C / C++ / MFC

Re: default implementation of assignment operator

14-Dec-07 19:04

Hello everyone,

I am wondering the default implementation of assignment operator (e.g. when we do not implement assignment operator in user defined class, what will be returned? temporary object? reference or const reference? deep copy or shallow copy is used in default assignment operator?)? I have the C++ Programming Book at hand, but can not find it from Index page.

thanks in advance,
George

CPallini15-Dec-07 0:45

CPallini

15-Dec-07 0:45

George_George wrote:
I am wondering the default implementation of assignment operator (e.g. when we do not implement assignment operator in user defined class, what will be returned? temporary object? reference or const reference?

A reference. (*this).

George_George wrote:
deep copy or shallow copy is used in default assignment operator?

Shallow copy.

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.

Re: default implementation of assignment operator

George_George15-Dec-07 4:01

non-const reference and const reference

15-Dec-07 4:01

Thanks CPallini,

My question is answerd! Cool!

have a nice weekend,
George

Thanks

CPallini15-Dec-07 5:21

CPallini

15-Dec-07 5:21

George_George wrote:
have a nice weekend

Thank you. Have a nice weekend you too.
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_George14-Dec-07 17:48

14-Dec-07 17:48

Hello everyone,

This is my understanding of non-const reference, const reference and their relationships with lvalue/rvalue. Please help to review whether it is correct and feel free to correct me. Thanks.

1. A const reference can be binded to a rvalue, for example, a temporary object. And the "life" of the temporary object is guaranteed to be extended and we can safely operate through the const-reference.

2. A non-const reference can not binded to a rvalue, I think the reason is rvalue is not addressable? And we can not change the rvalue through its reference? Are there any other reasons? I am not quite sure whether my understanding is fully correct. Since there are some non-modifiable lvalues (so we do not always need to modify values through its reference). I am still studying what is the reason in essence in compiler why a non-const reference can not be binded to a rvalue.

3. Both const and non-const reference can be binded to a lvalue.

thanks in advance,
George

Iain Clarke, Warrior Programmer

Iain Clarke, Warrior Programmer14-Dec-07 22:36

14-Dec-07 22:36

I've just answered your question. If you remember, you asked this question yesterday.

I've admit, todays question is better phrased, but still...

Have *SOME* patience.

Iain.

George_George15-Dec-07 4:02

15-Dec-07 4:02

I am sorry for any inconvenience, Iain! I am new to this topic. Smile | :)

have a nice weekend,
George

Iain Clarke, Warrior Programmer

Iain Clarke, Warrior Programmer15-Dec-07 4:17

15-Dec-07 4:17

Don't worry - some of your questions are much less simple than they look, and can be fun to answer.

I've now gone through all my operator= and copy constructors and const'ed them! They were about 50/50 depending on my mood when writing. If I tell you I can't see any good reason for them not to be consted, and good ones why they should, then I must do it to my code too!

ps, did my long answer to the "previous" question help?

Iain.

George_George15-Dec-07 19:27

Question about redeclaring header files

15-Dec-07 19:27

Sure, Iain!

I have read it and I want to read it again after dinner to have some further think, which is the time I have a clear mind.

Sometimes, we are working on upper layer technologies, like XML and we may forget the basic building blocks' technologies, and this is what I am interested.

regards,
George

(Steven Hicks)ⁿ⁺¹14-Dec-07 15:37

(Steven Hicks)ⁿ⁺¹

14-Dec-07 15:37

I'm having an issue when I attempt to do this:

testone.h:

#include "testtwo.h"

class testone
{
public:
testtwo* test;
}

testtwo.h:

#include "testone.h"

class testtwo
{
public:
testone* test;
}

There is a complaint about the pointer's type missing and the class definition is not valid. How would I be able to solve this problem?
I'm using Embedded Visual C++ 4. Is this a bug with C++ support?

-Steven Hicks

CPA

CodeProjectAddict

Actual Linux Penguins were harmed in the creation of this message.

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Re: Question about redeclaring header files

Iain Clarke, Warrior Programmer14-Dec-07 22:56

Iain Clarke, Warrior Programmer

14-Dec-07 22:56

Think about what you're written.

