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Hi,
A colleague of mine is using a syntactic construct I have never encountered before and I thought was incorrect. But, against all (my) odds, it appears to work.
The gist of it can be reduced to:
<br />
std::string function(void)<br />
{<br />
return "hello";<br />
}<br />
<br />
void sample(void)<br />
{<br />
std::string& reference = function();<br />
<br />
std::cout << reference + ", world!" << std::endl;<br />
}<br />
I have tested this with Visual Studio .NET, under Debug and Release build and no crash occurred. Since Visual Studio .NET is supposed to be standard compliant, it must be OK then.
Somehow, I don't feel it's correct, but I don't know how to make sure about this.
Can anyone give me some precisions?
Cheers
--
Maxime Labelle
maxime.labelle@freesurf.fr
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Maxime Labelle wrote:
Somehow, I don't feel it's correct, but I don't know how to make sure about this.
Well, you're right. It's not correct code, and is particularly dangerous. The fact that it didn't crash is purely luck. Basically, you can't take the address (which is what a reference boils down to) of a temporary object, such as the one returned by function(), because the compiler may destroy the object when it hits the next reparse point, in this case the end of the statement. I guess the compiler is recognising that the object is still referred to and is delaying the destruction of it, but I've never seen it happen before, and would strongly recommend against relying on it.
I'm not sure what the standard says about this, but it's certainly not safe code.
Ryan "Punctuality is only a virtue for those who aren't smart enough to think of good excuses for being late" John Nichol "Point Of Impact"
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Err,
as a matter of fact the compiler complains with warning level 4. But what about the following code, then:
<br />
const std::string& reference = function();<br />
Cheers.
--
Maxime Labelle
maxime.labelle@freesurf.fr
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What was the warning? I find it best to always set the warning level to 4 and to try and eliminate every warning.
Adding the const makes no difference, the code is still wrong and cannot be relied upon to work..
Mike
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Ryan Binns wrote:
Well, you're right. It's not correct code, and is particularly dangerous.
I don't have time to look it up now but I'm virtually certain there's an utterly obscure rule floating around in the spec somewhere: if a function returns a temporary object into a reference variable, that temporary object must stay alive for as long as the reference variable does.
I remember studying some funky magic written by one of those C++ gurus and he was relying on this behaviour.
[edit]
And yes, making the reference const may have had something to do with it
[/edit]
Lets be honest, isn't it amazing how many truly stupid people you meet during the course of the day. Carry around a pad and pencil, you'll have twenty or thirty names by the end of the day - George Carlin
Awasu 2.0.3 [^]: A free RSS reader with support for Code Project.
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Taka Muraoka wrote:
I'm virtually certain there's an utterly obscure rule floating around in the spec somewhere: if a function returns a temporary object into a reference variable, that temporary object must stay alive for as long as the reference variable does
There quite possibly is. I don't remember seeing it, but that doesn't mean it's not there  . I still think it's dangerous and shouldn't be relied on.
Taka Muraoka wrote:
I remember studying some funky magic written by one of those C++ gurus and he was relying on this behaviour.
The only reason I can see for doing this is to prevent a potentially time-consuming copy of a large object. In this case, though, I'd see it as more intuitive to return a pointer to a new'd object that would then need to be deleted when finished with. It's much easier for a maintenance programmer to see what's going on, even if it might be slightly less safe.
Ryan "Punctuality is only a virtue for those who aren't smart enough to think of good excuses for being late" John Nichol "Point Of Impact"
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hi,
for my evc++ project (runs in WINCE Emulator) which meant for a device , i hav to
display some chars like ▲▼ and left , right arrows in a edit box/ label reading from file.
i am just saving this symbols with some text in a unicode file.while
reading it is read as '?' and i am unable to display those symbols in a
edit box / static text .those symbols are just displayed as some garbage symbols ie box symbol always .
i am using CFile obj for reading from file and using setdlgitemtext method to display it ,CString obj i use to store text.
any project settings need to be changed to display unicode chars? or any conversion has to be made before displaying in wince emulator?
it will be very helpful to me if anybody can help in this regard.
expecting a reply,
R.soodamani
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Hello:
How can Profile the ISAPI Filter in VC.NET?
I'm able to Profile ISAPI DLL in Visual C++ 6.
