Dumb question... Copying derived class from base class pointer - C / C++ / MFC Discussion Boards

BlackKettle13-Oct-02 5:16

13-Oct-02 5:16

Hi,

I have a threads problem and here it is:
I am developing a program that will draw the graphic of a mathematical function but I want the draw to be done in other thread because for example if the multiplaier(the for is decreasing 'x' with 1/multiplaier) is too big, the program will draw the graphic slowly, and on every WM_PAINT message to my control the graphic has to be redrawn, and so if the user moves the window or whatever he doest to it, he will have to wait until the graphic is completly drawn, so I wanna make another thread to do this, but I have problem with the ThreadProc function:

I made a struct where all the parameters for the drawing are set:

<br />
typedef struct _DRAWERSUB {<br />
	HDC dc;<br />
	CRichEditCtrl *ctrlResults;<br />
	char * strFormula;<br />
	char * strX;<br />
	float nMultip;<br />
	int cx;<br />
	int cy;<br />
	double dPrecision;<br />
	BOOL bPrecision;<br />
	int nRootsFound;<br />
	double nDivs;<br />
	BOOL bCalc;<br />
} DRAWERSUB, *LPDRAWERSUB;<br />

this is the struct.
I initialize it in a function called by WM_PAINT as:

<br />
	DRAWERSUB dwsub;<br />
	dwsub.strFormula = (LPTSTR)(LPCTSTR)m_strFormula;<br />
	dwsub.strX = (LPTSTR)(LPCTSTR)m_strX;<br />
	dwsub.dc = dc->GetSafeHdc();<br />
	dwsub.nDivs = m_nDivisions;<br />
	dwsub.nMultip = m_nMultip;<br />
	dwsub.cx = cx;dwsub.cy = cy;<br />
	dwsub.bCalc = m_bCalc;<br />
	dwsub.bPrecision = m_bPrecission;<br />
	dwsub.dPrecision = precision;<br />
	m_trGraphicDrawer = AfxBeginThread(GraphicDrawerSub,&dwsub);

and this is the code for the ThreadProc:

<br />
UINT CGraphicFunc::GraphicDrawerSub(LPVOID pParam)<br />
{<br />
	LPDRAWERSUB dwsub = (LPDRAWERSUB)pParam;<br />
	CString strFormula((LPCTSTR)(LPTSTR)dwsub->strFormula), strX((LPCTSTR)(LPTSTR)dwsub->strX);<br />
	HDC hDC = dwsub->dc;<br />
	double nDivs = dwsub->nDivs;<br />
	float nMultip = dwsub->nMultip;<br />
	int cx = dwsub->cx, cy = dwsub->cy;<br />
	double dPrecision = dwsub->dPrecision;<br />
	BOOL bPrecision = dwsub->bPrecision;<br />
	BOOL bCalc = dwsub->bCalc;<br />
.....<br />
}<br />

but every time the ThreadProc is called, strFormula, strX and every value from it, has different value, sometimes the hDC is valid and sometimes it isn't...strFormula sometimes is "" and strX is something like this:
"U‹ìQƒeü"
What can I do for rapairing this problem???
I am not good at threads and I almost no experience with them Frown | :(

...

Thanks.

Re: Threads problem ...

Stephane Rodriguez.13-Oct-02 5:36

Stephane Rodriguez.

13-Oct-02 5:36

A quick hint, never use local variables in a thread proc. Just keep using dwsub-> even if this sounds not optimized.

Windows shared objects such like DCs can be locked using the WIN32 API.

Code sections can be locked for one thread only by using CriticalSection, Semaphore or Mutex. See MSDN.

She's so dirty, she threw a boomerang and it wouldn't even come back.

Re: Threads problem ...

BlackKettle13-Oct-02 6:43

BlackKettle

13-Oct-02 6:43

Thanks for the hint but it still dont works...the struct itself is some kind damaged because when I debug the application, in the thread proc, dwsub seems to be damaged ...

Re: Threads problem ...

Al Giv.13-Oct-02 7:08

Al Giv.

