Compilation errors with std::enable_shared_from_this<> on OS X

Question

A user has come to me with compilation problems on OS X:

http://fpaste.org/77628/39251593/

After ruling out the obvious; using gcc rather than clang, and the "right" standard library, it became apparent that the issue was with the std::enable_shared_from_this<> part of the class declaration

The class itself has a declaration of

class Expression : public std::enable_shared_from_this<Expression> {
// ...
};

Some investigation shows that this could be a problem with Xcode/clang itself: https://github.com/kripken/emscripten/issues/270

https://code.google.com/p/alembic/issues/detail?id=315

Can someone confirm this? Alternatively, is there a way to solve this?

Code is located here with the file in question located here

EDIT: The code in question was unneeded, so was removed. Link updated with a specific commit (r1047)

Answer 1

I believe this is a bug in libc++ that I've run into. I submitted a bug report for this a little while ago (http://llvm.org/bugs/show_bug.cgi?id=18843).

The issue arises because a shared_ptr to some const version of the class that derives from std::enable_shared_from_this is assigned a non-const pointer to that type. In libstdc++, this const is stripped away internally in the functions that do the assignment to the internal weak_ptr in std::enable_shared_from_this, but this does not occur in libc++. The following code shows an example of what will cause this error under the current libc++:

struct A : std::enable_shared_from_this<A> {};

int main()
{
  std::shared_ptr<A const> ptr( new A() );
}

You can work around this for now by doing something like:

std::shared_ptr<A const> ptr( std::const_pointer_cast<A const>( std::shared_ptr<A>( new A() ) ) );

Where the new A() is just the pointer you are trying to get into the shared_ptr.