Thanks
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I have a MDI application when it is created the memory occupied for that application in task manager is around 2000K i open some document in that MDI application, the usage of memory grows up. Now i minimize the main frame window and i notice in task manager the memory is decreased to around 1000K.
I think the resouces which were not used are released when the mainframe is minimized.
Please let me know if anybody knows....
Which function is called when the mainframe is minimized?
How the resources are released when it is minimized?
vidya
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What you are seeing is probably the result of RAM being swapped to disk (i.e., virtual memory). This is SOP with Windows.
"When I was born I was so surprised that I didn't talk for a year and a half." - Gracie Allen
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How to use VC++ with MsWord.olb to create a table in a *.doc?
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Hi,
I have been asked to possibly make modifications/tweaks to a program my colleague made about a year ago. It is all written in MFC etc... He was good at that and more importantly is no longer working for us, I am not good at it, never done it, never had the time As I was always busy writing C programs for diverse platforms. As I try to compile his software everything works except for it does not recognise a "CFileHeader" data type. Logically I would think this needs to be defined in some header file somewhere. However, I cannot find it anywhere. Can anybody tell me where this needs to be defined and where i can find/introduce the definition ??
Any help much appreciated.
Filip
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A 'brief' search of April 2004 MSDN did not bring up anything on the CFileHeader.
I think you have a source and/or header file missing from your project.
Use the 'Find in Files' menu command from Visual Studio to look for CFileHeader in the INL, H, and HPP files of your project. Then #include that file into the source file using the CFileHeader.
You might also want to make sure you have the CPP file with the CFileHeader as part of the project, or you will end up with link errors.
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Hi,
Thanks for your help, much appreciated. In the mean time I have found the required header file etc... It is part of a zip/unzip utility that was published on codeproject. Having located that one in a backup we had I have made some progress. However, now there is some other weird error message. I now suppose that my colleague had set up his machine/project in a particular way and did not mention that when he left. I guess we probably still have all the files and I will just have to figure it out the hard way. Still, as said before, your help is much appreciated.
All for now,
Filip
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Hi,
I want to a MFC date time control which can take milliseconds, seconds, minutes & hrs into consideration. I am aware of only CDateTimeCtrl in vc++, but this cannot take milliseconds. I also need to extract time from such a control and do <,>,== comparisions. Can anyone please help me?
Thanks
Madhavi
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Thanks for the inputs.
CTime does not take milliseconds..am I right? Anyway I will take a look at the other classes which you have mentioned.
madhavi
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You are right, CTime does not have milliseconds.
ARSALAN MALIK
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Hi,
I want to a MFC date time control which can take milliseconds, seconds, minutes & hrs into consideration. I am aware of only CDateTimeCtrl in vc++, but this cannot take milliseconds. I also need to extract time from such a control and do <,>,== comparisions. Can anyone please help me?
Thnaks
Madhavi
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Hello Friends,
Is it possible to create a windowsNT/2000 services without threads.The problem I am facing is that i cant set a timer inside a service.
Is there any workaround for this prob?
regards,
John
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Yeah,
if you don't mind i have a suggestion use Waitforsingleobject api inside the thread for wait for event which would set it self every time timer message arrived for example-->
your thread ->
void thread(lpvoid p)
{
while(1)
{
WaitForSingleObject(event);
----
your functioning
-------
-
--
}
}
and
OnTimer(UINT nIdEvent)//timer function
{
SetEvent(event);
}
simple and consise way for creating service
-----------------------------
"I Think It will Work"
Formerly Known As "Alok The Programmer" at CP
-----------------------------
Alok Gupta
visit me at http://www.thisisalok.tk
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I want to know:
There is any free compiler under windows for c++?
And, there is any free graphical develop under windows too?
Elfo,thanks
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GCC+Eclipse?
I have not used either, but have heard it is not bad, and can work together.
Igor Green
http://www.grigsoft.com
Compare It! + Synchronize It! : Files and folders comparison never was easier!
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yeah ther are two compiler i caome across
->DevShed C++ compiler
--> GNU compiler
or serach http://www.cpp-home.com for compiler
-----------------------------
"I Think It will Work"
Formerly Known As "Alok The Programmer" at CP
-----------------------------
Alok Gupta
visit me at http://www.thisisalok.tk
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