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Alan Griffiths <aGriffiths@ma.ccngroup.com> wrote:

>I've just tried porting some code to MSVC4 (it compiles and works with 
>SC7.1 and BC4.5).
>
>In effect it appears that the Microsoft compiler _requires_ anything 
>thrown as an exception to have an accessible copy constructor.  (I've 
>tended to make copy constructors private to ensure catch-by-reference
>and aviod slicing bugs.)
>
>Looking through the DWP I can't see any reason to require a copy 
>constructor, but there are references to copying temporaries.

          What DWP did you read? The January 96 version requires that the 
"throw" statement have access to the copy constructor and destructor of a 
thrown object.

                                    David
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On 05 Apr 96 02:58:28 GMT, Alan Griffiths <aGriffiths@ma.ccngroup.com>
wrote:

>I've just tried porting some code to MSVC4 (it compiles and works with 
>SC7.1 and BC4.5).
>
>In effect it appears that the Microsoft compiler _requires_ anything 
>thrown as an exception to have an accessible copy constructor.  (I've 
>tended to make copy constructors private to ensure catch-by-reference
>and aviod slicing bugs.)
>
>Looking through the DWP I can't see any reason to require a copy 
>constructor, but there are references to copying temporaries.
>
>Is MSVC4 correct?
>
>code:
>class MyBaseException {
>public:  MyBaseException() {}
>         virtual char* what() = 0;
>private: MyBaseException(const MyBaseException&);
>};
>
>class MyException1 : public  MyBaseException {
>public:     virtual char* what() { return "MyException1"; }
>};
>
>int main() {
>    try     {
>        throw MyException1();
>// error C2700: 'class MyException1' : cannot be thrown (use -W4 for more info)
>
>// warning C4671: 'MyException1' : the copy constructor is inaccessible
>    } catch (MyBaseException& e) {
>        cout << "OK - got here (" << __LINE__ << ") : have a " << e.what()
>		<< endl;
>    } catch (...) {
>        cout << "ERROR - got here (" << __LINE__ << ")" << endl;
>    }
>    return 0;
>}
>

Here's what BC++ 5.0 generates...

Borland C++ 5.0 for Win32 Copyright (c) 1993, 1996 Borland
International
MS-EXCPT.CPP:
Error MS-EXCPT.CPP 15: Compiler could not generate copy constructor
for class 'MyException1' in function main()
*** 1 errors in Compile ***

It appears we have an agreement in interpretation of the DWP.

Jason
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Alan Griffiths <aGriffiths@ma.ccngroup.com> writes:

>I've just tried porting some code to MSVC4 (it compiles and works with 
>SC7.1 and BC4.5).

>In effect it appears that the Microsoft compiler _requires_ anything 
>thrown as an exception to have an accessible copy constructor.  (I've 
>tended to make copy constructors private to ensure catch-by-reference
>and aviod slicing bugs.)

According to the draft standard, when you throw something, what is
thrown is actually a copy of that something. If the copy constructor
for that something is not accessible, the copy can't be made.

Under some circumstances the copy can be optimized away. The draft
standard was recently clarified to say that when logically a copy
is needed, the copy constructor must be accessible even if in a
particular case a compiler happens to optimize away the copy.

In your case, you will probably have to throw a pointer instead
of a value, if the type has a private copy constructor.
--
Steve Clamage, stephen.clamage@eng.sun.com
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Steve Clamage wrote:
> 
> Alan Griffiths <aGriffiths@ma.ccngroup.com> writes:
> 
> >I've just tried porting some code to MSVC4 (it compiles and works with
> >SC7.1 and BC4.5).
> 
> >In effect it appears that the Microsoft compiler _requires_ anything
> >thrown as an exception to have an accessible copy constructor.  (I've
> >tended to make copy constructors private to ensure catch-by-reference
> >and aviod slicing bugs.)
> 
> According to the draft standard, when you throw something, what is
> thrown is actually a copy of that something. If the copy constructor
> for that something is not accessible, the copy can't be made.
> [snip]

> In your case, you will probably have to throw a pointer instead
> of a value, if the type has a private copy constructor.

You'll have to implement an exception object management policy.  
(I know the exception is actually the pointer to some class, 
but I'm using "exception object" to mean the object pointed 
to by that exception.)  You must allocate your exception objects 
on the heap, and delete them in the final catch block.  That is, 
so long as you throw the pointer up the stack, as it were, you 
cannot delete what the pointer points to.  Only in the catch 
block in which you no longer rethrow the exception can you 
delete the memory (exception object) it points to.

-- 
Robert Stewart		| My opinions are usually my own.
Datalytics, Inc.	| stew@datalytics.com
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