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I agree Maxwell!
But could we come back to the original question please? Suppose we have to maintain some code using structured exception.
Why __try/__except does not work but try/catch work with structured exception?
regards,
George
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MSDN said that:
<br />
You should not use structured exception handling in functions that use objects with destructors. If an exception occurs, the destructor cannot be called. Use C++ exception handling instead.
I have never used SEH, c++'s exception handling is more powerful, I think
A Chinese VC++ programmer
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No zengkun100,
The destructor could be invoked even if it is structured exception, if you change my code from __try to try, and change __except to catch.
Here is the output,
constructing Foo
destructing Foo
access violation caught
You can see if we change the keyword, structured exception is caught and destructor is called before handler.
Not as you quoted -- "If an exception occurs, the destructor cannot be called.".
regards,
George
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George_George, the answer is very clear now.
Different keyword, different exception handling method.
Use C++ exception handling whenever possible
A Chinese VC++ programmer
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Any rule in C++ mentioned we can not use object with destructor in __try block with /EHa compile option? I can not find. So, I do not agree.
regards,
George
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George_George wrote: So, I do not agree.
You are getting stubborn.
George_George wrote: Any rule in C++ mentioned we can not use object with destructor in __try block with /EHa compile option?
The C++-Standard is not about MS-specific compiler internal like /EHa or __try
Which with its two leading underscores clearly says "I am vendor-specific".
Fact is, that you can tell the compiler to use C++ exceptions and you can tell the compiler to use the Standard-predating SEH.
Let's think the unthinkable, let's do the undoable, let's prepare to grapple with the ineffable itself, and see if we may not eff it after all. Douglas Adams, "Dirk Gently's Holistic Detective Agency"
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George_George wrote: Please read to get my analysis,
We are not here to do your work, George. Most of us have jobs, so we help here when we can, not every time you find something that you disagree with. A little due diligence on your part will go a long way.
"Normal is getting dressed in clothes that you buy for work and driving through traffic in a car that you are still paying for, in order to get to the job you need to pay for the clothes and the car and the house you leave vacant all day so you can afford to live in it." - Ellen Goodman
"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne
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Thanks all the same, DavidCrow!
I appreciate your participation.
regards,
George
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George_George wrote: So jhwurmbach, your point is __try and __except does not work with local object with destructor?
NO. His point is that C++ spec is not going to discuss vendor specific compiler options. I don't understand why anyone would even need to explain that, it seems so obvious.
George_George wrote: Is there a rule in MSDN covering this?
Zengkun100 already posted it. If you are not going to read peoples replies with the intention of listening to them and understanding them what is the point?
led mike
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Thanks led mike,
led mike wrote: NO. His point is that C++ spec is not going to discuss vendor specific compiler options. I don't understand why anyone would even need to explain that, it seems so obvious.
I agree and for myself, I will not use __try/__except, and I just need to maintain some legacy code.
have a good weekend,
George
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Try compiling without the /EHsc switch.
As opposed to SEH, C++ exception handling model supports "stack unwinding". It is aware of automatic objects, and it calls their destructors.
Basically, use C++ exception handling where it is possible, and fallback to SEH only when there are no other ways.
See here.
"Normal is getting dressed in clothes that you buy for work and driving through traffic in a car that you are still paying for, in order to get to the job you need to pay for the clothes and the car and the house you leave vacant all day so you can afford to live in it." - Ellen Goodman
"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne
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Hi DavidCrow,
I compile with /EHa in my question and experiment. If you have Visual Studio, you can have a try.
1. If you remove destructor, it is fine;
2. If you change to try/catch, it is fine.
I do not know why.
regards,
George
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George_George wrote: 2. If you change to try/catch, it is fine.
As opposed to SEH, C++ exception handling model supports "stack unwinding". It is aware of automatic objects, and it calls their destructors.
George_George wrote: I do not know why.
Now you do.