13-Oct-02 7:08

a possible solution:

in your example, the DRAWERSUB structure is defined on the stack...
So when you call the AfxBeginThread, the struct is valid. but before the thread starts, the method which called AfxbeginThread might end, and release the stack memory of the DRAWERSUB...
solution: either allocate it on the HEAP (using HeapAlloc) or make it a member variable of CGraphicFunc (but then you can only create one copy of the thread).

Share variable into DLL

Raphael Kindt13-Oct-02 5:06

Raphael Kindt

13-Oct-02 5:06

Hi world

I want to share a variable into a dll and all application using this dll handle the SAME variable (not a copy)..
How to do that?

Thanks in advance.

Hello World!!! Smile | :)

from Raphaël

Re: Share variable into DLL

Stephane Rodriguez.13-Oct-02 5:40

Stephane Rodriguez.

13-Oct-02 5:40

use global atoms. Link on MSDN[^].

Btw, as long as you don't cross process boundaries, all dlls share the same address space, thus every variable pointer is available from anywhere.

She's so dirty, she threw a boomerang and it wouldn't even come back.

Re: Share variable into DLL

Joaquín M López Muñoz13-Oct-02 8:48

Joaquín M López Muñoz

13-Oct-02 8:48

Use shared data segments. Phil MacGahan explains the stuff in his article How to share a data segment in a DLL[^].

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

Dumb question... Copying derived class from base class pointer

work_to_live13-Oct-02 3:44

work_to_live

13-Oct-02 3:44

I'm getting back into programming after a long time off, and may post a few... silly questions. OK, here goes... I'm creating a network simulator, using (among other things), a number of derived classes that share a common base class. Most access to these classes is obtained through the use of a pointer to the base class, accessing the derived class functionality. All well and good. I'm having trouble, when I want to make a copy of one of the derived classes with a pointer to the base class. What I get is a copy of the base class only!

Should I be able to make a copy of a derived class if I'm using a pointer to it's base class? If so, do I need to set up either the base class or the derived class in such a way as to enable this copy function?

Thanks.

Re: Dumb question... Copying derived class from base class pointer

Nish Nishant13-Oct-02 4:18

Nish Nishant

13-Oct-02 4:18

Hmmm

I couldn't reproduce your problem (I am using VC++.NET)

class B
{
public:
    B()
    {
        a=100;
    }
    int a;
};

class D : public B
{
public:
    D()
    {
        b=200;
    }
    int b;
};

int _tmain(void)
{
    //Create derived object and assign it to base variable
    B* b1 = new D();
    //copy object in base variable to another base variable
    B* b2 = b1;
    //copy copied base variable to derived variable
    D* d = static_cast<D*>(b2);
    //try accessing a member available only in D
    printf("%d",d->b);    
    //worked fine - printed 200 as expected
    delete b1;
    return 0;
}

Regards,
Nish

Author of the romantic comedy

Summer Love and Some more Cricket [New Win]

Review by Shog⁹
Click here for review[NW]

Re: Dumb question... Copying derived class from base class pointer

Nish Nishant13-Oct-02 4:23

Nish Nishant

13-Oct-02 4:23

By the way we are not actually creating copies here, rather we are just copying the pointers. There is actually just the one single object. Which is why there is just one delete

Regards,
Nish

Author of the romantic comedy

Summer Love and Some more Cricket [New Win]

Review by Shog⁹
Click here for review[NW]

Re: Dumb question... Copying derived class from base class pointer

Joaquín M López Muñoz13-Oct-02 4:29

Joaquín M López Muñoz

13-Oct-02 4:29

This is a very common situation when dealing with abstract classes. The usual solution is to define a virtual clone member function in the base class like this:

class Base{
public:
  virtual Base * clone()const=0; // =0 if no implementation is given in the Base class.
...
};
 
class Derived: public Base
{
public:
  virtual Base * clone()const
  {
    return new Derived(*this);
  }
};

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

MySQL Inserting data

Anonymous13-Oct-02 3:20

Anonymous

13-Oct-02 3:20

Hi, I'm having trouble inserting data into a MySQL table.

char myid[50]="testid";
char myip[50]="testip";
char myname[50]="testname";
char mymessage[50]="testmessage";

mysql_query(&mysql,"INSERT INTO mytable (id, ip, name, message) VALUES('myid','myip','myname','mymessage')")

This should insert the data which my vars contains, but instead it inserts myid myip myname and my mymessage into the table, not the data.
I think my syntax is wrong, since the mysql_query inserts 'myid' and no "testid" as it should..

