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Objects at runtime

General and Gameplay Programming Programming

Started by Endar February 16, 2005 01:33 AM

4 comments, last by iMalc 19 years, 2 months ago

Endar

668

Author

February 16, 2005 01:33 AM

Okay, this isn't really a question, its just to confirm something. After C++ classes are compiled, and a program is run with them, the member functions turn out like this:


// Start here
class A{
     void function1();
};

// End up here
void function1(A* aObject)
{
     // blah
}

Right?? So, if there was a situation like:


class B{
     bool value;
     int integer;
     float y;
     string hello;
     char c;

     void function1(){
          // blah
     }

     int function2(){
          // blah
     }
};

Then basically, the expression "sizeof(B)" would end up being the size of all the member variables. Right? okay, assuming that we have that right, which is good for me, but I was just struck with something else, what if in that class you have static variables? I assume that they wouldn't be counted with the 'sizeof' expression. So, what, are they just left to fend for themselves?

[size="2"][size=2]Mort, Duke of Sto Helit: NON TIMETIS MESSOR -- Don't Fear The Reaper

lucky_monkey

440

February 16, 2005 04:54 AM

Quote:Original post by Endar
Okay, this isn't really a question, its just to confirm something.

After C++ classes are compiled, and a program is run with them, the member functions turn out like this:

*** Source Snippet Removed ***

Right??

So, if there was a situation like:

*** Source Snippet Removed ***

Then basically, the expression "sizeof(B)" would end up being the size of all the member variables. Right?

okay, assuming that we have that right, which is good for me, but I was just struck with something else, what if in that class you have static variables? I assume that they wouldn't be counted with the 'sizeof' expression. So, what, are they just left to fend for themselves?

Yeah. So the following program outputs

sizeof(A): 8sizeof(B): 8

#include <cstdlib>#include <iostream>class A {	int a;	int b;	int func() {return a; };};class B {	int a;	int b;	static int c;	static int d;	int func() {return a; };};int B::c;int B::d;int main() {		std::cout << "sizeof(A): " << sizeof(A) << std::endl;	std::cout << "sizeof(B): " << sizeof(B) << std::endl;	system("pause");	return 0;}

Emmanuel Deloget

1,382

February 16, 2005 05:05 AM

Quote:Original post by Endar
Okay, this isn't really a question, its just to confirm something.

After C++ classes are compiled, and a program is run with them, the member functions turn out like this:
// Start hereclass A{     void function1();};// End up herevoid function1(A* aObject){     // blah}
Right??

Well, this is nearly true, but is perhaps oversimplified. In most compilers, this is not sent to the function as the other parameters. For example, VC++ moves this into ecx while it pushes the other parameters on the stack.

But this is only a slight (implementation) difference :)

Regards,

-- Emmanuel D. [blog, in French] [blog, very bad googlized translation]

nmi

978

February 16, 2005 05:09 AM

If you use virtual functions, your compiler adds a pointer to the vtable to the data of your class, so:

class A {
int a;
void func() {}
};

class B {
int b;
virtual void func() {}
};

sizeof(A) is 4
sizeof(B) is 8

BTW what does:
class C {}
sizeof(C) = ?

return ?

For gcc sizeof(C) is 1.

Oxyd

1,162

February 16, 2005 07:22 AM

Quote:Original post by nmi
BTW what does:
class C {}
sizeof(C) = ?

return ?

For gcc sizeof(C) is 1.

That's correct. The compiler inserts one byte there - nothing can be 0 bytes long - imagine an array of such 0-bytes-long objects - the address of 1st element would be the same as the address of 10000000th element. That would cause big troubles.

Oxyd

iMalc

2,466

February 16, 2005 12:23 PM

The static variables go in the globals data area.

"In order to understand recursion, you must first understand recursion."
My website dedicated to sorting algorithms

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