Warning: "Possible misuse of comma operator"

[thomthom]

I'm seeing a warning being emitted by this library when I'm using Xcode 10.3:

array/include/boost/array.hpp

Line 185 in 92f66a9

return i >= size() ? boost::throw_exception(std::out_of_range ("array<>: index out of range")), true : true;

Possible misuse of comma operator here

Initially I thought of making a PR to address it, but I don't really understand the expression at that line. Why is boost::throw_exception followed by , true ?

[mclow]

It's a false positive. In C++11, there were severe limitations on what a constexpr function could do. Effectively, they were limited to a single return statement.

Breaking this down, it looks like this:

return i >= size ()
   ? boost_throw_exception(...), true
   : true;

The , true on the second line there is necessary, because otherwise the two halves of the ternary (?:) expression would have different types, and the compiler would emit an error for that. Try removing the , true and you will see what I mean.

If you were writing this for C++14 and later, it would be clearer:

if (i >= size())
    throw boost_throw_exception(...);
return true;

but that won't compile for C++11

[thomthom]

In a build with warnings as errors this would break the build. What would be a good way to mute these warnings? Use #pragmas to push/pop disable this specific warning before including the boost header?

[mclow]

I don't know where that warning is coming from.
Is it coming from the compiler? If so, what are the options passed to the compiler, and what's the full error message?

[mclow]

This sample:

#include <boost/array.hpp>

int main () {
	boost::array<int, 5> arr;
	return arr.size();
	}

when compiled with Apple clang version 11.0.3 (clang-1103.0.32.62) as:
clang++ -Wall -Wextra -I $BOOST junk.cpp gives no warnings, no errors

[thomthom]

I can check when I'm back in office. But the warning I had to disable was -Wcomma.

[mclow]

Sounds like a bug in -Wcomma. (and I have verified that enabling that warning does throw a warning, with a completely incorrect "fix-it" suggestion)

[mclow]

Submitted https://bugs.llvm.org/show_bug.cgi?id=47566 against clang.

[shreegilliorkar]

Hey, i am Shree. New in open source contribution. My technical skills include C, C++, Python & Java. Can I please get some guidance for this project.