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C++ / Exceptions

 
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  #1
Feb 27th, 2007
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) 
  1. class A {
  2.     public:
  3.         A() {
  4.             if( error )
  5.                 throw MyExp;
  6.         }
  7. }

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) 
  1. int main() {
  2.     try {
  3.         A a_definition;
  4.     } catch( Exp error ) {
  5.         std::cout << error.message << endl;
  6.     }
  7.  
  8.     a_definition; // out of scope now...
  9. }

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) 
  1.     A *a_ptr = NULL;
  2.     try {
  3.         a_ptr = new A;
  4.     } 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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Re: C++ / Exceptions

 
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  #2
Feb 27th, 2007
keep the constructor simple , do some basic initialization which can not fail, move rest of initialization to another function say init
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Re: C++ / Exceptions

 
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  #3
Feb 27th, 2007
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Originally Posted by ithelp View Post
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?
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Re: C++ / Exceptions

 
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  #4
Feb 28th, 2007
First, there is nothing wrong with this code.

c Syntax (Toggle Plain Text) 
  1. int main() {
  2.     try {
  3.         A a_definition;
  4.     } catch( Exp error ) {
  5.         std::cout << error.message << endl;
  6.     }
  7.  
  8.     a_definition; // out of scope now...
  9. }

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) 
  1. do_that_thing_with_a_definition()
  2. {
  3.     A a_definition;
  4.     //a_definition is in scope.. a neat and clean function...
  5. }
  6. int main() {
  7.     try {
  8.         do_that_thing_with_a_definition() ;
  9.     } catch( Exp error ) {
  10.         std::cout << error.message << endl;
  11.     }
  12.  
  13.     //following is irrelevant now
  14.     //a_definition; // out of scope now...
  15. }
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Re: C++ / Exceptions

 
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  #5
Feb 28th, 2007
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.
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Re: C++ / Exceptions

 
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  #6
Mar 1st, 2007
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Originally Posted by joeprogrammer View Post
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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Re: C++ / Exceptions

 
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  #7
Mar 1st, 2007
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Originally Posted by thekashyap View Post
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) 
  1. int main() {
  2.     try {
  3.         do_that_thing_with_a_definition() ;
  4.     } catch( Exp error ) {
  5.         std::cout << error.message << endl;
  6.     }
  7.  
  8.     //following is irrelevant now
  9.     //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.
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Re: C++ / Exceptions

 
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  #8
Mar 2nd, 2007
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".
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Re: C++ / Exceptions

 
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  #9
Mar 2nd, 2007
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Originally Posted by thekashyap View Post
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.
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