C / C++ / MFC

Re: How to Delete List of files in a Folder

15-Dec-07 20:32

DeleteFile[^]?!

--
gleat
http://blogorama.nerdworks.in[^]
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Hamid_RT16-Dec-07 2:10

Hamid_RT

16-Dec-07 2:10

Of course I suggest use of SHFileOperation because it ask of you for confirm.

The type of *this

George_George15-Dec-07 19:50

15-Dec-07 19:50

Hello everyone,

I am confused what is the type of *this? Type of reference to instance or type of instance?

Looks like both code can compile, no warning messages. Any ideas?

class A{

	A& foo1()
	{
		return *this;
	}

	A foo2()
	{
		return *this;
	}
};

thanks in advance,
George

santhoshv8415-Dec-07 20:06

santhoshv84

15-Dec-07 20:06

Hi,

The this pointer is a pointer accessible only within the member functions of a class, struct, or union type. It points to the object for which the member function is called. Static member functions do not have a this pointer.

Smile | :)

// Example of the this pointer
void Date::setMonth( int mn )
{
month = mn; // These three statements
this->month = mn; // are equivalent
(*this).month = mn;
}

Thanks and Regards.
SANTHOSH V

foobar0715-Dec-07 20:19

foobar07

15-Dec-07 20:19

THANKS . I am also confused aboout 'this'.but now got it. Big Grin | :-D

George_George15-Dec-07 21:57

15-Dec-07 21:57

Thanks SANTHOSH,

My question is not what is this pointer, but when we return *this, should we treat return type as reference to object type, object type, or both are ok?

regards,
George

Rajasekharan Vengalil15-Dec-07 20:35

15-Dec-07 20:35

foo1 returns a reference to the object pointed at by this. foo2 returns a copy of the object pointed at by this.

--
gleat
http://blogorama.nerdworks.in[^]
--

George_George15-Dec-07 21:55

15-Dec-07 21:55

Thanks gleat,

You mean if we invoke,

1. fun1, like A& a = fun1(), then return a reference;
2. fun2, like A a = fun2(), return a new copy of instance (invokes assignment operator on A)

Both are good code, right?

regards,
George

Rajasekharan Vengalil15-Dec-07 22:03

15-Dec-07 22:03

Yep. That's the idea. As for whether its good code or not depends on what you want to do. Some simple thumb-rules are:

Don't pass around data heavy structures by value as the copying overhead can prove to be a performance drain. Be especially wary of passing them via STL (or any other) collection types. Imagine returning a vector<BigHeavyClass> containing 500 items by value!
When passing around objects by reference remember to use const if you do not intend to alter the object's state via that reference.

--
gleat
http://blogorama.nerdworks.in[^]
--

George_George15-Dec-07 22:11

15-Dec-07 22:11

Thanks for your advice, gleat!

My question is answered.

regards,
George

Gary R. Wheeler16-Dec-07 4:20

Gary R. Wheeler

16-Dec-07 4:20

George_George wrote:
(invokes assignment operator on A)

Not exactly true. It invokes the copy constructor, which is not the same thing.

Software Zen: delete this;

Fold With Us![^]

George_George16-Dec-07 20:27

16-Dec-07 20:27

Thanks Gary, you are correct. I want to confirm with you that,

1.

A a = fun2() will invoke copy constructor on a;

2.

A a;
a = fun2();

will invoke assignment operator on a.

Both are correct?

regards,
George

Gary R. Wheeler18-Dec-07 14:36

Gary R. Wheeler

18-Dec-07 14:36

Since foo1 returns a reference (which is really just a pointer), it can just return the address of the 'this' object.

The other function, foo2 actually returns an object of type A. In order to do that, it invokes the copy constructor for A, copying the contents of this to the destination.

Software Zen: delete this;

Fold With Us![^]

George_George18-Dec-07 18:49

18-Dec-07 18:49

Thanks Gary,

How about your comments on my two questions before?

--------------------
1.

A a = fun2() will invoke copy constructor on a;

2.

A a;
a = fun2();

will invoke assignment operator on a.

Both are correct?
--------------------

regards,
George

Gary R. Wheeler19-Dec-07 12:29

Gary R. Wheeler

19-Dec-07 12:29

I believe your statements are both true. The compiler is supposed to turn #1 into this:

A a(fun2());

automatically, which is a call to the copy constructor.

#2 obviously calls the assignment operator. Note that the default constructor for A is called first, when the variable is declared.

Software Zen: delete this;

Fold With Us![^]

George_George19-Dec-07 18:52

19-Dec-07 18:52

Thanks for your clarification, Gary!

regards,
George

CPallini15-Dec-07 22:14

CPallini

15-Dec-07 22:14

George_George wrote:

class A{

A& foo1()
{
return *this;
}

A foo2()
{
return *this;
}
};

Well, there's nothing special related to this in the above snippet:
you can do as well:

class B
{
  int i;
  int * p;
...
  int getiValue(){ retirn i;}
  int & getiRef(){ return i;}
  int getpValue(){ return *p;}
  int & getpRef(){ return *p;}
...
}

it is simply C++ syntax for dealing with references.

Smile | :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

George_George15-Dec-07 22:40

return reference to local variable, good code?

15-Dec-07 22:40

Cool, CPallini! Thanks, and my question is answered.

regards,
George

George_George15-Dec-07 18:46

Re: return reference to local variable, good code?

15-Dec-07 18:46

Hello everyone,

I am reading some code from other people, there are some code like this,

class Foo {

};

Foo& func()
{
	Foo foo;
	return foo;
}

int main()
{
	Foo& foo = func();

	return 0;
}

I want to confirm with you that it is not good code, since we return a reference to local object instance, right? Even if the code has expected function currently.

thanks in advance,
George

Rajasekharan Vengalil15-Dec-07 20:38

Re: return reference to local variable, good code?

15-Dec-07 20:38

Yep! Whatever else it is, it most certainly is NOT good code! The local instance of foo inside func will get deallocated when control returns from func. So the reference foo in main is undefined.

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gleat
http://blogorama.nerdworks.in[^]
--

George_George15-Dec-07 21:31