deny the Holy Spirit; burn me. http://blasphemychallenge.com/ <- Do you dare?
Akkernight is offline

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