Wednesday, November 2, 2011

empty throw ^ missing throw in c++

Background - all exception specs beside these 2 are deprecated in c++11. Only these 2 forms proved useful. Let's rephrase the question as "Real difference between 2 otherwise identical functions, one with empty throw() the other without throw".

Short Answer -- wrapping the empty-throw function call in try/catch is useless.

Short answer - empty throw means "should throw Nothing. Nothing expected";
Short answer - missing throw means the opposite -- "all exceptions expected"

Long answer -- suppose you wrap each function call in a try/catch (...) ie catch-all. For the function without throw, every single exception from it will fall into your catch(...). For the empty-throw function, every single exception always, always triggers unexpected(). Your try/catch is ignored by compiler -- could be removed during compilation.

Now we are ready to look at a function with throw(A). According the ARM, if at runtime this function throws unlisted exceptions, it's seen as a serious design fault, so serious that compiler will *disregard* your try/catch -- paper over (huge) cracks.

Here's a best practice from http://www.gotw.ca/publications/mill22.htm -- Moral #1: Never write an exception specification. If you follow this advice, then your try/catch will work, ie won't be stripped off by compiler

My advice -- "Either don't list the exceptions, or adhere to your list. If you bother to list the expected exceptions, then stick to it. Don't throw anything else. "

Note unexpected() is called always, always due to an explicit exception-spec i.e. an explicit throw suffix. "Unexpected" means "function author informed compiler to *expect* some exceptions, but something unexpected came out from the function".

Let's repeat the message -- whenever you notice unexpected() invoked, there's a function with a broken exception spec -- No smoke without fire. No throw suffix then no "unexpected()".

Further, these 2 conditions always lead to each other --

function has an exception list, but unlisted exceptions generated < ==> unexpected() invocation

What about terminate()? It's somewhat peripheral in the story of exception-spec. I will just describe one of the causes -- an expected (i.e. listed) exception is thrown but not caught. Also, unexpected() calls terminate() by default.

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