#scopeguard

[mylittlechangeling]

Here’s one of our winners for the charity. Thank you Scopeguard for donating <3 ( I am still working on the others!)

[lichray]

A simplified transctional ScopeGuard

Andrei's ScopeGuard[1] idiom has been implemented by lots of people with the std::function plus a lambda, though most of the time what they need is just a:

unique_ptr<FILE, decltype((fclose))> fguard(fp, fclose);

However, no matter how the ScopeGuard is implemented, it is a code smell to use it as an ad-hoc RAII replacement. The cleanup logic is always the same, and the same code should be refactored. The original article by Andrei already pointed out the main purpose of ScopeGuard: to ease the implementation of the transactional operations to gain the strong exception safety. A rollback logic is specific to a business logic, which are seldom `refactorable'.

So that is why I design the interface of a ScopeGuard like the following:

// creates an automatically named guard defer ( <exps | statements> ); // creates an guard with a <name> defer ( <exps | statements> ) namely ( <name> );

And, to support the cascading undo operations, multiple guards can be disabled within one statement:

defer (op1; op2) namely (undo1); /* operations may fail */ defer (op3; op4; op5) namely (undo2); /* more operations may fail */ undo1 = undo2 = false; // commit

Compared with Go's `defer' keyword, my deferxx[2] does not support `recover' since the exception in C++ is not reenterable. Compared with Boost.ScopeExit, deferxx ships with a built-in commit flag and a cleaner grammar (especially, without the ugly extra semi-colon after the curly braces). Compared with D's scope guard statement, deferxx is not exception-sensible, since the exception in C++ is not detectable (std::uncaught_exception() does not distinguish an exception propagating across a destructor from an exception thrown from the scope)[3]. However, I don't think it's a good idea to turn every failure into an exception. And, coincidentally, the success return code and errno of the C library functions are 0, so that you can just assign those values to the guards ;)

Links:

[1] Generic<Programming>: Change the Way You Write Exception-Safe Code — Forever. http://www.drdobbs.com/cpp/generic-change-the-way-you-write-excepti/184403758

[2] A Go's defer-like syntax scope guard idiom in C++11. https://github.com/lichray/deferxx

[3] GotW #47: Uncaught Exceptions. http://www.gotw.ca/gotw/047.htm