Review LazyObjectImpl::obj()

[alandefreitas]

Most compilers don't implement __cpp_lib_atomic_shared_ptr, on which LazyObjectImpl depends:

mrdocs/src/lib/Dom/Object.cpp

Lines 216 to 232 in a962db0

	ObjectImpl&
	LazyObjectImpl::
	obj() const
	{
	#ifdef __cpp_lib_atomic_shared_ptr
	auto impl = sp_.load();
	if(impl)
	return *impl;
	impl_type expected = nullptr;
	if(sp_.compare_exchange_strong(
	expected, construct().impl()))
	return *sp_.load();
	return *expected;
	#else
	return *sp_;
	#endif
	}

For this reason, the function above contains two implementations, depending on whether __cpp_lib_atomic_shared_ptr is supported.

Both implementations are problematic:

• The implementation is not thread-safe when __cpp_lib_atomic_shared_ptr is unsupported. The object would need to carry a pointer to mutex for that.
• The implementation when __cpp_lib_atomic_shared_ptr is supported is also unsafe because the load and exchange operations are independent. This means two concurrent threads will attempt to construct the object twice if they call the function before the object is constructed.

[vinniefalco]

sad