I'll define testtwo, which needs to know about testone, which needs to know about testtwo, which needs to know about testone, which needs to know about testtwo, which needs to know about testone, which needs to know about testtwo, which needs to know about testone, which needs to know about testtwo, which needs to know about testone, which needs to know about testtwo, which needs to know about testone, which needs to know about testtwo, which needs to know about testone, which needs to know about testtwo, which needs to know about testone, which needs to know about testtwo, which needs to know about testone, which needs to know about testtwo,.....

Ok, I'm fed up - and so was your compiler.

Which is probably which you use'd #pragma once, or
#ifndef TESTTWO_H
#define TESTTWO_H

class testtwo
... etc
#endif

But that just means...

I'll define testone, which needs about testone. So I'll include testone.h - which needs to know about testtwo. so, I'll include testone.h... which appears to be empty! whaaah!

So what you do is....

// guard against being re-included. therein lies madness.
#ifndef TESTTWO_H
#define TESTTWO_H

class testone;    // see outside pre block

class testtwo
{
    ....
    testone *one;
};

#endif

What we've done is tell the compiler "I'll get round to telling you all about testone later, but I'll tell you now that there is such a class."

And do vice versa in testone.h

Now you need to have both headers included into your .C file before you can actually DO anything useful with the pointer, but you've escaped your circular dependency!

Iain.

"Out Of Memory" When using CString GetBuffer() [modified]

nadiric14-Dec-07 14:09

Additional Info - "Out Of Memory" When using CString GetBuffer()

14-Dec-07 14:09

Hello all,

I am trying to track down the root cause of an out of memory error when using CString GetBuffer(). I appreciate any insight or constructive criticisms of poor methods.

The CString resides in an object of the "grp_vals" class:

class grp_vals{
public:
CStringA Name;
float Vals[MAX_VALS];
float& operator [] (int Idx){
return Vals[Idx];
}
};

The code where the problem arises is:

LPSTR p;
grp_vals *tmpvals;
...
tmpvals = new grp_vals;
...
p = tmpvals->Name.GetBuffer(32);//HERES WHERE THE PROBLEM STARTS
fgets(p,32,FileHandle);
tmpvals->Name.ReleaseBuffer();
tmpvals->Name.Remove('\n');

This code "works" for many iterations but eventually the error occurs and was traced to the "if( pNewData == NULL )" below (from the standard atlsimpstr.h)...sure enough, it's NULL but I'm not sure why only in this one case... the pOldData does not *seem* to vary from previous iterations that "worked".

ATL_NOINLINE void Fork( __in int nLength )
{
CStringData* pOldData = GetData();
int nOldLength = pOldData->nDataLength;
CStringData* pNewData = pOldData->pStringMgr->Clone()->Allocate( nLength, sizeof( XCHAR ) );
if( pNewData == NULL )
{
ThrowMemoryException();
}
....

modified on Friday, December 14, 2007 9:38:20 PM

nadiric14-Dec-07 15:15

Re: Additional Info - "Out Of Memory" When using CString GetBuffer()

14-Dec-07 15:15

My understanding from the MSDN docs on ReleaseBuffer() is that the memory that "p" points to is freed (invalidated), but when running in debug I can see that this is not the case. Is it possible that I am filling up memory by not explicitly freeing the memory that "p" points to before assigning it to a new string via GetBuffer()? Or is this a side-effect of running in debug mode?

nadiric14-Dec-07 15:37

Re: "Out Of Memory" When using CString GetBuffer()

14-Dec-07 15:37

Okay... the pointer is to the CString data, not a copy of it, so back to square one...

Am I the only one with nothing better to do on this fine Friday night? Laugh | :laugh:

Mark Salsbery15-Dec-07 8:18

Mark Salsbery

15-Dec-07 8:18

How are you handling string length?

If the data you read into the string buffer is NULL terminated, you're OK.

If the data is not NULL terminated, then you should be specifying the actual
string length when you call ReleaseBuffer().

Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

Re: "Out Of Memory" When using CString GetBuffer()

nadiric15-Dec-07 8:59