"Normal is getting dressed in clothes that you buy for work and driving through traffic in a car that you are still paying for, in order to get to the job you need to pay for the clothes and the car and the house you leave vacant all day so you can afford to live in it." - Ellen Goodman
"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne
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Hi DavidCrow,
But even if in my sample, we use try/catch, we are still dealing with structured exception -- access violation, and access violation is not a C++ exception.
So, when change to try/catch, I do not agree with you we are in C++ exception category. Any comments?
DavidCrow wrote: As opposed to SEH, C++ exception handling model supports "stack unwinding". It is aware of automatic objects, and it calls their destructors.
regards,
George
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George_George wrote: ...we use try/catch, we are still dealing with structured exception...
Not according to everything that I've read on the subject. Microsoft's __try /__catch is SEH.
Perhaps you should read this.
"Normal is getting dressed in clothes that you buy for work and driving through traffic in a car that you are still paying for, in order to get to the job you need to pay for the clothes and the car and the house you leave vacant all day so you can afford to live in it." - Ellen Goodman
"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne
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Yes, DavidCrow.
What I do not agree with you is you mentioned when there is structured exception, during stack unwinding, the local object's destructor is not called.
Through my experiment, it is not true.
You can try the following code and see if structured exception occurs, like access violation, during stack unwinding, destructor is also called.
So the question can be asked in two ways,
1. Why __try/__except does not work with structured exception with destructor;
2. Why try/catch work with structured exception with destructor.
Now we are talking about (2).
#include <iostream>
#include <excpt.h>
#include <windows.h>
using namespace std;
class Foo
{
public:
Foo()
{
cout << "constructing Foo" << endl;
}
virtual ~Foo()
{
cout << "destrucing Foo" << endl;
}
};
int main()
{
int* address = NULL;
try{
Foo foo1;
(*address) = 1024;
} catch (...)
{
cout << "access violation caught" << endl;
}
return 0;
}
</windows.h></excpt.h></iostream>
regards,
George
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Okay.
SEH is different from C++ exception.__try __catch block indicates that this is a SEH, and try catch indicates that this is a C++ exception.
MSDN said that :in Visual C++ 2005, all objects in scope when the asynchronous exception is generated will not be destroyed even if the asynchronous exception is handled.
where asynchronous exception is SEH. In your code, Foo's destructor will not be called, so compiler generates an error. If you comment this line cout << "destrucing Foo" << endl; int Foo's destructor, and your code will be compiled.
A Chinese VC++ programmer
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Great, zengkun100!
1.
zengkun100 wrote: Foo's destructor will not be called, so compiler generates an error. If you comment this line cout << "destrucing Foo" << endl; int Foo's destructor, and your code will be compiled.
No. You can try to comment out the statements in destructor and the compiler error still exists.
2.
try/catch can catch structured exception, in my case, access violation is not a C++ exception and is a structured exception, agree? But try/catch can catch it and calling destructor, unwinding stack quite well.
You can try (2) by changing my sample from __try to try and __except to catch(...).
regards,
George
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I don't know which compiler you use.
