Can't add a Persistent to vector>

Question

I'm migrating from v8 3.x to 4.y and I'm having problems with one of the functions defined in one of my header file.

//JavascriptBase.h
namespace Company {
    class Base {
      protected:
        void registerHandler(v8::Persistent<v8::Function>& func) {
            user_functions_.push_back(func);
        }
      private:
        std::vector<v8::Persistent<v8::Function>> user_functions_;
    }
}

When I try to compile I get the following errors:

third-party2/v8/4.7.39/gcc-4.8.1-glibc-2.17-fb/281a9e6/include/v8.h:667:53: error: assigning to 'v8::Object *volatile' from incompatible type 'v8::Primitive *'
    while (false) { *(static_cast<O* volatile*>(0)) = static_cast<Primitive*>(0); };
                                                    ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~
third-party2/v8/4.7.39/gcc-4.8.1-glibc-2.17-fb/281a9e6/include/v8.h:663:5: note: in instantiation of function template specialization 'v8::NonCopyablePersistentTraits<v8::Function>::Uncompilable<v8::Object>' requested here
    Uncompilable<Object>();
    ^
third-party2/v8/4.7.39/gcc-4.8.1-glibc-2.17-fb/281a9e6/include/v8.h:7175:6: note: in instantiation of function template specialization 'v8::NonCopyablePersistentTraits<v8::Function>::Copy<v8::Function, v8::NonCopyablePersistentTraits<v8::Function> >' requested here
  M::Copy(that, this);
     ^
third-party2/v8/4.7.39/gcc-4.8.1-glibc-2.17-fb/281a9e6/include/v8.h:729:5: note: in instantiation of function template specialization 'v8::Persistent<v8::Function, v8::NonCopyablePersistentTraits<v8::Function> >::Copy<v8::Function, v8::NonCopyablePersistentTraits<v8::Function> >' requested here
    Copy(that);
    ^
third-party2/libgcc/4.8.1/gcc-4.8.1-glibc-2.17-fb/8aac7fc/include/c++/4.8.1/ext/new_allocator.h:120:23: note: in instantiation of member function 'v8::Persistent<v8::Function, v8::NonCopyablePersistentTraits<v8::Function> >::Persistent' requested here
 { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }
                      ^
third-party2/libgcc/4.8.1/gcc-4.8.1-glibc-2.17-fb/8aac7fc/include/c++/4.8.1/bits/alloc_traits.h:254:8: note: in instantiation of function template specialization '__gnu_cxx::new_allocator<v8::Persistent<v8::Function, v8::NonCopyablePersistentTraits<v8::Function> > >::construct<v8::Persistent<v8::Function, v8::NonCopyablePersistentTraits<v8::Function> >, const v8::Persistent<v8::Function, v8::NonCopyablePersistentTraits<v8::Function> > &>' requested here
 { __a.construct(__p, std::forward<_Args>(__args)...); }
       ^
third-party2/libgcc/4.8.1/gcc-4.8.1-glibc-2.17-fb/8aac7fc/include/c++/4.8.1/bits/alloc_traits.h:393:4: note: in instantiation of function template specialization 'std::allocator_traits<std::allocator<v8::Persistent<v8::Function, v8::NonCopyablePersistentTraits<v8::Function> > > >::_S_construct<v8::Persistent<v8::Function, v8::NonCopyablePersistentTraits<v8::Function> >, const v8::Persistent<v8::Function, v8::NonCopyablePersistentTraits<v8::Function> > &>' requested here
 { _S_construct(__a, __p, std::forward<_Args>(__args)...); }
   ^
third-party2/libgcc/4.8.1/gcc-4.8.1-glibc-2.17-fb/8aac7fc/include/c++/4.8.1/bits/stl_vector.h:905:21: note: in instantiation of function template specialization 'std::allocator_traits<std::allocator<v8::Persistent<v8::Function, v8::NonCopyablePersistentTraits<v8::Function> > > >::construct<v8::Persistent<v8::Function, v8::NonCopyablePersistentTraits<v8::Function> >, const v8::Persistent<v8::Function, v8::NonCopyablePersistentTraits<v8::Function> > &>' requested here
     _Alloc_traits::construct(this->_M_impl, this->_M_impl._M_finish,
                    ^
./Company/JavaScriptBase.h:84:21: note: in instantiation of member function 'std::vector<v8::Persistent<v8::Function, v8::NonCopyablePersistentTraits<v8::Function> >, std::allocator<v8::Persistent<v8::Function, v8::NonCopyablePersistentTraits<v8::Function> > > >::push_back' requested here
    user_functions_.push_back(func);
                    ^
1 error generated.

I've done some googling and found that Persistent objects aren't assignable but I'm not sure what to do with this information. Any help would be appreciated.

Possible duplicate of [Storing handles to objects in a hashmap or set in Google's V8 engine](http://stackoverflow.com/questions/21239249/storing-handles-to-objects-in-a-hashmap-or-set-in-googles-v8-engine) — smirnoff, Feb 27 '16 at 19:41

score 3 · Accepted Answer · answered Jan 11 '16 at 19:31

3

If they aren't assignable then you (obviously) can't assign them. And since std::vector<> relies on copying values using the assignment operator, your approach won't work.

You can however store pointers to them instead, since copying pointers of anything is well defined.

answered Jan 11 '16 at 19:31

Blindy

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Technically, copying dangling/invalid pointers is undefined. But I suppose those are not pointers of things. :) – Lightness Races in Orbit Jan 11 '16 at 19:33
Is that true? I get that the standard is trying to pretend pointers are some kind of special magical construct and not just a bunch of numbers as it's most common nowadays, but what difference would the fact that it's an invalid pointer do? – Blindy Jan 11 '16 at 19:45
@SergeyA: You may debate it all you like :) – Lightness Races in Orbit Jan 11 '16 at 20:30
@LightnessRacesinOrbit, not me, no. But I've heard people arguing about that, i.e., is it really undefined behaviour to copy a value of invalid pointer to another pointer without dereferencing it or not. – SergeyA Jan 11 '16 at 20:36
@SergeyA: People argue about a lot of things. – Lightness Races in Orbit Jan 11 '16 at 20:37
@Blindy: Trap representations, for one. – Lightness Races in Orbit Jan 11 '16 at 20:38

xaxxon · Answer 2 · 2016-01-15T02:28:28.947

0

You can make a copyable persistent with this "typedef"

using CopyablePersistent = v8::Persistent<T, v8::CopyablePersistentTraits<T>>;

This assumes it's inside a templated class, but you can make the T whatever you want.

edit: apparently this is a "very bad" thing to do, but I don't know why.

edited Jan 15 '16 at 02:28

answered Jan 12 '16 at 02:23

xaxxon

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Why it's a "very bad" thing? Could you provide link to the source? – smirnoff Feb 27 '16 at 19:42
Nope. Just been told that by devs in #v8 on freenode. They said it will lead to intermittent crashes. It's really never needed, though. You can pass around Global& in your code and for use in STL containers or you can always make a new one with Global's normal constructors, too. – xaxxon Feb 27 '16 at 19:46

Can't add a Persistent to vector>

2 Answers2