C / C++ / MFC

How do you const your references?

18-Oct-11 7:33

here some info that might help you
WPF and Win32 Interoperation[^].

It has a section titled "Hosting WPF Content in a Microsoft Win32 Window". MFC is just a "light" wrapper around Win32.

And here's someone's blog about MFC and WPF Interop[^]

I found these by googling for wpf interop[^]

If your actions inspire others to dream more, learn more, do more and become more, you are a leader." - John Quincy Adams
You must accept one of two basic premises: Either we are alone in the universe, or we are not alone in the universe. And either way, the implications are staggering” - Wernher von Braun

User 5838525-Oct-11 18:35

User 583852

5-Oct-11 18:35

For years I've written

const Blah& b = GetMyBlah();

I was taught to read these right to left - ie a reference to a Blah which is const.

A colleague has pointed out that this is in fact incorrect as the const key word is left associative but most compilers allow it (gcc, intel, clang and VC in my case). So it should be written

Blah const& b = GetMyBlah();

When read right to left it's a reference to a const Blah - same thing.

He pointed me to the parashift c++ faq http://www.parashift.com/c++-faq-lite/[^] which was changed recently to put the const between the type and the reference.

Obviously pointers are different as both the pointer and the object pointed to (or both) can be const while a reference is always const.

modified 10-Oct-11 5:18am.

Stefan_Lang6-Oct-11 0:53

6-Oct-11 0:53

You're correct, both variants mean exactly the same thing. Whether you write const before or after the type is meaningless and just a matter of style - the latter variant may be a little clearer with respect to the rule of reading types right to left though.

If you wanted to express that your reference is const, then you could place const to the right of the reference operator, like this:

C++

Blah const& b const = GetMyBlah(); // const reference to a const Blah

However, as Herb Sutter points out (see point 13), this extra const is meaningless since references are always const. Therefore the extra const is usually omitted.

TheGreatAndPowerfulOz6-Oct-11 5:55

6-Oct-11 5:55

Good answer Stefan.

So I was trying out some stuff and I was surprised that the VC++ 2010 compiler gave me an error on the second of these saying that void GetBlah2(const Blah*) already has a body.

void GetBlah2(Blah const * blah) { } // #1 method taking pointer to const Blah
void GetBlah2(Blah const * const blah) { } // #2 method taking const pointer to const Blah

Yet I am able to declare these variables with the expected meaning:

const Blah* blah1 = new Blah(); // pointer to const Blah
Blah const * const blah2 = new Blah(); // const pointer to const Blah

Is this a bug in the compiler or is such a method (#2 above) disallowed by the C++ standard?

Stefan_Lang6-Oct-11 6:09

6-Oct-11 6:09

In the first function you passed a pointer, not a reference, and you passed that pointer by value. Passing a parameter of type X by value is the same as passing the same parameter as type const X - at least from the point of view of the caller - therefore the declaration of your second function is syntactically the same as the first (again, from the point of view of the caller). The compiler cannot disambiguate any calls to this function into one of the two versions, therefore it does not allow you to overload like that. Follow the link to GOTW I've posted above, IIRC that this was also covered there. In short, if you pass a parameter by value, you cannot overload it only by adding const to that type.

If you meant to pass your pointer by reference, do it this way (using underscore to highlight the change):

C++

void GetBlah2(Blah const *& blah) { } // #1 method taking pointer to const Blah
void GetBlah2(Blah const * const blah) { } // #2 method taking const pointer to const Blah

This should work.

TheGreatAndPowerfulOz6-Oct-11 6:37

6-Oct-11 6:37

I see. Your reply makes sense. It's like trying to declare a const reference (Blah & const). It's redundant.

Stefan_Lang6-Oct-11 6:48

6-Oct-11 6:48

Not quite. In case of a reference there really is no difference as there is no such thing as a non-const reference. In your example however there would be a difference, but it would only be visible within the code of your function: there, if you defined the parameter const, you could not modify it (locally), whereas if you didn't define it const you could. Of course, since you pass it by value the caller wouldn't notice the difference, basically you gain an additional local variable by not declaring it const.

C++

const Blah* GetBlah1(const Blah*& const b) {
   b = 0; // error: you cannot modify a const reference!
   return b;
}
const Blah* GetBlah2(const Blah*& b) {
   b = 0; // error: you cannot modify a reference, as it's always const!
   return b;
}
const Blah* GetBlah3(const Blah* const b) {
   b = 0; // error: you cannot modify a const pointer
   return b;
}
const Blah* GetBlah4(const Blah* b) {
   b = 0; // ok
   return b;
}
int main() {
   Blah* a = new Blah;
   Blah* b = GetBlah4(a); // this does not modify a
   assert(b==0);
   assert(a!=0);
   delete a;
   return 0;
}

TheGreatAndPowerfulOz6-Oct-11 7:07

6-Oct-11 7:07

I copied your code into VC++ 2010 and the first two errors do not occur.

