Is it possible to catch an exception raised by a member variable?

[Angus]

Hello

I have a class I am using which raises an exception in its constructor
if certain things aren't in place. I can easily create the situation
where an exception is raised.

If I use the create a member variable in a class using this class then
how do I catch the exception?

For now I have defined a member function as a pointer and in my
constructor in my class using the class which raises the exception, I
do a try catch block and do a new object. That works. But is it
possible to do a similar thing using a member variable?

Angus

[James Kanze]

On Sep 28, 4:33 pm, "Chris ( Val )" <chris...@gmail .comwrote:

On Sep 28, 10:52 pm, Pete Becker <p...@versatile coding.comwrote :

On 2007-09-28 05:30:31 -0400, "Chris ( Val )" <chris...@gmail .comsaid:

int main()
{
Base* B;

try {
B = new Base( "Oracle.driver. bar" );
}
catch( const std::exception& e )
{
B->Print();
delete B;
}

std::cin.get();
return 0;
}

I am interested to hear your, and the groups
thoughts on the validity of such a construct.

It really doesn't show anything. Replace the "B = new ..." with "throw
std::exception( );" and you'll probably get the same result. Calling
member functions on uninitialized pointers produces undefined behavior,
so anything you see is as valid as anything else.

I tried a non pointer version, and the results are the same.

Could you show it. If you replace the line with new Base...,
with something like:

Base b( "Oracle.driver. bar" ) ;

then b isn't even in scope in the catch block, and cannot be
accessd.

What I am curious about is at what point does the object actually
cease to exist? (which scope?)

Which object. The object whose destructor exits via an
exception never begins to exist, so the question of when it
ceases to exist is moot.

Is it not possible for it to even be partially constucted to
report such information back?

No. Or rather, it can put information in the exception, which
will propagate up. But if a constructor exits because of an
exception, all fully constructed sub-objects are immediately
destructed, and if the constructor was called as part of a new
expression, the allocated memory is immediately freed.

--
James Kanze (GABI Software) email:ja******* **@gmail.com
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[Chris ( Val )]

On Sep 29, 8:38 am, James Kanze <james.ka...@gm ail.comwrote:

On Sep 28, 11:30 am, "Chris ( Val )" <chris...@gmail .comwrote:

On Sep 28, 7:03 pm, James Kanze <james.ka...@gm ail.comwrote:

[...]

But you still don't have an object. You can use function try
blocks to remap the exception, or to treat it as a fatal error
(e.g. by calling abort or exit), but you cannot return normally
from the constructor, and the object that was being constructed
will not exist.

I have produced a crude example that will attempt to
prove otherwise:
# include <iostream>
# include <string>
# include <exception>
struct DataSource {
DataSource( std::string ds )
{
if( ds != "Oracle.driver. foo" )
throw "Could not connect to database";
}
};
class Base
{
private:
DataSource Ds;
public:
Base( std::string );
~Base() { std::cout << "Destructin g now...\n"; }
void Print()
{ std::cout << "I am still alive - Please try again.\n"; }
};
Base::Base( std::string ds )
try // function-try block
: Ds( ds ) {}
catch( const char* msg ) {
std::cout << "Exception Caught: \"" << msg << "\"" << '\n';
throw std::exception( );
}
int main()
{
Base* B;
try {
B = new Base( "Oracle.driver. bar" );
}
catch( const std::exception& e )
{
B->Print();

This is undefined behavior, since it accesses an uninitialized
pointer. If you get here, the assign in the try block has never
occured (and there is no Base object).

delete B;
}
std::cin.get();
return 0;
}
I am interested to hear your, and the groups
thoughts on the validity of such a construct.

Totally invalide.

The non-pointer version that I spoke of was
totally wrong. I thought about it after I
posted, amd realised I was just assigning
a new object in the catch block. It was late
and I totally confused myself :-)

Thanks to both you and Peter for clarifying it.

Cheers,
Chris Val

[Chris ( Val )]

On Sep 29, 12:57 am, Pete Becker <p...@versatile coding.comwrote :

On 2007-09-28 10:54:39 -0400, Pete Becker <p...@versatile coding.comsaid:

On 2007-09-28 10:33:00 -0400, "Chris ( Val )" <chris...@gmail .comsaid:

What I am curious about is at what point does the object actually
cease to exist? (which scope?)

Well, formally, it never existed, because its constructor didn't run to
completion. Mechanically, what happens is that when the exception
escapes from the try block

Sorry, I suspect that should be "from the catch clause". I'm a bit
pressed for time this morning, so I haven't looked it up.

No problem at all, I appreciate the help and example.

Thanks,
Chris Val