C / C++ / MFC

14-Mar-06 10:18

i am using VC++6 and MFC and using dialogs. I have a button that when clicked starts a method which creates an instance of a class, this object is then used to connect to the database and also does querys. how can i make this object seen by other methods and dialogs?

can this object be created globally? or what is best practice?

BadKarma14-Mar-06 10:29

BadKarma

14-Mar-06 10:29

Its best practice to have as little global objects as possible.

There is at least on global object theApp (the application) which can be accessed through the
function AfxGetApp() which returns a pointer to the application object.
You can make you database access class a member of the application class. Then you can access this class through the Application pointer from anywhere in your application.

codito ergo sum

viperlogic14-Mar-06 11:27

14-Mar-06 11:27

many thanks for your repply

could you further explain this with a code example please??

regards

Stephen Hewitt14-Mar-06 17:18

Stephen Hewitt

14-Mar-06 17:18

Too many globals are indeed bad, but that doesn't mean any use of globals is bad. Assuming you want a global variable here's one way to do it in C++:

// In a .h file.
extern int *g_pIntPointer;

// In a .c/.cpp file.
int *g_pIntPointer = NULL;

Now in any file where you add a #include "TheDotHFile.h" your can access you global.

There are other choices like putting the variable in a class and making it static. If you explain you situation more thoroughly I'm sure you can spark off a debate about the pros and cons of the various techniques.

Steve

ThatsAlok14-Mar-06 18:20

ThatsAlok

14-Mar-06 18:20

what about putting your object/pointer in stdafx.h file.. which is included in almost easch file in mfc application.

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow

cheers,
Alok Gupta
VC Forum Q&A :- I/ IV

ThatsAlok14-Mar-06 18:22

ThatsAlok

14-Mar-06 18:22

what about putting your object/pointer in stdafx.h file.. which is included in almost easch file in mfc application... i.e. give a dummy declaration of variable with extern keyword there and declare it in the place you actually using it...

i generally use Singleton class for this ... as it gurantee me a single object acceciable through out the project

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow

cheers,
Alok Gupta
VC Forum Q&A :- I/ IV

viperlogic14-Mar-06 23:23

14-Mar-06 23:23

im getting a bit confused now!

to further exlpain. i have a basic MFC dialog app that was created using the wizard. i want to connect to a database and execute querys, i am using a wrapper for this (mysqlwrapp). to make an object of the dbconnection class i use the following in a method that is invoked by a click of a button.

CSQLConnection m_conn; //create object
m_conn.Open(_T("localhost"), _T("salaries"), _T("root"), _T("root")); //connect

then in another method i want to execute the following, but it cant see the m_conn object. this new object allows querys to database
CSQLResult res(m_conn);
res.Query(_T("SELECT Name FROM house"));

to create these objects the "MySQLWrapp.h" is included in the dialog where the objects are created and used.

so what is the best way to make the m_conn object global. could you please give example code on where and how as most of this is new to me. thanks

Stephen Hewitt15-Mar-06 0:06

Stephen Hewitt

15-Mar-06 0:06

The "m_" wart is normally used to denote a class member variable and not a local variable as you’re using it. Follow these steps:
1. Remove the CSQLConnection m_conn; from the function.
2. Add it to your class definition. i.e.

class CYourDialog : public CDialog
{
public:
     // Blah, blah, blah....
 
protected:
     // Blah, blah, blah....
 
private:
     CSQLConnection m_conn;
     // Blah, blah, blah....
};

Notice I added the member variable as private, this is good form - As a rule of thumb always apply the most restrictive access permissions possible. You can always relax them later if need be.

Now you can access the variable "m_conn" anywhere in that class. It's not a global however as it can't be accessed from other classes or global functions.

Only use the "m_" wart on member variables (variables declared in the class definition) or else you'll confuse people.

