Re: reference to const int - more - C / C++ / MFC Discussion Boards

Re: reference to const int

Code-o-mat10-May-12 9:22

Code-o-mat

10-May-12 9:22

That probably does carry some truth Smile | :)

And that's why we love and hate C++

> The problem with computers is that they do what you tell them to do and not what you want them to do. <
> If it doesn't matter, it's antimatter.<

Re: reference to const int

Aescleal11-May-12 2:17

Aescleal

11-May-12 2:17

Responsible adult? You've never worked in the games industry Smile | :)

Biggest bunch of hackers (in the bad sense) out there.

Re: reference to const int

jschell11-May-12 13:21

jschell

11-May-12 13:21

Aescleal wrote:
Responsible adult? You've never worked in the games industry Biggest bunch of hackers (in the bad sense) out there.

I do however have extensive telecom and financial experience and the same thing applies there.

Re: reference to const int

CPallini9-May-12 22:53

CPallini

9-May-12 22:53

I don't know if (by the C++ specifications) that is the correct behaviour, however both the compiler provided by Visual Studio 2005 and g++ 4.4.3 gave me the same results you obtained.
Apparently (even with optimizations turned off) the compiler replaces each occurrence of i with the literal value 10, making all the changes you perform on the corrensponding memeory address irrelevant.
Compare the code generated for j and i in the cout lines:

j

ASM

cout << "j= " << j << endl;
0041143A  mov         eax,dword ptr [__imp_std::endl (4182B4h)]
0041143F  push        eax
00411440  mov         ecx,dword ptr [j] <=============== MEMORY ADDRESS
00411443  mov         edx,dword ptr [ecx]
00411445  push        edx
00411446  push        offset string "j= " (41570Ch)
0041144B  mov         eax,dword ptr [__imp_std::cout (4182BCh)]
00411450  push        eax
00411451  call        std::operator<<<std::char_traits<char> > (4110F0h)
00411456  add         esp,8
00411459  mov         ecx,eax
0041145B  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (4182C0h)]
00411461  mov         ecx,eax
00411463  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (4182C4h)]

ASM

cout << "i= " << i << endl;
00411469  mov         eax,dword ptr [__imp_std::endl (4182B4h)]
0041146E  push        eax
0041146F  push        0Ah  <================= LITERAL (10)
00411471  push        offset string "i= " (415710h)
00411476  mov         ecx,dword ptr [__imp_std::cout (4182BCh)]
0041147C  push        ecx
0041147D  call        std::operator<<<std::char_traits<char> > (4110F0h)
00411482  add         esp,8
00411485  mov         ecx,eax
00411487  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (4182C0h)]
0041148D  mov         ecx,eax
0041148F  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (4182C4h)]

Veni, vidi, vici.

Re: reference to const int - more

Richard MacCutchan10-May-12 0:32

Richard MacCutchan

10-May-12 0:32

If you change your reference statement to:

C++

const int& j = i; // do not cast away const-ness

you will not be allowed to increment the value of j as it is now a constant integer value and can only ever be 10.

Binding 100,000 items to a list box can be just silly regardless of what pattern you are following. Jeremy Likness

Re: reference to const int - more

Aabid10-May-12 1:06

Aabid

10-May-12 1:06

i know that, but i was trying to understand the previous behavior and that is still a mystery!

Re: reference to const int - more

Richard MacCutchan10-May-12 1:12

Richard MacCutchan

10-May-12 1:12

Aabid wrote:
the previous behavior and that is still a mystery!

Not really, in your original code you had:

C++

const int i = 10;
int& j = (int&)i;

so you are casting away the const-ness of i and effectively making j refer to a new variable, which starts out with the same value as i, but is modifiable since it is not a constant.

Binding 100,000 items to a list box can be just silly regardless of what pattern you are following. Jeremy Likness

Re: reference to const int - more

Aabid10-May-12 1:36

Aabid

10-May-12 1:36

But reference never creates a new variable, it is an alias to the same memory location, if you see the output of the code, you will see "i" and "j" have the same address.

ok, if the following statement creates a new variable, is there any way to get the address of that variable? i tried to print &(int&)i, it still gives the same address as "i" and "j";

int& j = (int&)i;

Re: reference to const int - more

Richard MacCutchan10-May-12 2:23

Richard MacCutchan

10-May-12 2:23

C++

const int i = 10;

This statement declares to the compiler that every time it sees the name i in a line of source code, it should replace it with the value 10. You have caused confusion in your source by declaring j as a reference to a non-const value of 10. The two are not the same. You cannot cast away the const-ness of a value in this way and expect it to remain constant.

Binding 100,000 items to a list box can be just silly regardless of what pattern you are following. Jeremy Likness

Re: reference to const int

Aescleal10-May-12 4:56

Aescleal

10-May-12 4:56

If you take the address or reference of a const object and assign it to a non-const pointer or reference and you then modify the value through that pointer or reference the compiler can do whatever it wants. Even laugh at you or fire nuclear missiles (if it's got any) while twirling it's moustache.

In C if you declare a constant it's just a read only variable. It has it's own memory location, everything's peachy and works the way you expect. In C++ it really is a constant for the rest of time. It'll never change. Wherever the compiler sees i it can (and probably does) just hardcode the value 10.

So what happens when you bind a non-const reference to a const value? Well anything can. But in your compiler's case it cooks up an anonymous temporary variable with the same lifetime as the reference and assigns whatever the constant is to it. This also happens when you take the address of a const as well and assign it to a non-const pointer.

