#37 Improvement for ScopeGuard

[zhnmju123]

Hi,

short version:
I ask for a LOKI_ON_BLOCK_EXIT that does not emit a compiler warning about an unused variable on GCC and MSVC.

long version:
one use of ScopeGuard is as a convenient RAII "cleanup" mechanism, that is without calling "Dismiss()" later.
The problem then is that GCC and MSVC emit compiler warnings about an unused variable.
This happens both when using a dummy variable without dismissing it and more critically when using "LOKI_ON_BLOCK_EXIT", which is specifically designed for that purpose.

As seen here, others have been looking for an answer, but with little success:
http://stackoverflow.com/questions/219770/dealing-with-c-initialized-but-not-referenced-warning-for-destruction-of-scop

For GCC, there is a nice and simple solution which could be implemented by the Loki library:
replace:
typedef const ScopeGuardImplBase& ScopeGuard;
by
__attribute__((unused)) typedef const ScopeGuardImplBase& ScopeGuard;

Unfortunately there is no MSVC equivalent.

Now with C++11, there might be light at the end of the tunnel:
One way to silence the warning about unused variable in both GCC and MSVC is to get rid of the const-reference:
typedef const ScopeGuardImplBase& ScopeGuard; -> typedef ScopeGuardImplBase ScopeGuard;
It's clear that this creates problems for the general ScopeGuard case where the type is dependent from the number and types of arguments passed. But for LOKI_ON_BLOCK_EXIT, this shouldn't matter. What's still missing is a way to "pass the value without copying". Here C++11 r-value-refences are the fitting solution.

You see, there are a few open questions, but my hope is that they can be figured out eventually.

Best regards, ZenJu

[zhnmju123]

> typedef const ScopeGuardImplBase& ScopeGuard; -> typedef ScopeGuardImplBase ScopeGuard;
The example is wrong, but the main idea is to get rid of the reference and move to a value type.

[zhnmju123]

Hi,

here is an actually working proposal for C++11: It's some more steroids for LOKI_ON_BLOCK_EXIT:

#define LOKI_ON_BLOCK_EXIT2(X) ::Loki::ScopeGuard LOKI_ANONYMOUS_VARIABLE(scopeGuard) = ::Loki::MakeGuard([&](){X;}); (void)LOKI_ANONYMOUS_VARIABLE(scopeGuard);

Resulting Syntax:

LOKI_ON_BLOCK_EXIT2(::CloseHandle(hDir));

Multiple statements are possible also:
LOKI_ON_BLOCK_EXIT2(::CloseHandle(hDir);
::CloseHandle(hDir));

And most importantly: no warning about unused variable anymore, both GCC and MSVC10!
Personally, I like the syntax a lot more than BOOST_SCOPE_EXIT (not to mention the bunch of header dependencies), so I would love to see something like this in Loki!

Regards, ZenJu