Re: exception question

From:
"mlimber" <mlimber@gmail.com>
Newsgroups:
comp.lang.c++
Date:
28 Jun 2006 11:15:33 -0700
Message-ID:
<1151518533.462071.50930@75g2000cwc.googlegroups.com>
junw2000@gmail.com wrote:

Hi,
Below is a simple code about exception.

#include <iostream>
using namespace std;

struct E {
  const char* message;
  E(const char* arg) : message(arg) { }
};

void my_terminate() {
  cout << "Call to my_terminate" << endl;
};

struct A {
  A() { cout << "In constructor of A" << endl; }
  ~A() {
  cout << "In destructor of A" << endl;
  throw E("Exception thrown in ~A()");
 }
};

struct B {
  B() { cout << "In constructor of B" << endl; }
  ~B() { cout << "In destructor of B" << endl; }
};

int main() {
  set_terminate(my_terminate);

  try {
    cout << "In try block" << endl;
    A a;
    B b;
    cout << "Leave try block" << endl;
   throw("Exception thrown in try block of main()"); //LINE1
  }
  catch (const char* e) { //LINE2
    cout << "Exception: " << e << endl; //LINE3
  }
  catch (...) {
    cout << "Some exception caught in main()" << endl;
  }

  cout << "Resume execution of main()" << endl;
}

The output of the code is below:
In try block
In constructor of A
In constructor of B
Leave try block
In destructor of B
In destructor of A
Call to my_terminate
Abort

My question is when LINE1 throws an exception, the exception is caught
by LINE2, right?
Why LINE3 does not output anything?

The last line of the output is "Abort". Why is it from? Does the
function my_terminate() output the "Abort"?


LINE2 would catch the char* exception you throw, except that before the
catch happens it must destroy all the automatic objects on the stack,
which includes A. Unfortunately, A's destructor itself throws an
exception, which is bad bad bad, and since you're already in the stack
unwinding process when that exception is thrown, you abort. That's how
the language is defined. See this FAQ, particularly the "Bang! You're
dead." part:

http://www.parashift.com/c++-faq-lite/exceptions.html#faq-17.3

The lesson here is: Don't throw exceptions in destructors.

Cheers! --M

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