What's actually happening is something like:

C++

int x = 10;

cout << "i= " << 10 << endl;
cout << "j= " << x << endl;
cout << "&i= " << &x << endl;
cout << "&j= " << &x << endl;

x++;

cout << "j= " << x << endl;
cout << "i= " << 10 << endl;

The only way you're going to change i is to hunt through the executable code and change the immediate value there Smile | :)

Incidentally how this happens is non-portable, it happens on VC++ and GCC but other compilers might have slightly different takes on it.

modified 10-May-12 11:51am.

Re: reference to const int

jschell10-May-12 9:23

jschell

10-May-12 9:23

Aabid wrote:
the code declares a const integer and a reference to it. incrementing a reference adds 1 to j and outputs as 11, that is fine.

I don't see that it is "fine".

The code is wrong - it shouldn't be written like that in the first place.
Once one starts with that then the resulting behavior no longer matters.

There are an infinite number of ways to write code incorrectly. There is a large, but finite, number of ways to write code correctly. Myself I don't care what the compiler does with incorrect code.

use mfc open "*.ppt"--can "use" the ppt

wangafei9-May-12 21:07

wangafei

9-May-12 21:07

Re: use mfc open "*.ppt"--can "use" the ppt

ThatsAlok9-May-12 21:08

ThatsAlok

9-May-12 21:08

are you talking about shellexecute win32 api verb?

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow
Never mind - my own stupidity is the source of every "problem" - Mixture

cheers,
Alok Gupta
VC Forum Q&A :- I/IV
Support CRY- Child Relief and You

Re: use mfc open "*.ppt"--can "use" the ppt

wangafei9-May-12 21:13

wangafei

9-May-12 21:13

yes,you can use win32 and api!
but it's best to use mfc
like:
you check a button,the dlg can show the "ppt" files

Re: use mfc open "*.ppt"--can "use" the ppt

David Crow10-May-12 2:37

David Crow

10-May-12 2:37

wangafei wrote:
but it's best to use mfc

MFC has no such mechanism for what you are after. Use ShellExecute() as has been suggested.

"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: use mfc open "*.ppt"--can "use" the ppt

Malli_S10-May-12 0:55

Malli_S

10-May-12 0:55

[Delegates] [Virtual Desktop] [Tray Me !]

-Malli...!

creating a dialog in non mfc application

Rajeev.Goutham9-May-12 21:01

Rajeev.Goutham

9-May-12 21:01

HI,

I have an application in which is not using mfc.But i got a requirement that i need to create and display a dialog box. Existing application is a Regular DLL.

Thanks & Regards,
Rajeev

Re: creating a dialog in non mfc application

ThatsAlok9-May-12 21:11

ThatsAlok

9-May-12 21:11

Since it regular DLL, i am guessing you are looking for pure win32 based solution (NOT ATL based): some of these article might help you!

Writing Win32 Apps with C++: V2 - part 1[^]
Scrollable Dialog in Pure Win32 API[^]
Win32 Tips and Tricks[^]

cheers,
Alok Gupta
VC Forum Q&A :- I/IV
Support CRY- Child Relief and You

Re: creating a dialog in non mfc application

wangafei9-May-12 21:15

wangafei

9-May-12 21:15

it's a good idea,but mfc si the best!

Re: creating a dialog in non mfc application

Rajeev.Goutham9-May-12 21:22

Rajeev.Goutham

9-May-12 21:22

HI Alok,

Problem is existing application in non mfc. for displaying a small dialog do i need to make that existing application also Mfc one. please guide.

Thanks,
Rajeev

Re: creating a dialog in non mfc application

ThatsAlok9-May-12 22:57

ThatsAlok

9-May-12 22:57

Rajeev.Goutham wrote:
Problem is existing application in non mfc. for displaying a small dialog do i need to make that existing application also Mfc one. please guide.

No not necessary, you can create dialog box in non-mfc environment also. MFC is just wrapper around win32 based apis

cheers,
Alok Gupta
VC Forum Q&A :- I/IV
Support CRY- Child Relief and You

Re: creating a dialog in non mfc application

wangafei10-May-12 16:40

wangafei

10-May-12 16:40

yes! mfc can use api and win32
so you can do anything without mfc
but it's not easy

Re: creating a dialog in non mfc application

ThatsAlok10-May-12 19:57

ThatsAlok

10-May-12 19:57

wangafei wrote:
yes! mfc can use api and win32

so you can do anything without mfc

but it's not easy

Agreed, however it's the project choice to utilize services of MFC or not. if the project is targeted to basic win32, then let it be.. anybody who can program in MFC can program in win32 too.

cheers,
Alok Gupta
VC Forum Q&A :- I/IV
Support CRY- Child Relief and You

Re: creating a dialog in non mfc application

Rajeev.Goutham9-May-12 21:24

Rajeev.Goutham

9-May-12 21:24

Thanks Alok, will check that.

Thanks,
Rajeev

Drawimage

john56329-May-12 2:09

john5632

9-May-12 2:09

I am trying to draw image using bitblt method. When I try to

CBitmap bitmap;
bitmap.LoadBitmap(IDB_BITMAP1);

it works and image is getting drawn but when I use

Bitmap *bitmap = Bitmap::FromFile(L"d:\\Projects\\res\\state.bmp");

It does not draw any image...

Pleas help to sort out the problem. I need to draw image directly from path, not from resource and using bitblt only..

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