I need help on this Smile | :)

Oh by the way, I'm not using the MySQL++ wrapper.

Thanks for any help Smile | :)

Re: MySQL Inserting data

Nish Nishant13-Oct-02 3:31

Nish Nishant

13-Oct-02 3:31

Anonymous wrote:
mysql_query(&mysql,"INSERT INTO mytable (id, ip, name, message) VALUES('myid','myip','myname','mymessage')")

That line is all wrong friend.

Use something like this :-

char sqlstr[512];<br />
sprintf(sqlstr,"insert into tablename(id,ip) values ('%s','%s')",myid,myip);

Now do :-

mysql_query(&mysql,sqlstr);

Regards,
Nish

Author of the romantic comedy

Summer Love and Some more Cricket [New Win]

Review by Shog⁹
Click here for review[NW]

Re: MySQL Inserting data

Anonymous13-Oct-02 10:45

Anonymous

13-Oct-02 10:45

Thanks for your help. I never thought of that Smile | :)

Getting DNS Server IP

Al Giv.13-Oct-02 0:19

Al Giv.

13-Oct-02 0:19

I want to get the IP of the DNS server on a local computer.
is there a general way to do this that will work on all windows flavours?

At First it thought of IPHlpApi.lib, but Ip Helper Library is not supported in win95...

obviously it is possible, because WinIPCFG displays that information...

please help, thanks!

Re: Getting DNS Server IP

Anders Molin13-Oct-02 0:29

Anders Molin

13-Oct-02 0:29

I think the easiest way is to read it from the registry. But even then you have to read different places on different versions of windows. Hmmm | :|

- Anders

Money talks, but all mine ever says is "Goodbye!"

Query about how to pass a control's Identification through its Message parameters

12-Oct-02 22:17

any expert pls kindly help me, Smile | :)

i've created a new control "uiCtrlListCtrl" from the window control "CListCtrl". and i've defined some new message handlers.

such as:
void uiCtrlListCtrl::OnItemchanged(NMHDR* pNMHDR, LRESULT* pResult){
...
CWnd* pwndParent = GetParent();
pwndParent->SendMessage(UI_WM_ITEMCHANGING);
...
}

i have two such new list controls in a dialog(m_lsclList1 and m_lsclList2), how can i tell which control is it from when i received such an UI_WM_ITEMCHANGING message?

my message map is defined as below:
ON_MESSAGE( UI_WM_ITEMCHANGING, OnItemChanging)

the message handling function is defined as:
void uiDlgFrm::OnItemChanging(WPARAM wParam, LPARAM lParam){...}

afraid haven't expressed my problem clear enough: how can i pass something through the message's parameters to know it's from m_lsclList1 or m_lsclList2?

thx a lot!

A beginner's confusion.

zhoujun12-Oct-02 21:33

zhoujun

12-Oct-02 21:33

In c++,I define a class named Tdate,
I can define object using "Tdate td;","Tdate td1(1);","new Tdate;","new Tdate(1);",
And I know I can't do "Tdate td();" because it looks like function,
But Can I do "new Tdate();"?

And more,in visual basic(sorry I am just for contrast),
"Dim td as new Tdate()" is popularly used,and I did not see "dim td as new Tdate"?

I want to find the answer in some programming books,but they always did not illustrate clearly.
So I want others' help,or could you introduce me some articles or documents that elaborate on object define.
Thanks.

this is my signature for forums quoted from shog*9:
I can't help but feel, somewhere deep within that withered, bitter, scheming person, there is a small child, frightened, looking a way out.

Re: A beginner's confusion.

