Ambigious call to overloaded function when using lambda

Question

Never used lambdas before and I can't understand where I'd have to add it.

My Error is "Show: Ambigious call to overloaded function"

Show() can take 2 types CustomizeToast and CustomizeToastAsync. So I guess I need to specify CustomizeToast somewhere but I can't for the life of me see where.

This is my current code:

ToastContentBuilder()
    .AddText(L"Hello World!")
    .Show([](ToastNotification const& toast) -> void
    {
       toast.Tag(L"1");
    });

Error:

error C2668: 'winrt::impl::consume_Microsoft_Toolkit_Uwp_Notifications_IToastContentBuilderClass<winrt::Microsoft::Toolkit::Uwp::Notifications::IToastContentBuilderClass>::Show': ambiguous call to overloaded function
Microsoft.Toolkit.Uwp.Notifications.0.h(2159,60): message : could be 'auto winrt::impl::consume_Microsoft_Toolkit_Uwp_Notifications_IToastContentBuilderClass<winrt::Microsoft::Toolkit::Uwp::Notifications::IToastContentBuilderClass>::Show(const winrt::Microsoft::Toolkit::Uwp::Notifications::CustomizeToastAsync &) const'
Microsoft.Toolkit.Uwp.Notifications.0.h(2158,31): message : or       'auto winrt::impl::consume_Microsoft_Toolkit_Uwp_Notifications_IToastContentBuilderClass<winrt::Microsoft::Toolkit::Uwp::Notifications::IToastContentBuilderClass>::Show(const winrt::Microsoft::Toolkit::Uwp::Notifications::CustomizeToast &) const'
message : while trying to match the argument list '(winrt::MyApplication::implementation::Toast::ShowToastNotification::<lambda_786678859ea03b85c00686eebdcb39db>)'

Project is an UWP project using the Microsoft.Toolkit.Uwp.Notifications

Please provide full error message, in addition to minimal reproducible example. — Algirdas Preidžius
– Algirdas Preidžius, Commented May 19, 2021 at 12:19
if Show can either take a CustomizeToast or a CustomizeToastAsync and both can be constructed from the lambda, then the compiler cannot know which one to pick. I have to guess, but the complete compiler error should tell all this in a much more concise way. Without code and complete error message we can only guess — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented May 19, 2021 at 12:32
Added Error Message, this is all I have in my ShowToastNotification function and should work fine with an empty UWP Solution and the Microsoft.Toolkit.Uwp.Notification Package — indiehjaerta
– indiehjaerta, Commented May 19, 2021 at 12:42

YSC · Accepted Answer · 2021-05-19 12:53:58Z

1

Show() can take 2 types CustomizeToast and CustomizeToastAsync.

This is clearly an oversight on the API's developpers end. Now since both classes can be constructed from a lambda the compiler doesn't know which one to use, so you have to guide it:

ToastContentBuilder()
    .AddText(L"Hello World!")
    .Show(CustomizeToast{[](ToastNotification const& toast) -> void
    {
       toast.Tag(L"1");
    }});

This is less readable and cumbersome, but constructing syntaxic suggar for it would be error-prone and a bit convoluted.

answered May 19, 2021 at 12:53

YSC

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9 Comments

IInspectable Over a year ago

"This is clearly an oversight on the API's developpers end." - Or, just something one has to live with when C++' overly complex overload resolution kicks in. If you cannot help it but need to point fingers, don't ignore what a mess C++ has turned into.

YSC Over a year ago

@IInspectable I stand by my words.

MSalters Over a year ago

Don't think you can really blame C++ here. It would be trivial for the API developers to add an ShowCustomizeToast method, not overloaded. This will not break any existing code, and be a pure extension. The overloaded Show(CustomizeToast) would then be a simple shorthand, and probably an inline wrapper.

IInspectable Over a year ago

The API developer doesn't know about C++, nor lambda expressions. Nor about crazy constructors that take function-like objects.

YSC Over a year ago

@IInspectable I'm lost. Is it not a C++ API? Shouldn't its devs know of a major, 10-years-old C++ feature?

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