how to init static 2d arrays of a class

[kvikram]

Hi guys,
i have written a class:

class myclass{
private:
	int a;
public:
	static int b;
	static int c[10][10];

	myclass(){
		a = 0;
	}
};

i have couple of questions:
1. when i initialize b outside main using int myclass::b = 4; i get no erros
but when i use the same inside main i get this error: "error C2086: 'int myclass::b' : redefinition" why is that?


2. ok even if the above works, i am not able to initialize c at all (either outside or inside main)
like writing "int myclass::c[2][2] = 23;" gives error:
'initializing' : cannot convert from 'int' to 'int [2][2]'
does that mean i cant declare 2d static arrays?
is there any other way of making an array that is common to all objects of that class?

thanks.

[chinalinux]

1. because of the modifier myclass::, if you remove it, like this: int b=0; you will get no error too. but if you did this, it means you define a local variable in the main, not the initilization of the variable b in the myclass.

2. int myclass::c[2][2] = {{23,23}}; here is a correct example to intilize the c[0][0] to (23,23). hope you got that already.

[Gaminic]

Your code doesn't make sense. You're trying to assign values to a Class variable (which doesn't exist). Only Objects of a class have variables. Before you can manipulate those values, you have to instantiate an Object of that Class.

myclass myObject = myObject();
myObject.b = 4;

That'll work.

In the case that class-variable-assigning does for some reason work, you'd still have to drop the 'int' part of your statement. You only use that for declaring new variables.

[mik2718]

I think you need the global int myclass::b = 4; to define the variable.

If you have done this in global scope you can then access the variable in main using myclass::b = 3; (whatever)
(since this is effectively a global variable and you have made it public in the class)

chinalux is right r.e. the global myclass::c variable. You need to use C style array initialisation

[kvikram]

thanks guys,
using this works:

#include <iostream.h> 
#include <stdafx.h>
using namespace std; 

class myclass{
private:
	int a;
public:
	static int b;
	static int c[2];

	myclass(){
		a = 0;
	}
};


int myclass::b = 4;
int myclass::c[2] = {1,2};

int main(){
	myclass ob;
	
}

but i dont understand why i must define those variables b and c outside main.. why does it give me an error if i define them inside main.

[mik2718]

When you have static data in a class this is just the same as global data.
(except the name is myclass::b instead of just b)

Global data is all initialised before main even starts. If fact the variable int myclass::b = 4; will already have a space in the
executable file with its initialised value.

Thats why you get a redefinition error when you try to create the variable in main.

[kvikram]

ok I understood that. but what if my static array 'c' was a huge one, of say size 100, and i did not wish to initialize it directly using {1,2,3,....100} but instead wanted to use a for loop.
how will i do that?

i obviously cant use a for loop outside main to initialize the array, so i will have to use a function like this:

class myclass{
private:
	int a;
public:
	static int b;
	static int c[2];

	myclass(){
		a = 0;
	}
};


void func(){
	int myclass::b = 4;
	for(int i = 0 ; i < 100; i ++)
		int myclass::c[i] = 5;
}

int main(){
	myclass ob;
	func();
	
}

but as you might expect i get errors in this too.. (redefinition and cant convert int to int[] etc)

[mik2718]

In C if there are fewer initialisers then the array size then it is filled with zeros

int c[100] = { 0 };

should give you an array of 100 zeros

class myclass{
private:
	int a;
public:
	static int b;
	static int c[100];

	myclass(){
		a = 0;
	}
};

// need to declare globally
int myclass::b = 0;
int myclass::c[100] = { 0 };

void func(){
	myclass::b = 4;  // access global variable
	for(int i = 0 ; i < 100; i ++)
		myclass::c[i] = 5; // access global variable
}

int main(){
	myclass ob;
	func();
	
}

You only initialise once in the global space. After that you can change the value programmatically if you need to

[kvikram]

Thank a lot mik2718! that answers my question!