Clang throws unused variable warnings

[Alexander Grund]

Using clang 3.5

contrib/turtle/include/turtle/detail/function_impl_template.hpp:66:18: error: unused variable '_' [-Werror,-Wunused-variable]
            lock _( mutex_ );
                 ^
contrib/turtle/include/turtle/detail/function_impl_template.hpp:83:18: error: unused variable '_' [-Werror,-Wunused-variable]
            lock _( mutex_ );
                 ^
contrib/turtle/include/turtle/detail/function_impl_template.hpp:199:18: error: unused variable '_' [-Werror,-Wunused-variable]
            lock _( mutex_ );
                 ^
contrib/turtle/include/turtle/detail/function_impl_template.hpp:206:18: error: unused variable '_' [-Werror,-Wunused-variable]
            lock _( mutex_ );
                 ^
contrib/turtle/include/turtle/detail/function_impl_template.hpp:215:18: error: unused variable '_' [-Werror,-Wunused-variable]
            lock _( mutex_ );
                 ^
contrib/turtle/include/turtle/detail/function_impl_template.hpp:251:18: error: unused variable '_' [-Werror,-Wunused-variable]
            lock _( mutex_ );
                 ^
contrib/turtle/include/turtle/detail/function_impl_template.hpp:261:18: error: unused variable '_' [-Werror,-Wunused-variable]
            lock _( impl.mutex_ );

[Mathieu Champlon]

That seems indeed a little inconvenient, what is the usual way to deal with an RAII guard like this with clang?
I suppose there is a better way than just disabling that warning for the whole code base, isn't there?

[Alexander Grund]

Maybe template <class T> inline void ignore_unused_variable_warning(T const&) {}

[Mathieu Champlon]

Yes, that rings a bell, there is even something in Boost for this : https://www.boost.org/doc/libs/1_68_0/libs/core/doc/html/core/ignore_unused.html

[Alexander Grund]

Yeah. But as this is such a simple thing I'd like you to avoid this quite recent addition. E.g. again we are stuck with boost 1.55 ATM

[Alexander Grund]

That seems indeed a little inconvenient, what is the usual way to deal with an RAII guard like this with clang?

From what I found clang "knows" when ctors have side effects so it doesn't warn for such scoped locks. However for the non-thread-safe variant, the ctor of lock class is empty so that instance does in fact not do anything at all. Hence the warning.

[Alexander Grund]

I'd like to fix this. Would you rather like to add the ignore_unused_variable_warning function or use a macro #define MOCK_SCOPED_LOCK(mutex) lock _( mutex ); which is empty for non-threadsafe builds?

[Mathieu Champlon]

I'd go with the macro otherwise all users of the library would have to add ignore_unused_variable_warning themselves…
Thanks!

[Alexander Grund]

Done. Seems like this is only for older clangs as your CI clangs do not show this behaviour. I also could not remove the dummy lock class as it is used as a member in a class. Besides that the lock/unlock from the dummy mutex cannot be removed either due to https://github.com/mat007/turtle/blob/master/include/turtle/detail/functor.hpp#L40

[Mathieu Champlon]

Thank you!