Question

Consider an exception class with a copy constructor with side-effects.

Can a compiler skip calling the copy constructor here:

try {
    throw ugly_exception();
}
catch(ugly_exception) // ignoring the exception, so I'm not naming it
{ }

What about this:

try {
    something_that_throws_ugly_exception();
}
catch(ugly_exception) // ignoring the exception, so I'm not naming it
{ }

(yes, I know this is all very ugly, this was inspired by another question)

Answer 1

Yes, it can be elided both during throwing and catching. For catching it can be elided only when the type specified in the catch clause is the same (save for cv-qualifications) as the type of the exception object. For more formal and detailed description see C++11 12.8/31.

…This elision of copy/move operations, called copy elision, is permitted in the following circumstances (which may be combined to eliminate multiple copies):

…

• in a throw-expression, when the operand is the name of a non-volatile automatic object (other than a function or catch-clause parameter) whose scope does not extend beyond the end of the innermost enclosing try-block (if there is one), the copy/move operation from the operand to the exception object (15.1) can be omitted by constructing the automatic object directly into the exception object

…

• when the exception-declaration of an exception handler (Clause 15) declares an object of the same type (except for cv-qualification) as the exception object (15.1), the copy/move operation can be omitted by treating the exception-declaration as an alias for the exception object if the meaning of the program will be unchanged except for the execution of constructors and destructors for the object declared by the exception-declaration.