My IDE is VS2008 beta 2, after I commented the line, the program compiled successfully.MSDN said that
<br />
Specifies the model of exception handling to be used by the compiler and destroys C++ objects that will go out of scope as a result of the exception. If /EH is not specified, the compiler will catch structured and C++ exceptions, but will not destroy C++ objects that will go out of scope as a result of the exception.<br />
<br />
<br />
/EH{s|a}[c][-]<br />
<br />
<br />
Arguments<br />
a<br />
The exception-handling model that catches asynchronous (structured) and synchronous (C++) exceptions.<br />
<br />
s<br />
The exception-handling model that catches C++ exceptions only and tells the compiler to assume that extern C functions do throw an exception.<br />
<br />
c <br />
If used with s (/EHsc), catches C++ exceptions only and tells the compiler to assume that extern C functions never throw a C++ exception. /EHca is equivalent to /EHa.<br />
<br />
Remarks<br />
Use /EHs to specify the synchronous exception handling model (C++ exception handling without structured exception handling exceptions). If you use /EHs, then your catch clause will not catch asynchronous exceptions. Also, in Visual C++ 2005, all objects in scope when the asynchronous exception is generated will not be destroyed even if the asynchronous exception is handled. Under /EHs, catch(...) will only catch C++ exceptions. Access violations and System..::Exception exceptions will not be caught.<br />
<br />
Use /EHa to specify the asynchronous exception handling model (C++ exception handling with structured exception handling exceptions). /EHa may result in a less performant image because the compiler will not optimize a catch block as aggressively, even if the compiler does not see a throw.<br />
<br />
Use /EHa if you want to catch an exception raised with something other than a throw. The following sample will generate an exception:<br />
<br />
Copy Code <br />
#include <iostream><br />
#include <excpt.h><br />
using namespace std;<br />
<br />
void fail() {
try {<br />
int i = 0, j = 1;<br />
j /= i;
}<br />
catch(...) {
cout<<"Caught an exception in catch(...)."<<endl;<br />
}<br />
}<br />
<br />
int main() {<br />
__try {<br />
fail(); <br />
}<br />
<br />
__except(EXCEPTION_EXECUTE_HANDLER) { <br />
cout << "An exception was caught in __except." << endl;<br />
}<br />
}<br />
<br />
<br />
</excpt.h></iostream>
And access violation is called hard exception, NOT structured exception. I have said that __try __catch block indicates a structured exception.
Jeffery Richter's great book «Programming Applications for Microsoft Windows»has a good explanation about SEH
A Chinese VC++ programmer
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Thanks zengkun100,
Your reply is great!
Two more comments,
1.
zengkun100 wrote: Use /EHs to specify the synchronous exception handling model (C++ exception handling without structured exception handling exceptions). If you use /EHs, then your catch clause will not catch asynchronous exceptions. Also, in Visual C++ 2005, all objects in scope when the asynchronous exception is generated will not be destroyed even if the asynchronous exception is handled. Under /EHs, catch(...) will only catch C++ exceptions. Access violations and System..::Exception exceptions will not be caught.
It means under /EHs mode, structured exception still exists, but we can not catch it and destructor of local object is not called during structured exception triggered stack unwinding. Right?
BTW: but I have proved it is not true. I have posted my sample below and if you compile with /EHsc and you can see the output is -- means destructor is still called during stack unwinding, not the same as the above statement from MSDN.
--------------------
constructor
destructor
--------------------
2.
I have wrote some code to verify that when using /EHa, during exception triggered stack unwinding, the destructor of local object is invoked. Could you help to review my code to see whether both my conclusion and the code are correct please?
#include <iostream>
using namespace std;
class Foo
{
public:
Foo()
{
cout << "constructor" << endl;
}
virtual ~Foo()
{
cout << "destructor" << endl;
}
};
int main ()
{
int i = 1;
int j = 0;
try{
Foo foo;
i = i / j;
} catch (...)
{
cout << "catch structured exception -- division by zero" << endl;
}
}
My output:
--------------------
constructor
destructor
catch structured exception -- division by zero
--------------------
have a good weekend,
George
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this occurs while iam trying to use domodal()
only in debug mode..
it works fine in release mode. help me to get rid of this...
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If it happens in debug mode, then have you tried debugging it? Work backwards from the exception, and find out what wrong information you're giving to MFC?
Iain.
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Maybe posting the code where exception happens will help.
I can guess it is an ASSERT though.
If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.
[my articles]
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Its in appcore.cpp
// CWinApp diagnostics
#ifdef _DEBUG
void CWinApp::AssertValid() const
{
CWinThread::AssertValid();
ASSERT(afxCurrentWinApp == this);
ASSERT(afxCurrentInstanceHandle == m_hInstance);
if (AfxGetThread() != (CWinThread*)this)
return; // only do subset if called from different thread
if (m_pDocManager != NULL)
ASSERT_VALID(m_pDocManager);
}
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