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Compiler Error C3779

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'function': a function that returns 'auto or decltype(auto)' cannot be used before it is defined

The auto return type of the specified function call couldn't be deduced. The compiler didn't have enough information at the call site.

Remarks

This error can occur when you call a forward-declared member function that has an auto return type but no body or trailing return type. You can also get this error when the compiler can't find a candidate return type when it instantiates a template specialization. A common cause of this error is a missing interface header. The missing type can often be determined from the typename mentioned in a note that follows this error. To resolve this issue, for every type you use, include the header for the namespace the type is in.

Examples

The following C++/WinRT sample generates C3779:

// C3779.cpp
#include <winrt/Windows.Gaming.Input.h>

void CheckGamepads()
{
    auto gamepads =
        winrt::Windows::Gaming::Input::Gamepad::Gamepads();
    for (auto&& gamepad : gamepads)
    {
        check(gamepad);
    }
}

This code reports an error:

C3779.cpp(8): error C3779: winrt::impl::consume_Windows_Foundation_Collections_IIterable<D,winrt::Windows::Gaming::Input::Gamepad>::First': a function that returns 'auto' cannot be used before it is defined
with
[
    D=winrt:::Windows:::Gaming:::Input:::Gamepad
]
note: see declaration of 'winrt::impl::consume_Windows_Foundation_Collections_IIterable<D,winrt::Windows::Gaming::Input::Gamepad>::First'
with
[
    D=winrt::Windows::Gaming::Input::IVisualCollection
]

The note implies that the return type definition depends on Windows::Foundation::Collections::IIterable. To resolve the issue, include the header for its containing namespace:

// C3779b.cpp
#include <winrt/Windows.Foundation.Collections.h>
#include <winrt/Windows.Gaming.Input.h>

void CheckGamepads()
{
    auto gamepads =
        winrt::Windows::Gaming::Input::Gamepad::Gamepads();
    for (auto&& gamepad : gamepads)
    {
        check(gamepad);
    }
}

See also

Consume APIs with C++/WinRT
Why does my C++/WinRT project get errors of the form "consume_Something: function that returns 'auto' cannot be used before it is defined"?