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    <p>In both cases, since you catch by reference, you are effectively altering the state of the original exception object (which you can think of as residing in <a href="https://web.archive.org/web/20110824215142/http://cutebugs.net/files/cpp/ar01s02.html" rel="noreferrer">a magical memory location which will stay valid during the subsequent unwinding</a> -- <code>0x98e7058</code> in the example below). However,</p> <ol> <li>In the first case, since you rethrow with <code>throw;</code> (which, unlike <code>throw err;</code>, preserves the original exception object, with your modifications, in said "magical location" at <code>0x98e7058</code>) <strong><em>will</em> reflect the call to append()</strong></li> <li>In the second case, since you throw something explicitly, a <strong>copy</strong> of <code>err</code> will be created then thrown anew (at a different "magical location" <code>0x98e70b0</code> -- because for all the compiler knows <code>err</code> could be an object on the stack about to be unwinded, like <code>e</code> was at <code>0xbfbce430</code>, not in the "magical location" at <code>0x98e7058</code>), so <strong>you will lose derived-class-specific data</strong> during the copy-construction of a base class instance.</li> </ol> <p>Simple program to illustrate what's happening:</p> <pre><code>#include &lt;stdio.h&gt; struct MyErr { MyErr() { printf(" Base default constructor, this=%p\n", this); } MyErr(const MyErr&amp; other) { printf(" Base copy-constructor, this=%p from that=%p\n", this, &amp;other); } virtual ~MyErr() { printf(" Base destructor, this=%p\n", this); } }; struct MyErrDerived : public MyErr { MyErrDerived() { printf(" Derived default constructor, this=%p\n", this); } MyErrDerived(const MyErrDerived&amp; other) { printf(" Derived copy-constructor, this=%p from that=%p\n", this, &amp;other); } virtual ~MyErrDerived() { printf(" Derived destructor, this=%p\n", this); } }; int main() { try { try { MyErrDerived e; throw e; } catch (MyErr&amp; err) { printf("A Inner catch, &amp;err=%p\n", &amp;err); throw; } } catch (MyErr&amp; err) { printf("A Outer catch, &amp;err=%p\n", &amp;err); } printf("---\n"); try { try { MyErrDerived e; throw e; } catch (MyErr&amp; err) { printf("B Inner catch, &amp;err=%p\n", &amp;err); throw err; } } catch (MyErr&amp; err) { printf("B Outer catch, &amp;err=%p\n", &amp;err); } return 0; } </code></pre> <p>Result:</p> <pre><code> Base default constructor, this=0xbfbce430 Derived default constructor, this=0xbfbce430 Base default constructor, this=0x98e7058 Derived copy-constructor, this=0x98e7058 from that=0xbfbce430 Derived destructor, this=0xbfbce430 Base destructor, this=0xbfbce430 A Inner catch, &amp;err=0x98e7058 A Outer catch, &amp;err=0x98e7058 Derived destructor, this=0x98e7058 Base destructor, this=0x98e7058 --- Base default constructor, this=0xbfbce430 Derived default constructor, this=0xbfbce430 Base default constructor, this=0x98e7058 Derived copy-constructor, this=0x98e7058 from that=0xbfbce430 Derived destructor, this=0xbfbce430 Base destructor, this=0xbfbce430 B Inner catch, &amp;err=0x98e7058 Base copy-constructor, this=0x98e70b0 from that=0x98e7058 Derived destructor, this=0x98e7058 Base destructor, this=0x98e7058 B Outer catch, &amp;err=0x98e70b0 Base destructor, this=0x98e70b0 </code></pre> <p>Also see:</p> <ul> <li><a href="https://stackoverflow.com/questions/1654150/scope-of-exception-object-in-c">Scope of exception object in C++</a> </li> <li><a href="https://stackoverflow.com/questions/2274661/throwing-non-const-temporaries-by-reference">Throwing ... "by reference"</a></li> </ul>
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