C++: Throwing an exception invokes the copy constructor?

Question

We have an custom error class that is used whenever we throw an exception:

class AFX_CLASS_EXPORT CCLAError : public CObject

It has the fol

Answer 1

The type of the object thrown must be copyable because the throw expression may make a copy of its argument (the copy may be elided or in C++11 a move may take place instead, but the copy constructor must still be accessible and callable).

Rethrowing the exception using throw; will not create any copies. Throwing the caught exception object using throw e; will cause a copy of e to be made. This is not the same as rethrowing the exception.

Your "updated" code does not work as you expect. catch (CCLAError&) will not catch an exception of type CCLAError*, which is the type of the exception thrown by throw new CCLAError(...);. You would need to catch CCLAError*. Do not do this, though. Throw exceptions by value and catch by reference. All exception types should be copyable.