when to use "new"

[Rv5]

Rookie c++ question, but Ive spent the last 5 years doing Java, where
everytime I created an object I used new. In c++ I can create my objects
without and its confusing me just a little.

I have a class called polynomial. Its a nothing little class right now,
with just int variables, a basic container class. Im using it as I go
through some tutorials, but in this particular tutorial its telling me to do
polynomial *first = new polynomial();
but before I found this site I was just doing
polynomial first;
Im also struggling through pointers. I understand the basics, but fail to
see the advantage using them with my objects so quickly. Is one way better
than the other?

Thanks

[Gernot Frisch]

"Sam" <ma*******@hotm ail.com> schrieb im Newsbeitrag
news:cn******** **@kermit.esat. net...

"Gernot Frisch" <[e-mail] >
дÈëÏûÏ¢ÐÂÎÅ:2v *************@u ni-berlin.de...

<snip>

Now we get a problem. You can write the following Java codes:
Polynomial createNew() {
return new Polynomial();
}

<snap>

Why does one want to do this:

Polynomial* p1 = OtherPoly.creat eNew();

instead of:
Polynomial p1 = OtherPoly;

I didn't mean that createNew() is a method of class Polynomial.
However, no matter whether it is or not. It's not my point. I was
just saying that, you can easily create a new object in a function
in Java and return its handle(or reference, or pointer) to the
caller so that the caller can control that object(by calling its
method). But in C++, you can't do this easily because the object you
create within a function as a local variable is destroyed
automatically after the function returns. Thus the pointer it
returns doesn't point to a valid object any longer. So you have to
use the new expression if you want an object to exist after the
function that creates it returns.
For example, suppose you have another class called MySystem, you do
this in Java:
MySystem sys = new MySystem();
Polynomial poly = sys.createNew() ;
It works if you have the method createNew() in class MySystem:
Polynomial createNew() {
return new Polynomial();
}

In C++ I'd write:

MySystem sys;
Polynomial poly; sys.CreateNew(p oly); // Just set initial values

See - no new/delete required.
Avoid it where you can. It's more time efficient and if you come from
Java you don't get into memory leak trouble.

Just my .02$,
-Gernot

[John Harrison]

"Sam" <ma*******@hotm ail.com> wrote in message
news:cn******** **@kermit.esat. net...

"Gernot Frisch" <[e-mail] >
дÈëÏûÏ¢ÐÂÎÅ:2v *************@u ni-berlin.de...

<snip>

Now we get a problem. You can write the following Java codes:
Polynomial createNew() {
return new Polynomial();
}

<snap>

Why does one want to do this:

Polynomial* p1 = OtherPoly.creat eNew();

instead of:
Polynomial p1 = OtherPoly;

I didn't mean that createNew() is a method of class Polynomial. However,
no matter whether it is or not. It's not my point. I was just saying that,
you can easily create a new object in a function in Java and return its
handle(or reference, or pointer) to the caller so that the caller can
control that object(by calling its method). But in C++, you can't do this
easily because the object you create within a function as a local variable
is destroyed automatically after the function returns. Thus the pointer it
returns doesn't point to a valid object any longer. So you have to use the
new expression if you want an object to exist after the function that
creates it returns.
For example, suppose you have another class called MySystem, you do this
in Java:
MySystem sys = new MySystem();
Polynomial poly = sys.createNew() ;
It works if you have the method createNew() in class MySystem:
Polynomial createNew() {
return new Polynomial();
}

But in C++, if you write:
MySystem sys;
Polynomial* p = sys.createNew() ;

you must have the mothod createNew() in class MySystem as follows
Polynomial* MySystem::creat eNew()
{
return new Polynomial;
}

rather than
Polynomial* MySystem::creat eNew()
{
Polynomial po;
return &po; // oops! a bad-point-to-be
}

But you can simply write

Polynomial MySystem::creat eNew()
{
Polynomial po;
return po;
}

There is no need for pointers with all the trouble that they bring. If you
are worried about the cost of copying a Polynomial object then the answer is
to make that class efficiently copyable (by using smart pointers for
instance) not to avoid copying by introducing pointers.

john

[Sam]

> In C++ I'd write:

MySystem sys;
Polynomial poly; sys.CreateNew(p oly); // Just set initial values It's just an example showing some differences between Java and C++. Actually
You cannot always avoid pointers in C++. Polynomial may be an abstract base
class here if I write a createNew() method for it. If I wanna just set
initial values, I should call it initPoly() something. And it is probable
that I wanna keep holding that new object after the function calling
createNew() returns.

