Does an exception use move semantics when thrown in C++11?

Question

http://www.drdobbs.com/cpp/practical-c-error-handling-in-hybrid-env/197003350?pgno=4

In this article Herb Sutter explains that throwing an exception requires a copy of the exception as it's created as a temporary and therefore uses an std::auto_ptr to get round the copy overhead. In light of move semantics being made available in C++11 is this still necessary?

Answer 1

I have just checked, and the Standard allows

• omitting the copy or move of an object specified by the operand of a throw expression into the exception object
• omitting the copy or move of the exception object into the catch clause variable of the same type as the exception object if you don't otherwise change the meaning of the program (i.e if you would rethrow and subsequent catches would suddenly see a changed exception object changed by the previous catch block).

Since these omissions are allowed, the spec requires to first regard the source of the copy or move as an rvalue. So this means that the respective objects will be moved if possible. Of course copy and move elision are still allowed as the first choice.

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Update

I was notified that the consideration of the exception object initializer of a catch clause parameter as an rvalue initializer will probably be dropped from the Standard (because in general it is not possible for all cases to detect when the behavior of the program is unchanged when omitting a copy/move), so I recommend to not rely on this (second bullet above).

What you can still rely about is the move of a local variable into the exception object, as in throw x; (first bullet above).

Answer 2

Move from exception objects is not mandatory now.

It's a defect of C++11. See CWG1493 .