[BUG] type_info equality check on same class objects is failing when objects are from two different libraries

[pendyalasyam]

Dear Team,

We have following piece of code in our project which compares the two typeinfos. This is working properly with GNUSTL but failing with libc++.

template<typename T>
bool AreSameTypes(AnyType &any)
{
        //AnyType is capable of storing any data type & TypeInfo() returns 
       // type_info of the data it is holding
    if (typeid(T) == any.TypeInfo())
        return true;
    return false;
}

Here is the scenario:

1. Above piece of code is present in shared library LIB1.

   From with in this library, there is a call to this function with argument of type std::__ndk1::basic_string<wchar_t, wc16::wchar16_traits, std::__ndk1::allocator> like below.

   Function1(){ ....... std::__ndk1::basic_string<wchar_t, wc16::wchar16_traits, std::__ndk1::allocator> obj; AreSameTypes<std::__ndk1::basic_string<wchar_t, wc16::wchar16_traits, std::__ndk1::allocator>>(obj); ....... }

   Since we are applying typeid() on std::__ndk1::basic_string<wchar_t, wc16::wchar16_traits, std::__ndk1::allocator>, compiler is generating the typeinfo for this class. I can see the typeinfo for class by applying readelf command on the shared library LIB1.

2. Within LIB1 itself, there is another function like below

   bool Function2(AnyType &anyTypeObj){ //some code return AreSameTypes<std::__ndk1::basic_string<wchar_t, wc16::wchar16_traits, std::__ndk1::allocator>>(obj); }

3. There is one more shared libary LIB2. LIB2 is creating an object of type std::__ndk1::basic_string<wchar_t, wc16::wchar16_traits, std::__ndk1::allocator> and calling Function2. I did readelf on LIB2 and found typeinfo for std::__ndk1::basic_string<wchar_t, wc16::wchar16_traits, std::__ndk1::allocator> here as well.

With GNUSTL, Function2 is returning true. But with libc++, it is returning false.

To debug the issue, I put log statement inside AreSameTypes and printing the address,name of template argument, received object in order and following is the output

SyamDebug2: 0xc6bc5fc0 NSt6__ndk112basic_stringIwN4wc1614wchar16_traitsENS_9allocatorIwEEEE 0xb9b18380 NSt6__ndk112basic_stringIwN4wc1614wchar16_traitsENS_9allocatorIwEEEE

It seems GNUSTL is comparing the contents whereas libc++ is comparing addresses. Could you please help me how to fix this issue. As of now I can think of two ways to solve this issue.

Solution#1: Make lib2 to use typeinfo generated in lib1 itself. When it is compared, since they are same typeinfos (wrt addresses), it will return true. But I dont know how to make lib2 to depend on lib1 for typeinfo of std::__ndk1::basic_string<wchar_t, wc16::wchar16_traits, std::__ndk1::allocator>. Since ours is product code, touching the code and making logical changes is very difficult.

Solution#2. Make change in typeinfo file in ndk to compare contents instead of addresses. Not sure what are the complications.

Kindly suggest.

[DanAlbert]

https://android.googlesource.com/platform/ndk/+/master/docs/user/common_problems.md#rtti_exceptions-not-working-across-library-boundaries

You can find lots of other discussions of this by searching this bug tracker for "type_info" as well.

libc++ complies with the C++ ABI, gnustl does not. Rationale discussed here: https://lists.llvm.org/pipermail/llvm-dev/2014-June/073465.html

[DanAlbert]

One of those other bugs was even filed by you: https://github.com/android-ndk/ndk/issues/899

[pendyalasyam]

Hi Dan, initially I thought of adding the key function. But std::__ndk1::basic_string<wchar_t, wc16::wchar16_traits, std::__ndk1::allocator> is explicit instantiation of basic_string defined by ndk. So I can not add virtual function to basic_string class.

[DanAlbert]

RTTI doesn't work for non-polymorphic types. You want std::is_same for non-polymorphic types.

[pendyalasyam]

As far as I understand, std::is_same is compile time type verification. But in my case, I am basically doing type verification at runtime.

bool isOfStringType(AnyType obj) //AnyType is local implementation of std::any { //Step1: get object type at runtime. The only way I know as of now is obj.type().name() return string_type == type obtained in above step. }

could you please help me to code "how to check if an object stored in std::any(/AnyType) is of type X or not, where X is non-polymorphic type and objects are passed from different shared library."

[DanAlbert]

The C++ ABI does not support this aiui.

[pendyalasyam]

So, Can I assume C++ ABI can not identify whether two run time objects are of same type if two objects are passed from two different shared libraries (because two objects typeid wont match)?

[DanAlbert]

That's my understanding, yes.

[pendyalasyam]

In that case, we may need to correct our logic. Thanks @DanAlbert for your support.