question--A function returns class object(comparin g C++ & JAVA)

[Xiaoshen Li]

Dear Sir,

I am a little puzzled about a function returning a class object, for
example, suppose I hava a class Money and a method:

Money lastYear(Money aMoney)
{
Money tempMoney;
...
return tempMoney;
}

Because in C++ the RETURNED is actually a new copy of the object(kind of
like passing by value), here a copy of tempMoney is returned, so
tempMoney will be useless, so after the statement of return, the object
tempMoney will be destroyed. Is this correct?

Now, if in JAVA(sorry, not offending), since always passing by
reference, if above code is Java, the RETURNED is the MEMORY ADDRESS of
the object tempMoney. So after the exit of the function, the object
tempMoney is referenced by other variable. So it stays existing. Is this
correct?

Thank you very much.

[Victor Bazarov]

Thomas J. Gritzan wrote:

Xiaoshen Li schrieb:

ni*****@micro soft.com wrote:

In this case, tempMoney is still destroyed and the reference
returned by the function is bound to a non-existent object.
Unlike in Java, the existence of a reference has no impact on
the lifetime of the referenced object. Likewise, a pointer does
not affect the lifetime of the pointee. It's up to you to make
sure there are no dangling references or pointers.

I became so interested in seeing this so I wrote code to test it. BUT,
the results is not what you said. (I truely believe you are correct!).
With following function:

Money& lastYear(Money aMoney)
{
Money tempMoney;
tempMoney = aMoney;
return tempMoney;
}

Money bMoney = lastYear(aMoney );

Here you are copying the object into another object so that...

Nope. By the time 'lastYear' returns [to initialise 'bMoney'], the
reference it attempts to return has already become invalid, since the
object to which it referred has gone out of existence at the closing
curly brace of the 'lastYear's body.

bMoney.output ();

This operates on a completly valid object.

Nope, since the initialisation of it had undefined behaviour, this point
in the program exists in a parallel universe, with completely different
laws of physics.
[..]

V

[Pete Becker]

Xiaoshen Li wrote:

Dear Sir,

I am a little puzzled about a function returning a class object, for
example, suppose I hava a class Money and a method:

Money lastYear(Money aMoney)
{
Money tempMoney;
...
return tempMoney;
}

Because in C++ the RETURNED is actually a new copy of the object(kind of
like passing by value), here a copy of tempMoney is returned, so
tempMoney will be useless, so after the statement of return, the object
tempMoney will be destroyed. Is this correct?

Now, if in JAVA(sorry, not offending), since always passing by
reference, if above code is Java, the RETURNED is the MEMORY ADDRESS of
the object tempMoney.

No. It returns a null reference. There is no object of type Money in
that code. To return an object you have to create one:

Money tempMoney = new Money;

Similarly, in C++ you can use pointers:

Money *lastYear(Money *aMoney)
{
Money *tempMoney = new Money;
return tempMoney;
}

--

Pete Becker
Dinkumware, Ltd. (http://www.dinkumware.com)