Making an agile version of a Windows Runtime delegate in C++/WinRT, part 8

This article discusses handling exceptions in the context of C++/WinRT delegates, particularly focusing on the construction order of lambda captures and potential reference leaks. It emphasizes the importance of precreating deleters to avoid issues with raw pointers.

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Last time, we fixed the problem of an exception thrown from the custom deleter’s constructor resulting in a reference leak . But wait, there’s another source of exceptions.

if (d.try_as<::INoMarshal>()) { in_context_deleter del; void* p; if constexpr (std::is_reference_v<Delegate>) { p = winrt::detach_abi(d); } else { winrt::copy_to_abi(d, p); } return [p = std::unique_ptr<void, in_context_deleter>(p, std::move(del)), token = get_context_token()](auto&&...args) { if (token == get_context_token()) { std::remove_reference_t<Delegate> d; winrt::copy_from_abi(d, p.get()); d(std::forward<decltype(args)>(args)...); } else { throw winrt::hresult_error(CO_E_NOT_SUPPORTED); } }; } Precreating the deleter means that an exception in its construction happens before we do any funny business with the…

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