Steve

viperlogic15-Mar-06 2:22

how to detect hyperlink from word file

15-Mar-06 2:22

perfect

many thanks

mk_le14-Mar-06 9:48

mk_le

14-Mar-06 9:48

hello,

how can i detect a hyperlink in word file?

thanks for any help

Example on creating an item identifier list

Chris Meech14-Mar-06 9:33

Chris Meech

14-Mar-06 9:33

I'm trying to use the shell function SHGetPathFromIDList but not having luck constructing the first parameter properly. It requires an item identifier list and in my case, I'm needing the path to just the 'My Documents' folder. If someone can post an example or a link to an example that explains how to create this list with just that entry in it and how it's to be freed, if necessary, that would help me out. Thanks.

Chris Meech

I am Canadian. [heard in a local bar]

When I want privacy, I'll close the bathroom door. [Stan Shannon]

BAD DAY FOR: Friendly competition, as Ford Motor Co. declared the employee parking lot at its truck plant in Dearborn, Mich., off limits to vehicles built by rival companies. Workers have to drive a Ford to work, or park across the street. [CNNMoney.com]

Nice sig! [Tim Deveaux on Matt Newman's sig with a quote from me]

Michael Dunn14-Mar-06 10:28

Michael Dunn

14-Mar-06 10:28

To get the conventional file path, use SHGetSpecialFolderPath(CSIDL_PERSONAL), no need to mess around with PIDLs.

--Mike--
Visual C++ MVP Cool | :cool:

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Chris Meech15-Mar-06 2:37

Chris Meech

15-Mar-06 2:37

Thanks Mike. That's going to work much better. Smile | :)

Chris Meech

I am Canadian. [heard in a local bar]

When I want privacy, I'll close the bathroom door. [Stan Shannon]

BAD DAY FOR: Friendly competition, as Ford Motor Co. declared the employee parking lot at its truck plant in Dearborn, Mich., off limits to vehicles built by rival companies. Workers have to drive a Ford to work, or park across the street. [CNNMoney.com]

Nice sig! [Tim Deveaux on Matt Newman's sig with a quote from me]

Graham Bradshaw14-Mar-06 12:05

Graham Bradshaw

14-Mar-06 12:05

Looks like Michael's answered your immediate question, but for future reference, here's some code that gets the Application Data folder - just change the CSIDL_COMMON_APPDATA to whatever you want.

CString strCommonDataStorageFolder;
LPITEMIDLIST pidl;
if (SUCCEEDED(SHGetSpecialFolderLocation(NULL, CSIDL_COMMON_APPDATA, &pidl)))
{
	LPMALLOC lpMalloc;

	if (::SHGetMalloc(&lpMalloc) == NOERROR)
	{
		TCHAR szPath[MAX_PATH];

		if (SHGetPathFromIDList(pidl, szPath))
			strCommonDataStorageFolder = szPath;

		lpMalloc->Free(pidl);
		lpMalloc->Release();
	}
}

Problem comparing 'double' values

Chris Meech15-Mar-06 2:40

Chris Meech

15-Mar-06 2:40

Thanks very much, Graham. The example you provided is exactly what I was after, but I'm also going to look into Mike's suggestion too. Appreciate the help. Smile | :)

Wheatbread14-Mar-06 9:23

Wheatbread

14-Mar-06 9:23

For some reason, when I compare two identical double values, its failing.

double val = func->GetValue() // returns a double
double val1 = func2->GetOldVal() // returns a double

if (val != val1)
doFooA()
else
doFooB().

In my case, val == val1, but its passing and calling doFooA(). I've tried 'if ( (double) val != (double) val1) )' and 'if ((float) val != (float) val1)'. Although I know the latter will lose precision, I"m trying anything.

Any ideas why this is happening?
TIA
-C

Maximilien14-Mar-06 10:06

Maximilien

14-Mar-06 10:06

this is normal.

in short, a computer cannot store exact floating point numbers in memory; and will rely on an approximation.

That's why you'll see things like 10.00000001 or 9.9999999999

to compare 2 double ( or float ) numbers you need to check if the difference between them is smaller than a small value.

Maximilien Lincourt
Your Head A Splode - Strong Bad

Wheatbread14-Mar-06 10:52

Wheatbread

14-Mar-06 10:52

Is there an easy way to do this without subtracting one from the other? Maybe some fancy math function already built?

Maximilien14-Mar-06 12:49

Maximilien

14-Mar-06 12:49

I wish I knew !!!

Maximilien Lincourt
Your Head A Splode - Strong Bad