Syntax Error with Q_ASSERT

[A Former User]

I just came up with something that looks like a solution for this to me. What do guys think?

#if !defined(MY_ASSERT)
#  ifndef QT_NO_DEBUG
#    define MY_ASSERT_FIRST_ARGUMENT(A, ...) A
#    define MY_ASSERT(...) ((!MY_ASSERT_FIRST_ARGUMENT(__VA_ARGS__)) ? qt_assert(#__VA_ARGS__,__FILE__,__LINE__) : qt_noop())
#  else
#    define MY_ASSERT(...) qt_noop()
#  endif
#endif

[mrjj]

@Wieland
where is the docs? ;)
Looks cool. its cryptic enough that it might actually work :)

[koahnig]

@mrjj said in Syntax Error with Q_ASSERT:

Looks cool. its cryptic enough that it might actually work :)

:D :D

[A Former User]

It's almost the same as the current implementation of Q_ASSERT, just with a variadic macro. So it's C++11 only.

[Chris Kawa]

@mrjj good one :)
@Wieland Could you explain how it is suppose to work? From what I can decrypt it just checks the first argument passed, so for your original exampleMY_ASSERT(std::is_same<int, int>::value); it would expand to something like ((!std::is_same<int) ? ... which doesn't make much sense? Or am I missing something?

Btw. I get compiler errors for this:
with gcc: in definition of macro MY_ASSERT_FIRST_ARGUMENT wrong number of template arguments (1, should be 2)
with clang: error: expected > MY_ASSERT(std::is_same<int, int>::value);
it compiles in VS2015 U3 although their macro expansion is broken to bits so I wouldn't trust it does what it should.

[A Former User]

@Chris-Kawa Damn, I only tested it with MSVC and it works there :-(

[A Former User]

@Chris-Kawa My idea hope was that the preprocessor would be smarter when I confront it with a variadic macro.

[Chris Kawa]

@Wieland I don't think it works. It just compiles ;)
Well you could use something simpler:

define MY_ASSERT(...) (!(__VA_ARGS__) ? qt_assert(#__VA_ARGS__,__FILE__,__LINE__) : qt_noop())

and that should be ok, but it has the same drawback I described earlier - it changes the expression it tests by adding extra () around it. Admittedly it's not a big deal and it should work as expected most of the time.

[A Former User]

@Chris-Kawa Yes it works; funny that it doesn't work for you. Who knows why..

[Chris Kawa]

@Wieland Maybe I messed up something. http://ideone.com/XKJUnV

[A Former User]

@Chris-Kawa Ha! Just found out that #define FIRST(A, ...) A doesn't work as expected (with MSVC): It doesn't give us the first argument only, instead it just gives all arguments (like __VA_ARGS__). Maybe I'm wrong here again, but that looks like a bug in MSVC to me and it also explains why the code works.

Edit: Maybe it's really a bug in VC's preprocessor, at least someone on SO says so. And the workaround he presents actually seems to fix it, so now my code doesn't compile with MSVC anymore.

[A Former User]

Thanks everyone for watching me stumbling around like a clown :)

[mrjj]

@Wieland
Well to be fair, fooling around with Variadic Macros takes more balls than entertaining
clueless children - so it was educational to see that even in 2016, you cannot trust the preprocessor
to work the same across compilers. :)