ex.what() changes in nested try-catch

Question

I have a nested try-catch code like below:

void A()
{
    try
    {
        //Code like A = string(NULL) that throws an exception
    }
    catch(std::exception& ex)
    {
        cout<<"in A : " << ex.what();
        throw ex;
    }
}

void B()
{
   try
   {
       A();
   }
   catch(std::exception& ex)
   {
       cout<<"in B : " << ex.what();
   }
}

After running this I got this result:

in A: basic_string::_M_construct null not valid
in B: std::exception

As you can see, ex.what() works OK in function A and tell me the correct description, but in B ex.what() tells me just std::exception. Why does this happen?

Am I throwing something different or wrong in the catch clause of function A? How do I throw a nested exception so that I can get the exact exception description in B?

See: http://stackoverflow.com/questions/1481612/what-is-the-difference-between-throw-and-throw-with-arg-of-caught-exception, http://stackoverflow.com/questions/1833982/in-c-is-there-a-difference-between-throw-and-throw-ex — Cody Gray - on strike, Dec 07 '16 at 18:38

score 41 · Accepted Answer · edited May 23 '17 at 12:09

41

Replace throw ex; with throw;.

Doing the latter will re-throw the exception ex, by reference, so obviating the hazards in attempting to make a value copy: see What is object slicing?

(Note that you are allowed to modify ex even if you write throw).

edited May 23 '17 at 12:09

Community

1
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answered Dec 07 '16 at 12:30

Bathsheba

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with all respect to @StoryTeller answer, i accept yours because of extra link and your point at the end. – dev-masih Dec 07 '16 at 14:54
3

Nitpick, but I assume you mean `throw;`, as (AFAIK) `throw` on its own isn't valid. – Nic Dec 08 '16 at 05:48

StoryTeller - Unslander Monica · Answer 2 · 2016-12-07T12:32:40.220

32

You are throwing a copy of the exception ex in A. Which causes object slicing that turns the concrete exception into std::exception.

To rethrow the actual exception you caught polymorphically, use the throw; statement.

It's worth keeping in mind what Scott Meyers says in his book. You throw by value, and should catch by reference.

edited Dec 07 '16 at 12:32

answered Dec 07 '16 at 12:30

StoryTeller - Unslander Monica

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score 7 · Answer 3 · edited May 23 '17 at 12:32

7

You're slicing the original exception object, try with

try {
  throw std::runtime_error("Hello world");
}
catch (std::exception& ex)
{
  cout << "in A : " << ex.what();
  throw; // Rethrows the original exception - no copy or slicing
  ^^^^^^
}

edited May 23 '17 at 12:32

Community

1
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answered Dec 07 '16 at 12:32

Marco A.

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ex.what() changes in nested try-catch

3 Answers3