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Feb 27th, 2007

C++ / Exceptions

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I'm having some trouble understanding how Exceptions thrown from constructors work...

Let's say I have a class like this:

C++ Syntax (Toggle Plain Text)
class A {
    public:
        A() {
            if( error )
                throw MyExp;
        }
}
class A {
    public:
        A() {
            if( error )
                throw MyExp;
        }
}

If I try to define a variable of type "A" within a try {} catch {} block, I run into problems with scope:

C++ Syntax (Toggle Plain Text)
int main() {
    try {
        A a_definition;
    } catch( Exp error ) {
        std::cout << error.message << endl;
    }
 
    a_definition; // out of scope now...
}
int main() {
    try {
        A a_definition;
    } catch( Exp error ) {
        std::cout << error.message << endl;
    }

    a_definition; // out of scope now...
}

Wrapping the whole program in a try/catch block doesn't seem very appealing to me! And while I could do something like this:

C++ Syntax (Toggle Plain Text)
    A *a_ptr = NULL;
    try {
        a_ptr = new A;
    } catch( Exp error ) { // so on...
    A *a_ptr = NULL;
    try {
        a_ptr = new A;
    } catch( Exp error ) { // so on...

It doesn't look very clean; I don't need a pointer, other than for the work-around.

Is there an easy way to do this?

Last edited by Darkmeerkat; Feb 27th, 2007 at 1:05 am. Reason: Didn't know I could make the code blocks language-specific!

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Darkmeerkat

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Feb 27th, 2007

Re: C++ / Exceptions

keep the constructor simple , do some basic initialization which can not fail, move rest of initialization to another function say init

ithelp

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Feb 27th, 2007

Re: C++ / Exceptions

Click to Expand / Collapse Quote originally posted by ithelp ...

keep the constructor simple , do some basic initialization which can not fail, move rest of initialization to another function say init

Thanks, I can do that.
Just out of curiosity, is there any way to write this with more of a "RAII"-ish behavior, without having to split acquisition and initialization up?

Darkmeerkat

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Feb 28th, 2007

Re: C++ / Exceptions

First, there is nothing wrong with this code.

c Syntax (Toggle Plain Text)
int main() {
    try {
        A a_definition;
    } catch( Exp error ) {
        std::cout << error.message << endl;
    }
 
    a_definition; // out of scope now...
}
int main() {
    try {
        A a_definition;
    } catch( Exp error ) {
        std::cout << error.message << endl;
    }

    a_definition; // out of scope now...
}

a_definition is indeed out of scope at teh place you mention, but so what ?! You are anyway using it inside the try-catch block (I hope) so you anyway don't it outside the try catch, so let it go out of scope. Now if you really want a code that gives a fake feeling of cleanliness you can do this:

c Syntax (Toggle Plain Text)
do_that_thing_with_a_definition()
{
    A a_definition;
    //a_definition is in scope.. a neat and clean function...
}
int main() {
    try {
        do_that_thing_with_a_definition() ;
    } catch( Exp error ) {
        std::cout << error.message << endl;
    }
 
    //following is irrelevant now
    //a_definition; // out of scope now...
}
do_that_thing_with_a_definition()
{
    A a_definition;
    //a_definition is in scope.. a neat and clean function...
}
int main() {
    try {
        do_that_thing_with_a_definition() ;
    } catch( Exp error ) {
        std::cout << error.message << endl;
    }

    //following is irrelevant now
    //a_definition; // out of scope now...
}

thekashyap

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Feb 28th, 2007

Re: C++ / Exceptions

Your second example is even worse because the object will go out of scope as soon as do_that_thing() returns, so any usage of it inside the try ... catch block will now be illegal.

Back on topic:
If you really need to use code outside the constructor, use pointers, although it is a bad design if you are forced to use an object out of its try catch block.

John A

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Re: C++ / Exceptions

Click to Expand / Collapse Quote originally posted by joeprogrammer ...

Your second example is even worse because the object will go out of scope as soon as do_that_thing() returns, so any usage of it inside the try ... catch block will now be illegal.

May be I wasn't clear. What was meant is that create the object and do whatever you wanna do with the object "inside" do_that_thing_with_a_definition(). I thot the name of the function will make it clear. Anyway..

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Mar 1st, 2007

Re: C++ / Exceptions

Click to Expand / Collapse Quote originally posted by thekashyap ...

May be I wasn't clear. What was meant is that create the object and do whatever you wanna do with the object "inside" do_that_thing_with_a_definition(). I thot the name of the function will make it clear. Anyway..

Well, except that here:

C++ Syntax (Toggle Plain Text)
int main() {
    try {
        do_that_thing_with_a_definition() ;
    } catch( Exp error ) {
        std::cout << error.message << endl;
    }
 
    //following is irrelevant now
    //a_definition; // out of scope now...
int main() {
    try {
        do_that_thing_with_a_definition() ;
    } catch( Exp error ) {
        std::cout << error.message << endl;
    }

    //following is irrelevant now
    //a_definition; // out of scope now...

}

You basically implied that a_definition would be in scope out of the try...catch if you used do_that_thing() function. However, that's not the case. It suffers the same fate as the previous problem, so putting it in a function doesn't really do much good. In fact, you've effectively reduced the scope of the object by allocating it in a function.

John A

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Mar 2nd, 2007

Re: C++ / Exceptions

Problem stmt is: "It doesn't look very clean; I don't need a pointer". So the solution is such that it is "clean" and "doesn't need a pointer".

thekashyap

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Mar 2nd, 2007

Re: C++ / Exceptions

Click to Expand / Collapse Quote originally posted by thekashyap ...

Problem stmt is: "It doesn't look very clean; I don't need a pointer". So the solution is such that it is "clean" and "doesn't need a pointer".

Clean as it might be, it still doesn't solve the OP's original problem -- which is to be able to use the object outside of the try...catch block.

John A

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