[Newbie Question] Static members

[Manuel]

Hi!
Anyone can explain me why this work:

--------------------------
class Rect
{
public:
Rect()
{num ++;}

~Rect()
{num --;}

static void pippo(){num ++;};

protected:
static int num;

};
int num = 0;

main(){}
-------------------------

and this not work?

-------------------------
class Rect
{
public:
Rect()
{num ++;}

~Rect()
{num --;}

static void pippo();

protected:
static int num;

};
int num = 0;
void Rect:ippo() {num ++;}
main(){}

 

[Marchello]

and this not work?

-------------------------
class Rect
{
public:
Rect()
{num ++;}

~Rect()
{num --;}

static void pippo();

protected:
static int num;

};
int num = 0; // Maybe errors occurs here ?!
void Rect:ippo() {num ++;}
main(){}

try to write:
int Rect::num = 0;

 

[Jim Langston]

Hi!
Anyone can explain me why this work:

--------------------------
class Rect
{
public:
Rect()
{num ++;}

~Rect()
{num --;}

static void pippo(){num ++;};

protected:
static int num;

};
int num = 0;

main(){}
-------------------------

and this not work?

-------------------------
class Rect
{
public:
Rect()
{num ++;}

~Rect()
{num --;}

static void pippo();

protected:
static int num;

};
int num = 0;
void Rect:ippo() {num ++;}
main(){}

Actually, it's fairly simple. Rect:ippo() is static. Which means it's
only supposed to be used on static variables. They have no this pointer.
Neighter of those should work.

change num to static and it should work.

static int num;

then outside the class

int Rect::num = 0;

I can not tell you why your first version compiled. It shouldn't of as far
as I'm aware.

 

[Manuel]

try to write:
int Rect::num = 0;

It work!
Thanks you very much!

 

[majianan]

I have seen the first program in the <Effective C++> : Item 14:Make
sure base classes have virtual destructors.

 

[Manuel]

I have seen the first program in the <Effective C++> : Item 14:Make
sure base classes have virtual destructors.

What's <Effective C++> ?
A template for debug?

 

[Manuel]

change num to static and it should work.

static int num;

This was already done...or you mean I should write this in some other
places?

then outside the class

int Rect::num = 0;

Yes. This was the error!
Thanks,

Manuel

 

[Bob Hairgrove]

What's <Effective C++> ?
A template for debug?

One of several very useful books by Scott Meyers.

 

[Manuel]

One of several very useful books by Scott Meyers.

Ooops...sorry for stupid question...

 

[bvatsa]

U must use Class scope, while acessing Static class members,...
I think u need to brush-up c++ once...