More C++ Idioms/Polymorphic Exception

Polymorphic Exception

Intent

• To create an exception object polymorphically
• To decouple a module from the concrete details of the exceptions it may throw

Motivation

Dependency Inversion Principle (DIP), a popular object-oriented software design guideline states that higher level modules should not depend directly on lower level modules. Instead, both should depend on common abstractions (captured in the form of well-defined interfaces). For example, an object of type Person (say John) should not create and use an object of type HondaCivic but instead John should simply commit to a Car interface, which is an abstract base class of HondaCivic. This allows John to upgrade to a Corvette easily in future without any changes to the class Person. John can be "configured" with a concrete instance of a car (any car) using dependency injection technique. Use of DIP leads to flexible and extensible modules that are easy to unit test. Unit testing is simplified by DIP because real objects can be easily replaced with mock objects using dependency injection.

However, there are several occasions when DIP is violated: (1) while using the Singleton pattern and (2) while throwing exceptions! Singleton pattern breaks DIP because it forces the use of the concrete class name while accessing the static instance() function. A singleton should be passed as a parameter while calling a function or a constructor. A similar situation arises while dealing with exceptions in C++. The throw clause in C++ requires a concrete type name (class) to raise an exception. For example,

throw MyConcreteException("Big Bang!");

Any module that throws exceptions like this immediately results into a violation of DIP. Naturally, it is harder to unit test such a module because real exception objects cannot easily be replaced with mock exception objects. A solution like the one below fails miserably as throw in C++ uses static typing and knows nothing about polymorphism.

struct ExceptionBase { };
struct ExceptionDerived : ExceptionBase { };
 
void foo(ExceptionBase& e)
{
   throw e; // Uses static type of e while rasing an exception.
}
int main (void)
{
  ExceptionDerived e;
  try {
    foo(e);
  }
  catch (ExceptionDerived& e) {
    // Exception raised in foo does not match this catch.
  }
  catch (...) {
    // Exception raised in foo is caught here.
  }
}

Polymorphic exception idiom addresses the issue.

Solution and Sample Code

Polymorphic exception idiom simply delegates the job of raising the exception back to the derived class using a virtual function raise()

struct ExceptionBase 
{ 
  virtual void raise() { throw *this; }
  virtual ~ExceptionBase() {} 
};
struct ExceptionDerived : ExceptionBase 
{ 
  virtual void raise() { throw *this; }
};
 
void foo(ExceptionBase& e)
{
   e.raise(); // Uses dynamic type of e while raising an exception.
}
int main (void)
{
  ExceptionDerived e;
  try {
    foo(e);
  }
  catch (ExceptionDerived& e) {
    // Exception raised in foo now matches this catch.
  }
  catch (...) {
    // not here anymore!
  }
}

The throw statement has been moved into virtual functions. The raise function invoked in function foo is polymorphic and selects the implementation in either ExceptionBase or ExceptionDerived class depending upon what is passed as a parameter (dependency injection). Type of *this is obviously known at compile-time, which results into raising a polymorphic exception. The structure of this idiom is very similar to that of Virtual Constructor idiom.

Propagating a polymorphic exception

Quite often an exception is handled in multiple catch statements to treat it differently in different layers of the program/library. In such cases, the earlier catch blocks need to rethrow the exception so that the outer catch blocks, if any, may take the necessary actions. When a polymorphic exception is involved, inner catch block(s) may modify the exception object before passing it on to the catch blocks up in the stack. In such cases, care must be taken to ensure that the original exception object is propagated. Consider a seemingly innocuous looking program below, which fails to do that.

try {
    foo(e); // throws an instance of ExceptionDerived as before.
  }
  catch (ExceptionBase& e) // Note the base class. Exception is caught polymorphically.
  {
    // Handle the exception. May modify the original exception object.
    throw e; // Warning! Object slicing is underway.
  }

The throw e statement does not throw the original exception object. Instead, it throws a sliced copy (only ExceptionBase part) of the original object because it considers the static type of the expression in front of it. Silently, the derived exception object was not only lost but also translated into the base type exception object. The catch blocks up in the stack do not have access to the same information this catch had. There are two ways to address the problem.

• Simply use throw; (without any expression following it). It will rethrow the original exception object.
• Use polymorphic exception idiom again. It will throw a copy of the original exception object because the raise() virtual function uses throw *this.

Use of throw; should be strongly preferred in practice, because depending on the implementation, it may preserve the original throw-location in case the exception goes unhandled and the program dumps core, thus easing post mortem analysis of the problem.

try {
    foo(e); // throws an instance of ExceptionDerived as before.
  }
  catch (ExceptionBase& e) // Note the base class. Exception is caught polymorphically.
  {
    // Handle the exception. May modify the original exception object.
    // Use only one of the following two.
    throw;      // Option 1:  Original derived exception is thrown.
    e.raise();  // Option 2:  A copy of the original derived exception object is thrown.
  }

Related Idioms

Virtual Constructor

References

• How do I throw polymorphically? (parashift)
• How do I throw polymorphically? (NCTU)