Programming Forums > Application Development > C++

Harakim · May 12th, 2006, 1:46 PM

In c++, you have to use destructors. I'm having some problems with this as mine never seem to get called.

Anyway, my main class, `Game`, is used until the return statement of the main() return statement. If my operating system is just going to delete all my program memory, I figure I may as well let the operating system destruct it for me.

So I have three questions:
1) Does the operating system free all memory allocated by a program on exit?
2) Would this method work on all major operating systems?

Twilight · May 12th, 2006, 3:12 PM

A Windows and Linux system will only deallocate static memory. If you create by doing this

(Toggle Plain Text)

variable something = new something

then the operating system will not delete it. When an OS deallocates a class though, it will call your destructors for you. But if there is nothing in the dtor to delete it, it will not get deleted.

As for number 2, I know this is how it works for Windows and Linux, presumably it would work the same way on other OS's. It likely has more to do with the compiler really.

Cache · May 12th, 2006, 4:03 PM

The only time an object' destructor would not be called is when it's constructor did not fully construct the object. I imagine you'd know about it if that was happening. If you want to dynamically allocate an object without needing to explicitly call delete, then consider using std::auto_ptr.

ex:

(Toggle Plain Text)

#include <memory>

class MyClass
{
};

typedef std::auto_ptr<MyClass> MyClass_ptr;

int main()
{
	MyClass_ptr myClass( new MyClass );
	return 0;
}

Just make sure you understand auto_ptr's first. Don't use them in standard containers ect.

DaWei · May 12th, 2006, 4:58 PM

&c ...

Harakim · May 12th, 2006, 6:57 PM

Wow. Thanks for the information guys. I came back hoping someone answered and already three people did.

The Dark · May 12th, 2006, 7:36 PM

Quote:

Originally Posted by Twilight

A Windows and Linux system will only deallocate static memory. If you create by doing this

(Toggle Plain Text)

variable something = new something

then the operating system will not delete it. When an OS deallocates a class though, it will call your destructors for you. But if there is nothing in the dtor to delete it, it will not get deleted.

As for number 2, I know this is how it works for Windows and Linux, presumably it would work the same way on other OS's. It likely has more to do with the compiler really.

Under Windows and Linux, any memory you allocate using new will be freed when the program exits. The destructors for objects allocated in this way will not be called without a call to delete.
That said, you should call delete for each new in your program, just in case you ever want to include the code in some other program, or put a loop around it.

Jason Isom · May 12th, 2006, 7:41 PM

Quote:

Originally Posted by Harakim

In c++, you have to use destructors. I'm having some problems with this as mine never seem to get called.

Anyway, my main class, `Game`, is used until the return statement of the main() return statement. If my operating system is just going to delete all my program memory, I figure I may as well let the operating system destruct it for me.

So I have three questions:
1) Does the operating system free all memory allocated by a program on exit?
2) Would this method work on all major operating systems?

Before you start using auto_ptr as the end-all fix for this, you might want to look into why you think the destructors aren't getting called.

I think you'll find that they are being called, it's just that the method you're using to determine if it reached the destructor is invalid. Are you using std::cout or some form of Message Box? The reason I ask, is because the order that objects are destroyed isn't defined, so I'd imagine for you to "test" if the destructor is called try doing the following:

(Toggle Plain Text)

int main()
{
    {
        Game g;
    } // this guarantees that g's destructor will be called before your program exits
    return 0;
}

I know it looks silly, but by forcing the block-scope for your Game instance, it will go out of scope and get destructed before the rest of your program does.

(If I'm way off base, someone please correct me. It's been awhile since I've used C++ but I remember running into a similar issue when I was learning it)

Cache · May 12th, 2006, 7:52 PM

Quote:

Originally Posted by Jason Isom

the order that objects are destroyed isn't defined

Maybe not in C#. In C++ objects are destructed in reverse order to there construction. FILO, if I remeber correctly.

Jason Isom · May 12th, 2006, 8:20 PM

Quote:

Originally Posted by Cache

Maybe not in C#. In C++ objects are destructed in reverse order to there construction. FILO, if I remeber correctly.

Well, that makes my entire post invalid so please ignore.

Harakim · May 13th, 2006, 2:15 AM

Quote:

Originally Posted by Jason Isom

(Toggle Plain Text)

int main()
{
    {
        Game g;
    } // this guarantees that g's destructor will be called before your program exits
    return 0;
}

I know it looks silly, but by forcing the block-scope for your Game instance, it will go out of scope and get destructed before the rest of your program does.

(If I'm way off base, someone please correct me. It's been awhile since I've used C++ but I remember running into a similar issue when I was learning it)

I actually have read this method several places and I am glad that you posted in case I hadn't known it.
As for my constructor not being called, it's one of those things you don't like to tell anyone, but:
I was editting source in one directory and compiling in another. No matter how I changed the program, it was still using the old code. Heh, live and learn.

May 12th, 2006, 1:46 PM	#1
Harakim Hobbyist Programmer Join Date: May 2006 Location: West Jordan, Utah, United States Posts: 176 Rep Power: 3	Destructors and how the OS handles program memory In c++, you have to use destructors. I'm having some problems with this as mine never seem to get called. Anyway, my main class, `Game`, is used until the return statement of the main() return statement. If my operating system is just going to delete all my program memory, I figure I may as well let the operating system destruct it for me. So I have three questions: 1) Does the operating system free all memory allocated by a program on exit? 2) Would this method work on all major operating systems?

May 12th, 2006, 3:12 PM	#2
Twilight Programmer Join Date: Apr 2006 Location: Calgary, Alberta Posts: 67 Rep Power: 3	A Windows and Linux system will only deallocate static memory. If you create by doing this (Toggle Plain Text) variable something = new something variable something = new something then the operating system will not delete it. When an OS deallocates a class though, it will call your destructors for you. But if there is nothing in the dtor to delete it, it will not get deleted. As for number 2, I know this is how it works for Windows and Linux, presumably it would work the same way on other OS's. It likely has more to do with the compiler really.

May 12th, 2006, 4:03 PM	#3
Cache Hobbyist Join Date: Sep 2005 Posts: 266 Rep Power: 4	The only time an object' destructor would not be called is when it's constructor did not fully construct the object. I imagine you'd know about it if that was happening. If you want to dynamically allocate an object without needing to explicitly call delete, then consider using std::auto_ptr. ex: (Toggle Plain Text) #include <memory> class MyClass { }; typedef std::auto_ptr<MyClass> MyClass_ptr; int main() { MyClass_ptr myClass( new MyClass ); return 0; } #include <memory> class MyClass { }; typedef std::auto_ptr<MyClass> MyClass_ptr; int main() { MyClass_ptr myClass( new MyClass ); return 0; } Just make sure you understand auto_ptr's first. Don't use them in standard containers ect.

May 12th, 2006, 4:58 PM	#4
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	&c ... __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

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May 12th, 2006, 6:57 PM	#5
Harakim Hobbyist Programmer Join Date: May 2006 Location: West Jordan, Utah, United States Posts: 176 Rep Power: 3	Wow. Thanks for the information guys. I came back hoping someone answered and already three people did.

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Programming Forums > Application Development > C++

Destructors and how the OS handles program memory