Shog⁹12-Oct-02 21:38

Shog⁹

12-Oct-02 21:38

Tdate td;
and
Tdate td();
are equivilant.

new Tdate td();
and
new Tdate td();
are equivilant.
The compiler is able to recognize they are not function calls by the context they are used in.

In VB, DIM td as new Tdate() creates an (unsized) array of Tdate objects.

Shog⁹
~~------~~

And on the pedestal, these words appear:
"My name is Ozymandias, King of Kings,
Look on my Works, ye Mighty, and despair!"
Nothing beside remains.

Re: A beginner's confusion.

zhoujun12-Oct-02 22:00

zhoujun

12-Oct-02 22:00

Thanks Shog. Smile | :)

In a book I have just read said "Tdate td()" is not allowed.

Is it right that "Tdate td()" equals "Tdate td",and "new Tdate" equals "new Tdate()"?

In VB,what is the difference of the following three:
Dim td as new Tdate();
Dim td as new Tdate;
Dim td as Tdate;

Thanks.

this is my signature for forums quoted from shog*9:
I can't help but feel, somewhere deep within that withered, bitter, scheming person, there is a small child, frightened, looking a way out.

Re: A beginner's confusion.

Shog⁹13-Oct-02 5:08

Shog⁹

13-Oct-02 5:08

zhoujun wrote:
In a book I have just read said "Tdate td()" is not allowed.

As Joaquín also pointed out, that is correct; i was wrong. (i guess i should probably stick to sleeping at 2:00am...)

new Tdate;
is equivilant to
new Tdate();
however.

zhoujun wrote:
In VB,what is the difference of the following three:
Dim td as new Tdate();
Dim td as new Tdate;
Dim td as Tdate;

The first one declares an array, but gives it no initial size (parentheses are used for arrays as well as function calls in VB)
Actually, i'm not possitive that the first line will work; the syntax seems a bit odd, even for VB, as new is used to instantiate objects, but as the array is initially empty, there is nothing to instantiate.

The second one declares and instantiates a Tdate object.

The third one declares, but does not instantiate a Tdate object; it will need to later be instantiated using new or CreateObject().

Generally in VB6 the use of new in a declaration line is discouraged, as though it's function is to instantiate an object, it does not actually do so until it is first used, at which point it is difficult to gracefully handle failures. In VB.NET, this has been corrected.

(The VB forum would probably be the place to get more confident answers to VB questions...)

Shog⁹
~~------~~

And on the pedestal, these words appear:
"My name is Ozymandias, King of Kings,
Look on my Works, ye Mighty, and despair!"
Nothing beside remains.

Re: A beginner's confusion.

Joaquín M López Muñoz13-Oct-02 4:34

Joaquín M López Muñoz

13-Oct-02 4:34

Unfortunately, Tdate td; and Tdate td(); are not equivalent. The latter, as the original poster rightly pointed out, is interpreted by the compiler as the declaration of a function with no arguments returning a Tdate, regardless of the context. It is a quirk of the language we have to live with Frown | :(

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

Re: A beginner's confusion.

Shog⁹13-Oct-02 5:09

Shog⁹

13-Oct-02 5:09

Ooops! Thanks Joaquín; not sure what i was thinking last night... Blush | :O

Shog⁹
~~------~~

And on the pedestal, these words appear:
"My name is Ozymandias, King of Kings,
Look on my Works, ye Mighty, and despair!"
Nothing beside remains.

UrlEscape Error

John Bosko12-Oct-02 21:04

John Bosko

12-Oct-02 21:04

Hi,
with regard to UrlEscape function, why do I get "'UrlEscape' : undeclared identifier" when I compile?
I do have the required version of "Shlwapi.dll".

I am running on Win2000.

Kindly Help.
Thanks,
John.

Re: UrlEscape Error

Stephane Rodriguez.12-Oct-02 21:33

Stephane Rodriguez.

12-Oct-02 21:33

It's declared in shlwapi.h (5.0 and higher).

She's so dirty, she threw a boomerang and it wouldn't even come back.

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