Simple question

[Factors]

What determines the size of a class within an object when that class is being used as the object schematic's/type.
Example:

#include <iostream>
using namespace std;

class test{
	  public:
	  		 int test1,test2,test3;
	  		 test(){test1=0;test2=0;test3=0;}
};

int main(){
	test ob;int i;
	cout<<sizeof(ob)<<endl;
	cin>>i;
}

What is it within the class /data type test that determines the sizeof object ob?. Or a better question would be, what is the basic layout/blueprints of an object?.

[oonej]

the size of yours ...

ints are 4 bits ... so 3 * 4 = size of

variables, add up,

arrays are size of the array (number of elements) * the type..

[Factors]

Is it only that? what about function's ?.

[ui uiho]

NEVER make variable public.

this is an example of a class

class myclass{
private:
int number;
char letter;

public:

int returnnumber(){
return number;
}

char returnletter(){
return letter;
}

myclass(int,char);
};

myclass::myclass(int i, char c){    //constructor 
number = i;
letter = c;
}

[Factors]

I know that using public: to initialize variable is a horrible thing but it was just for the sake of the example.
Ps. If a variable is initialized in private, does that mean that the object will be affected by it(memory wise)?.

[Gaminic]

What? Public members are a feature for a reason. Saying "never" is stupid. There are good reason to use them.

Anyway, object size is slightly more complex than "count the numbers" due to alignment. Most (?) systems are aligned on a 4byte boundary. To enforce this, padding is added to align the member variables.

struct myStruct {
     int a; // 4bytes
     char b; // 1byte + 3bytes padding
     int c; // 4bytes
     bool d; // 1byte + 3bytes padding
     int e; // 4bytes
}

struct myStruct2 {
     int a; // 4bytes
     int c; // 4bytes
     int e; // 4bytes
     char b; // 1byte
     bool d; // 1byte + 2bytes padding
}

An object of the first class will have a size of 20bytes. An object of the second class will be 16bytes in size.

This "packing" of variables can make significant differences. If you're going to have a million objects of a certain type, saving 4bytes per object is pretty neat.

Secondly, this also means you sometimes have "free space". I can add 2bytes of data to myStruct2 without increasing the size of the objects. That's a free short, or two chars, or two bools, ...

[ui uiho]

i say never because if you just want a set of variables struct works great. and if you want the value of a variable do

datatype returnvalue(){
return value;
}

easy enough...

[Gaminic]

Congratulations, you've managed to get the exact same result in 40 more characters. What's the point of making something private if you're just going to provide accessors anyway?

Don't make something private "BECAUSE I MUST". Do so because it makes sense, or don't because it doesn't. Anyone who brings up a rule that begins with "never" or "always" deserves a kick in the groin.

[zb0S216C]

Factors wrote:

Is it only that? what about function's ?.

Member functions do not contribute to the overall size of a class. However, a virtual table does. Member functions are merely functions that are associated with a class.

ui uiho wrote:

NEVER make variable public.

A class doesn't have to encapsulate its members. It doesn't break the rules of the class if data members are public. A class can be used to ensure that something is accomplished during construction/destruction.

_Wazzak