Question

Will the following append() in the catch cause the rethrown exception to see the effect of append() being called?

try {
  mayThrowMyErr();
} catch (myErr &err) {
  err.append("Add to my message here");
  throw; // Does the rethrow exception reflect the call to append()?
}

Similarly, if I rewrite it this way, will bit slicing occur if the actual exception is derived by myErr?

try {
  mayThrowObjectDerivedFromMyErr();
} catch (myErr &err) {
  err.append("Add to my message's base class here");
  throw err; // Do I lose the derived class exception and only get myErr?
}

Answer 1

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 magical memory location which will stay valid during the subsequent unwinding — 0x98e7058 in the example below). However,

1. In the first case, since you rethrow with throw; (which, unlike throw err;, preserves the original exception object, with your modifications, in said “magical location” at 0x98e7058) will reflect the call to append()
2. In the second case, since you throw something explicitly, a copy of err will be created then thrown anew (at a different “magical location” 0x98e70b0 — because for all the compiler knows err could be an object on the stack about to be unwinded, like e was at 0xbfbce430, not in the “magical location” at 0x98e7058), so you will lose derived-class-specific data during the copy-construction of a base class instance.

Simple program to illustrate what’s happening:

#include <stdio.h>

struct MyErr {
  MyErr() {
    printf("  Base default constructor, this=%p\n", this);
  }
  MyErr(const MyErr& other) {
    printf("  Base copy-constructor, this=%p from that=%p\n", this, &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& other) {
    printf("  Derived copy-constructor, this=%p from that=%p\n", this, &other);
  }
  virtual ~MyErrDerived() {
    printf("  Derived destructor, this=%p\n", this);
  }
};

int main() {
  try {
    try {
      MyErrDerived e;
      throw e;
    } catch (MyErr& err) {
      printf("A Inner catch, &err=%p\n", &err);
      throw;
    }
  } catch (MyErr& err) {
    printf("A Outer catch, &err=%p\n", &err);
  }
  printf("---\n");
  try {
    try {
      MyErrDerived e;
      throw e;
    } catch (MyErr& err) {
      printf("B Inner catch, &err=%p\n", &err);
      throw err;
    }
  } catch (MyErr& err) {
    printf("B Outer catch, &err=%p\n", &err);
  }
  return 0;
}

Result:

  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, &err=0x98e7058
A Outer catch, &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, &err=0x98e7058
  Base copy-constructor, this=0x98e70b0 from that=0x98e7058
  Derived destructor, this=0x98e7058
  Base destructor, this=0x98e7058
B Outer catch, &err=0x98e70b0
  Base destructor, this=0x98e70b0

Also see:

• Scope of exception object in C++
• Throwing … “by reference”