Tuesday, February 21, 2012

eg - c++ exception-passing vs param-passing

Update -- [[c++primer]] points out the key concept of ExceptionObject (exo for short) in a try/catch. This is the object thrown, assuming we don't throw a pointer.

exo isn't the object created ("object1") and passed to the "throw" statement. That object1 can have a short lifespan and destroyed much sooner than the exo, which needs a long lifespan -- described on P1039. Note exo is copy-constructed from object1. See R1 below.

exo isn't the object caught either, unless you catch by reference. P1037.

exo IS the object re-thrown if you use the empty throw statement.
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Based on [[More effC++]] item on the difference between exception-passing and function-parameter-passing. Illustrates a few rules -- R1 R2 R3

class B: public std::exception{};
class C: public B{}; //and so on
class D: public C{}; //and so on
class K: public I{}; //and so on

f(){ D* ex = new K();
  throw *ex; // Copy#1 created [R1] and thrown, sliced [1] to D
}
main(int){
  try{ f();}
  catch (exception * ex){} // silently ignored because no "address" is thrown -- type mismatch [R2]

  //catch (B ex){} //sliced Copy#2a created, just like function parameter pbclone
  // we will leave the above catch clause commented without further discussion.

  catch(B & exCaught){ //Copy#1 caught without slicing
      //throw; //throws Copy#1 [R3], type D
      throw exCaught; //Copy#2b created [R1] and thrown, sliced[1] to B
}

[1] slicing by copy-ctor.

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