Stefan_Lang wrote:

const Blah* GetBlah1(const Blah*& const b) {
   b = 0; // not an error: you're assigning a new pointer value to b which will survive once returning from the method
   return b;
}
const Blah* GetBlah2(const Blah*& b) {
   b = 0; // not an error: again you're merely assigning a new pointer to b which will survive once returning from the method
   return b;
}

Also, this line gets an error because you can't assign a const value to a non-const value:

Stefan_Lang wrote:

Blah* b = GetBlah4(a); // this does not modify a

Anyway, thanks for your help. I understand I think.

Stefan_Lang6-Oct-11 22:07

6-Oct-11 22:07

Ah, my bad, you are right in all three cases: in the first functions the assignments don't change the reference, only the value they reference. So these examples were not valid for the purpose I wanted to demonstrate. Also, when I tried to compile GetBlah1() I got the warning warning C4227: anachronism used : qualifiers on reference are ignored. This leads me to believe that it is not actually possible to pass a parameter by const reference, and the rightmost const in the signature is ignored, therefore b is not const and may be modified. I wasn't aware of that restriction before.

The function call to GetBlah4() doesn't work because it tries to assign the (const) return value to a non-const variable. Add a const at the start of the line and it should work.

PostMessage with WPARAM does not work

«_Superman_»6-Oct-11 6:05

«_Superman_»

6-Oct-11 6:05

In the case of pointers, the following 2 lines are the same, where the contents cannot be changed while the pointer can be made to point to another object.

const Blah* p = GetMyBlahPtr();
Blah const* p = GetMyBlahPtr();

Whereas in the following line, the contents can be changed, but the pointer itself cannot be pointed to another object.

Blah* const p = GetMyBlahPtr();

«_Superman_»
I love work. It gives me something to do between weekends.

Microsoft MVP (Visual C++)

Polymorphism in C

Vaclav_5-Oct-11 17:43

Vaclav_

5-Oct-11 17:43

I can do this in my code
PostMessage(WM_KEYDOWN);

The message gets properly intercepted, no problem.

I need to detect right arrow so I add wParam like this
PostMessage(WM_KEYDOWN,VK_RIGHT,0);

The message is nowhere to be found.
WindowProc does not even detects WM_KEYDOWN.

All this in MFC.
What am I missing?

Thanks for your help.

Re: PostMessage with WPARAM does not work

Richard MacCutchan5-Oct-11 21:39

Richard MacCutchan

5-Oct-11 21:39

You may need to add some detail in your LPARAM variable as described here[^]. Also check the status returned from your PostMessage() call.

Unrequited desire is character building. OriginalGriff

Re: You can't simulate keyboard input with PostMessage

Software_Developer5-Oct-11 21:50

Software_Developer

5-Oct-11 21:50

explained here[]

solution: try this[] or this[]

MSDN:SendInput function[]

Re: PostMessage with WPARAM does not work

Vaclav_5-Oct-11 23:21

Vaclav_

5-Oct-11 23:21

I have changed to SendMessage and get expected results.
Also adding GetLastError processing AFTER the PostMessage I get expected results.
Can anybody explain why is this happening?

Especialy when it works when only WM_KEYDOWN is used without the wParam.

The lParam should specify number of repeats of the keyboard, right arrow in my case and should be 0.

I am not using any other language, only US English.

Re: PostMessage with WPARAM does not work

enhzflep6-Oct-11 1:10

enhzflep

6-Oct-11 1:10

I just dropped the following code into a program to test the interpretation of

"Specifies the repeat count. The value is the number of times the keystroke is repeated as a result of the user holding down the key."

C++

case WM_KEYDOWN:
    sprintf(szText, "repeat count = %d", lParam & 0xFF);
    MessageBox(NULL, szText, "Guess What?", MB_OK|MB_ICONQUESTION);
    return 1;

While I agree with your interpretation, the message box says that the repeat count is 1.

