Complete code in try/catch block

Question

I want to know, is it a good practice to place complete code inside a try block or I should place only the code which I feel it will cause a specific exception?
And should I catch basic Exception always

Code 1: complete code in try block

myFunction(){
 try{
   .........
   Code with chance of OneException 
   .............
 }catch(OneException e){
  ............
 }catch(Exception e){
   ..............
 }
}

Code 2: Only the Code with chance of Exception in try block

myFunction(){
  .......
  try{
   Code with chance of OneException 
  }catch(OneException e){
  ............
  }
  ............
}

Code 3:Should I catch Exception always

    myFunction(){
      .......
      try{
       Code chance of OneException 
      }catch(OneException e){
      ............
      }catch(Exception e){
       ..............
      }
      ........
   } 

Out of this (code1, code2 and code3) which one is the best?
I'm mainly concern with java and C++ coding

Answer 1

Generally speaking, you should only catch exceptions you're interested in and which you can handle. That is...catch an exception where you can do something s.t. the user doesn't perceive the problem or when it is explicitly necessary to tell the user about the problem.
For all other exceptions, let them pop up with all their details (stacktrace etc..) which you obviously log. Note, obviously this doesn't mean the user should also see that exception output but rather a generic error.

Told this, I assume that when you write "Code chance of OneException" you know how to handle OneException, but not Exception, right? So then...only handle OneException.

Answer 2

Always catch exactly what you have to and no more. No matter how much we try, we cannot make our code completely "idiot proof". If someone passes you something which will cause some random error, then it is their job to handle it. If our code handles someone else's exception that has far too much risk of being an unexpected side-effect.

As far as what code to place where: code before the line which could throw the Exception will be run either way, so it does not really make sense to have it inside the try block and before the code which throws. Code after the potential exception should be placed between try and catch if and only if it depends on the exception generating code. So, if your database connection call can fail, place all of the database queries inside the try block.

Limiting the "time" spent in a try...catch makes it easier to read and less prone to accidental catching. I can't tell you how many hours have been lost because someone decided to catch an Exception which should have propagated.