See - no new/delete required.
Avoid it where you can. It's more time efficient and if you come from Java
you don't get into memory leak trouble.

Just my .02$,
-Gernot

[Sam]

> But you can simply write

Polynomial MySystem::creat eNew()
{
Polynomial po;
return po;
}

There is no need for pointers with all the trouble that they bring. If you
are worried about the cost of copying a Polynomial object then the answer
is to make that class efficiently copyable (by using smart pointers for
instance) not to avoid copying by introducing pointers.

john

Yes. You're right. But may I argue that you need pointers for polymorphism
at least. "References " in c++ are more confused than pointers, I think.

[Howard]

"John Harrison" <jo************ *@hotmail.com> wrote in message
news:2v******** *****@uni-berlin.de...

"Rv5" <rm*****@adelph ia.net> wrote in message
news:6a******** ************@ad elphia.com...

Rookie c++ question, but Ive spent the last 5 years doing Java, where
everytime I created an object I used new. In c++ I can create my objects
without and its confusing me just a little.

I have a class called polynomial. Its a nothing little class right now,
with just int variables, a basic container class. Im using it as I go
through some tutorials, but in this particular tutorial its telling me to
do
polynomial *first = new polynomial();
but before I found this site I was just doing
polynomial first;

It's likely that you are right and the tutorial is wrong.

Hello...? Did it occur to you that the purpose of that particular tutorial
might be to *teach* using pointers? One does not learn how to use pointers
by avoiding them.

Pointers are useful in many situations, but they are most often useful when
storing polymorphic objects in containers. That is, storing pointers to
base class objects in a vector (for example), when those pointers actually
point to dynamically created instances of derived class objects. Of course,
that's something you wouldn't tackle until later, if you're just starting on
pointers, but it's an example of where they are commonly used.

All that said, I do agree that if you don't actually *need* pointers, don't
bother using them. You'll find out soon enough if there's a case where you
*do* need a pointer. (And just because a function you're calling requires a
pointer as a parameter, doesn't mean that you have to pass it a pointer
variable...you more likely want to pass the address of an existing object.)

At some point, you'll also want to look at using "smart" pointers. They
really help make code better in those cases where you *do* need pointers.

-Howard

[Gernot Frisch]

> At some point, you'll also want to look at using "smart" pointers.

They really help make code better in those cases where you *do* need
pointers.

....depending on what you call "better" code. But, I agree, there's
situations where smarties are more appropreate than simple pointers...
-Gernot

[Rv5]

Thanks for all the info guys. Lot to absorb. Im going to have to read each
post carefully, maybe ever a couple times to really grab what is going on
here. C++ is definitely a lot more indepth then java is.
"Sam" <ma*******@hotm ail.com> wrote in message
news:cn******** **@kermit.esat. net...

But you can simply write

Polynomial MySystem::creat eNew()
{
Polynomial po;
return po;
}

There is no need for pointers with all the trouble that they bring. If
you are worried about the cost of copying a Polynomial object then the
answer is to make that class efficiently copyable (by using smart
pointers for instance) not to avoid copying by introducing pointers.

john

Yes. You're right. But may I argue that you need pointers for polymorphism
at least. "References " in c++ are more confused than pointers, I think.

[Karl Heinz Buchegger]

Rv5 wrote:

Thanks for all the info guys. Lot to absorb. Im going to have to read each
post carefully, maybe ever a couple times to really grab what is going on
here. C++ is definitely a lot more indepth then java is.

Honestly. Not a good idea.
A better idea is to get a book. There are so many pitfalls and
things to know in C++, that you can't discover them on your own
without major frustration.

--
Karl Heinz Buchegger
kb******@gascad .at

[DaKoadMunky]

> Only use a

pointer when you need to control the lifetime of an object.

What if the type of object to be created is not known until run-time?

What if the number of objects to be created is not known until run-time?

At the lowest level I believe the new operator would be required.

Of course at a higher level one could use abstractions to avoid having to
explicitly use new and delete.

[Jason Heyes]

"DaKoadMunk y" <da*********@ao l.com> wrote in message
news:20******** *************** ****@mb-m11.aol.com...

Only use a
pointer when you need to control the lifetime of an object.

What if the type of object to be created is not known until run-time?

Such an object has it's lifetime managed by the programmer. What's your
point?