I'd suppose that when you send WM_KEYDOWN without the wParam, the helper-function that you're calling sets up an appropriate one before calling the 'real' SendMessage, as defined by:

C++

LRESULT SendMessage(

    HWND hWnd,	// handle of destination window
    UINT Msg,	// message to send
    WPARAM wParam,	// first message parameter
    LPARAM lParam 	// second message parameter
   );

Hi there all again! This time with an improvement! (and more clear question about C++ ADO) :)

symeramon5-Oct-11 5:21

symeramon

5-Oct-11 5:21

C++

RecordsetPtr rsUrun;
rsUrun.CreateInstance(__uuidof(Recordset));
	try{

_variant_t Holder;

CString cQry=_T("");
CString cTmp=_T("");
_stUrun stTmp;
vUrun.clear();

cQry=_T("Select * From urun order by adi");
//AfxMessageBox(cQry);
rsUrun->Open(_variant_t(cQry),AdoTest.GetInterfacePtr(),adOpenDynamic,adLockOptimistic,adCmdText);
CStringArray caTmp;
int nPs=0;
while(!rsUrun->adoEOF)
{
stTmp.cUrunAd=_T("");
stTmp.dBirimf=0;
stTmp.nDBID=0;

	Holder = rsUrun->GetCollect("urunid");//id
	if(Holder.vt!=VT_NULL) stTmp.nDBID=atoi((char*)_bstr_t(Holder));

	Holder = rsUrun->GetCollect("adi");//name
	if(Holder.vt!=VT_NULL) stTmp.cUrunAd=(char*)_bstr_t(Holder);

	Holder = rsUrun->GetCollect("birimf");//cost
	if(Holder.vt!=VT_NULL) stTmp.dBirimf=atof((char*)_bstr_t(Holder));

	vUrun.push_back(stTmp);
	CString adi = rsUrun->GetCollect("adi");
	listbo1.AddString(_T(adi));
	int gfujf = listbo1.GetItemData(listbo1.GetCount()-1);
rsUrun->MoveNext();

SO here is the code, could you please help me on how can I get the listbox item's id in order to use it in an update query?? Here is my button below, I did some work on it but seems no good Frown | :(

C++

void CTestADSSQLDlg::OnBnClickedButton1()
{
	_RecordsetPtr rsUrun;
rsUrun.CreateInstance(__uuidof(Recordset));
CString cQry=_T("Update urun set adi='%s' where urunid='%d'", editbo1;

rsUrun->Open(_variant_t(cQry),AdoTest.GetInterfacePtr(),adOpenDynamic,adLockOptimistic,adCmdText);
}

Re: Hi there all again! This time with an improvement! (and more clear question about C++ ADO) :)

David Crow5-Oct-11 8:10

David Crow

5-Oct-11 8:10

symeramon wrote:
CString cQry=_T("Update urun set adi='%s' where urunid='%d'", editbo1;

Seems a parenthesis has gone AWOL.

symeramon wrote:
SO here is the code, could you please help me on how can I get the listbox item's id...

Umm, since you are using MFC, why not just assign it a CListBox member variable? Use ClassWizard (or equivalent).

"One man's wage rise is another man's price increase." - Harold Wilson

"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons

"Show me a community that obeys the Ten Commandments and I'll show you a less crowded prison system." - Anonymous

Re: Hi there all again! This time with an improvement! (and more clear question about C++ ADO) :)

symeramon6-Oct-11 0:47

symeramon

6-Oct-11 0:47

Thanks David, at least you answered my question... But the main problem is how can I get the id from vector to use it in the button click event?

Re: Hi there all again! This time with an improvement! (and more clear question about C++ ADO) :)

David Crow6-Oct-11 2:55

David Crow

6-Oct-11 2:55

symeramon wrote:
But the main problem is how can I get the id from vector...

What does a vector have to do with any of this?

symeramon wrote:
...use it in the button click event?

BEGIN_MESSAGE_MAP(CMyDialog, CDialog)
    //{{AFX_MSG_MAP(CMyDialog)
    ON_BN_CLICKED(IDC_SOME_BUTTON, OnButtonClicked)
END_MESSAGE_MAP()
...
void CMyDialog::OnButtonClicked() 
{
    m_listbox.AddString(_T("Hello World!"));
}

Re: Hi there all again! This time with an improvement! (and more clear question about C++ ADO) :)

symeramon7-Oct-11 5:33

symeramon

7-Oct-11 5:33

Thanks for your reply David... Finally I got it, by SetItemData! Smile | :)

MFC usage

mysmax5-Oct-11 2:01

mysmax

5-Oct-11 2:01

1, Window Design
2, Skining the Dialog
3, Other plug in usage

if any one have answers with simple snippets please refer me.

Re: MFC usage

Albert Holguin5-Oct-11 2:53

Albert Holguin

5-Oct-11 2:53

Are you trying to ask how its used? MFC is basically an objected oriented layer around the WinAPI, but it's not just that, it provides a framework for development. As a framework, it provides base abstraction layers for creating objects, windows, threads, lists, arrays, strings, controls, and many more objects common in desktop development.

Re: MFC usage

Stefan_Lang6-Oct-11 1:32