{"id":106415,"date":"2022-03-30T07:00:00","date_gmt":"2022-03-30T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=106415"},"modified":"2022-03-30T07:18:48","modified_gmt":"2022-03-30T14:18:48","slug":"20220330-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20220330-00\/?p=106415","title":{"rendered":"Why am I getting the error message “This is only for weak ref support” when I try to create a C++\/WinRT weak reference?"},"content":{"rendered":"

A customer tried to obtain a weak reference from within a C++\/WinRT class implementation by calling get_weak()<\/code>. This normally works, but for some reason, the customer was getting the error<\/p>\n

winrt\\base.h(7594,47): error C2338: This is only for weak ref support.\r\nwinrt\\base.h(7593): message : while compiling class template member function 'winrt::impl::IWeakReferenceSource *winrt::impl::root_implements<D,IWidget>::make_weak_ref(void) noexcept'\r\n        with\r\n        [\r\n            D=Widget\r\n        ]\r\nwinrt\\base.h(7500): message : see reference to function template instantiation 'winrt::impl::IWeakReferenceSource *winrt::impl::root_implements<D,IWidget>::make_weak_ref(void) noexcept' being compiled\r\n        with\r\n        [\r\n            D=Widget\r\n        ]\r\nwinrt\\base.h(7803): message : see reference to class template instantiation 'winrt::impl::root_implements<D,IWidget>' being compiled\r\n        with\r\n        [\r\n            D=Widget\r\n        ]\r\nWidget.h(15): message : see reference to class template instantiation\r\n<\/pre>\n
The message “This is only for weak ref support” is rather enigmatic. It sounds like an internal note from the authors of the C++\/WinRT library to their future selves, telling them that they made some sort of internal error.<\/p>\n
But let’s see if we can figure out what it’s saying. This is another segment in the “Behind C++\/WinRT series” (and the larger topic of decoding template metaprogramming).<\/p>\n
The error message comes from  make_weak_ref<\/a>, which is a helper function  called from get_weak<\/a>:<\/p>\n
static_assert(is_weak_ref_source::value, \"This is only for weak ref support.\");\r\n<\/pre>\n
What is is_weak_ref_source<\/code>? Let’s look for it:<\/p>\n
using is_inspectable = std::disjunction<std::is_base_of<Windows::Foundation::IInspectable, I>...>;\r\nusing is_weak_ref_source = std::conjunction<is_inspectable, std::negation<std::disjunction<std::is_same<no_weak_ref, I>...>>>;\r\n<\/pre>\n
We saw in an earlier installment  how to read std::disjunction<\/code>, std:conjunction<\/code>, and std::negation<\/code><\/a>.<\/p>\n
First, let’s untangle is_inspectable<\/code>: It checks each of the implemented interfaces I<\/code> to see if it has Windows::Foundation::IInspectable<\/code> as a base class, and then uses std::disjunction<\/code> to “or” all the results together via a template parameter pack expansion. Therefore, is_inspectable<\/code> is true if any of the implemented interfaces has Windows::Foundation::IInspectable<\/code> as a base class, which means that at least one of the interfaces is or is derived from IInspectable<\/code>.<\/p>\n
Next, we calculate is_weak_ref_source<\/code>. The parameter pack expansion checks whether any of the implemented interfaces is the marker type no_weak_ref<\/code>. Therefore, through the power of reading code<\/i>, we determine that in order for a type to support weak references, it must<\/p>\n