Are std::exception objects created when an exception is thrown?

Question

I have a class CustomException, that implements std::exception, in which I explicitely deleted the copy and move constructors. When I throw an exception of that class, there are compiling errors for calling the deleted constructors.

Are CustomException instances being created somewhere? What objects are created when the exception is thrown?

Answer 1

When you throw, an exception object is constructed that has the same type as the operand of throw with top-level cv-qualifiers removed (if you throw an array or function, they also decay to their corresponding pointers).

So what you did is a no-go, I'm afraid.

C++ standard chapter [except.throw] §5:

When the thrown object is a class object, the copy/move constructor and the destructor shall be accessible, even if the copy/move operation is elided (12.8).

Answer 2

Before unwinding stack a throw operator (except throw; without an argument, used for rethrowing) creates an exception object in a special memory area. Depending on circumstances, the object is initialized in different ways: constructor, copy constructor, move constructor (https://en.cppreference.com/w/cpp/language/copy_elision) using what was provided to the throw operator. So the information is available from the exception object, which is alive until completing handling the exception, though what was provided to the throw operator is destroyed when the stack is unwound).

In your case the compiler needs the functions you deleted to either initialize the exception object when throwing the exception or to initialize the catch-clause argument, or both - since it is how the compiler does it by design (using the functions).