Re: compile error about destructor

From:

"Ben Voigt [C++ MVP]" <rbv@nospam.nospam>

Newsgroups:

microsoft.public.vc.language

Date:

Tue, 19 Feb 2008 19:35:18 -0600

Message-ID:

<OXiuZC2cIHA.5900@TK2MSFTNGP02.phx.gbl>

George wrote:

Hi Ben,

I have written a sample below to verify you need not worry about stack
unwinding -- even if for structured exception, during stack unwinding,
destructor is called.

This is a different example, now you don't even have a __try block.

The __except clause is much more powerful than catch (...), for example with
__except I can print out a full stack trace of the instruction that caused
the exception. If you want to use both __except and destructors, they must
be in different functions. Then everything will unwind correctly (because,
as you note, structured exceptions unwinding cause C++ destructors to run).

Here is my code and output, let me know if you have any comments.
Thanks. :-)

Output:

Constructor
Destructor
catch C++ and structured exception

Code:

#include <iostream>

using namespace std;

class Foo {
public:
Foo()
{
cout << "Constructor " << endl;
}

virtual ~Foo()
{
cout << "Destructor " << endl;
}
};

int main()
{
try{

Foo f;
int i = 100;
int j = 0;
j = i / j;
}catch (...)
{
cout << "catch C++ and structured exception " << endl;
}

return 0;
}

regards,
George

"Ben Voigt [C++ MVP]" wrote:

George wrote:

Thanks Ben,

1.

With /EHa, SEH and C++ exceptions may be mixed, but not in the same

I agree and this is what we proved in his thread before.

2.

function. Separate the __try block body into a separate function.

What do you mean separate the __try block into another function? We

George, why do you always misquote people? I did not say "separate
the __try block into another function", but "Separate the __try
block body into a separate function."

For example, this can't work:

void f(void)
{
    __try {
        this_could_fail();
        auto_ptr<T> temp = new T();
        temp->something_likely_to_fail();
    }
    __finally {
    }
}

The problem is the mixing of C++ destructors with SEH, because both
need unwinding. Whether the destructor for temp should be called
depends on whether the constructor completed. So we make the body
of the __try block a separate function:

void fhelper(void)
{
        this_could_fail();
        auto_ptr<T> temp = new T();
        temp->something_likely_to_fail();
}

void f2(void)
{
    __try {
        fhelper();
    }
    __finally {
    }
}

Now the local inside fhelper can be properly unwound when a
structured exception is thrown, because they exist in a unique stack
frame that the compiler support for SEH knows how to deal with.

always provide try/catch or __try/__except into the same function. I
am confused. Could ytou provide some pseudo code to show your points
please?

regards,
Geprge

"Ben Voigt [C++ MVP]" wrote:

Igor Tandetnik wrote:

"George" <George@discussions.microsoft.com> wrote in message
news:625FF32F-F1B8-4989-B476-8272396CA81A@microsoft.com

I compile with /EHa, and not /EHsc, and this is why I have
confusion there is such error.

Double-check your project settings (perhaps you have a file-level
override). The documentation claims C2712 error should not be
produced under /EHa.

With /EHa, SEH and C++ exceptions may be mixed, but not in the same
function. Separate the __try block body into a separate function.