IntrusiveUnsetOptionalState.h

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// Copyright Epic Games, Inc. All Rights Reserved.
 
#pragma once
 
#include "Misc/OptionalFwd.h"
#include "Concepts/DecaysTo.h"
#include "Concepts/SameAs.h"
 
namespace UE::Core::Private
{
    struct FOptional;
}
 
struct FIntrusiveUnsetOptionalState
{
    // Defined in the global namespace for convenience and consistency
    // with TOptional which is also global.
 
    template <typename>
    friend struct TOptional;
 
    friend struct UE::Core::Private::FOptional;
 
private:
    explicit FIntrusiveUnsetOptionalState() = default;
};
 
template <typename T>
constexpr bool HasIntrusiveUnsetOptionalState()
{
    // Ensure the type has a nested bHasIntrusiveUnsetOptionalState, and as derived types are not guaranteed to have
    // an intrusive state even if the base does, ensure IntrusiveUnsetOptionalStateType matches the type in the optional.
    if constexpr (requires{ { T::bHasIntrusiveUnsetOptionalState } -> UE::CDecaysTo<bool>; requires UE::CSameAs<const typename T::IntrusiveUnsetOptionalStateType, const T>; })
    {
        return T::bHasIntrusiveUnsetOptionalState;
    }
    else
    {
        return false;
    }
}