Missing traits for std::vector<bool>

[robinchrist]

Hi,

We do crossbuilds for OSX and I stumbled upon a few issues.
I'll provide details later, but they are all related to the usage of std::vector<bool> and type traits not working.

This is most probably an OSX SDK issue though, but I think we might find a generic solution, because this could happen with different implementations too.

[ColinH]

That's strange, we do CI builds on macOS, and my main development platform is macOS, and usually everything works. Which compiler and C++ library are you using? Are you actually cross-compiling?

[robinchrist]

We use Clang 7.0.1 together with cmake-ts on Linux to crosscompile our native node addons for Windows, Linux and Mac.

[ColinH]

We do CI builds with Clang 7, so it shouldn't be the compiler. Are you using libc++ or libstdc++?

[robinchrist]

The offending line was

templObj.emplace("FrontFacing", templ.getFrontFacing());

where

const std::vector<bool>& getFrontFacing() const;

Full error log:
https://pastebin.com/Xn04DyUu

My first workaround attempt was a helper function

tao::json::value booleanVecToArray(const std::vector<bool>& vec) {
    tao::json::value arr = tao::json::empty_array;
    for(const auto& val : vec) {
        arr.push_back(val);
    }

    return arr;
}

Which does not work too
https://pastebin.com/TDDdBXmd

Changing to

tao::json::value jsonValue = val;
arr.push_back(std::move(jsonValue));

doesn't work either

../../../../../secretprojectname/src/util/jsonutil.cpp:135:13: error: no matching member function for call to 'push_back'
        arr.push_back(val);
        ~~~~^~~~~~~~~
../../../../../secretprojectname/include/tao/json/binding/internal/../../basic_value.hpp:892:12: note: candidate function not viable: no known conversion from 'const std::__1::__bit_const_reference<std::__1::vector<bool, std::__1::allocator<bool> > >' to 'const tao::json::basic_value<traits>' for 1st argument
      void push_back( const basic_value& v )
           ^
../../../../../secretprojectname/include/tao/json/binding/internal/../../basic_value.hpp:898:12: note: candidate function not viable: no known conversion from 'const std::__1::__bit_const_reference<std::__1::vector<bool, std::__1::allocator<bool> > >' to 'tao::json::basic_value<traits>' for 1st argument
      void push_back( basic_value&& v )

Finally

for(const bool val : vec) {
        tao::json::value jsonValue = val;
        arr.push_back(std::move(jsonValue));
    }

So I'd say there are some serious issues with const-refs to std::vector<bool> with the libc++ OSX SDK

[ColinH]

So the issue is that there is no tao::json::traits< std::vector< bool > > specialisation?

[robinchrist]

@ColinH see my updated comment #54 (comment)

[ColinH]

In the first work-around, try arr.push_back(bool(val)), since val will be the weird proxy object instead of the actual bool.

[robinchrist]

In the first work-around, try arr.push_back(bool(val)), since val will be the weird proxy object instead of the actual bool.

I'm sure this would work. However, I'm more concerned by the fact, that the OSX std::vector<bool> implementation is broken and delivers some weird proxy object (std::__1::__bit_const_reference<...>)

Actually nlohmann/json had the same problem, see nlohmann/json#494

[ColinH]

That's not broken, std::vector< bool > is strange by (ancient) design, in other words it's not a bug it's a feature!

https://en.cppreference.com/w/cpp/container/vector_bool

[ColinH]

What's missing is a specialisation of the traits for std::vector< bool >, shouldn't take too long, I'll see when I get around to it.

[robinchrist]

I know of the peculiarities of std::vector<bool>. I was really surprised, that it worked on Linux and Windows, but not on OSX...

[ColinH]

That is somewhat surprising.

[d-frey]

Is that macOS vs Linux or is it libstdc++ vs libc++?

…

On 16. Jul 2019, at 13:26, Dr. Colin Hirsch ***@***.***> wrote: That is somewhat surprising. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub, or mute the thread.

[ColinH]

@robinchrist Can you please test whether the latest commit fixes the issue?

[robinchrist]

@ColinH Sure, will report back ASAP

[robinchrist]

@ColinH CI build works fine!

[ColinH]

Great. Did you see which library you are using on the different platforms?

[robinchrist]

As far as I know it's libstdc++ for linux and standard CRT for Windows