SharedObjects.h

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/** @noop License Comment
 *  @file
 *  @copyright
 *  This Source Code is subject to the terms of the Mozilla Public License, v2.0.
 *  If a copy of the MPL was not distributed with this file You can obtain one at
 *  https://mozilla.org/MPL/2.0/
 *  
 *  @author David Mórász
 *  @date 2025
 */
 
#pragma once
 
#include "CoreMinimal.h"
#include "Mcro/FunctionTraits.h"
#include "Mcro/Construct.h"
 
/** @brief Utilities for TSharedPtr/Ref and related */
namespace Mcro::SharedObjects
{
    using namespace Mcro::Concepts;
    using namespace Mcro::FunctionTraits;
 
    /** @brief Copy thread-safety from other shared object type */
    template <CSharedOrWeak T>
    using TSharedPtrFrom = TSharedPtr<typename T::ElementType, T::Mode>;
 
    /** @brief Copy thread-safety from other shared object type */
    template <CSharedOrWeak T>
    using TSharedRefFrom = TSharedRef<typename T::ElementType, T::Mode>;
 
    /** @brief Copy thread-safety from other shared object type */
    template <CSharedOrWeak T>
    using TWeakPtrFrom = TWeakPtr<typename T::ElementType, T::Mode>;
 
    /**
     *  @brief  Concept describing an object which provides a deferred initializer for shared objects.
     *  
     *  This works around the annoyance of TSharedFromThis objects cannot use their shared pointers in their
     *  constructor, braking RAII in some cases. Of course this is only achievable if the object cooperates and
     *  implements an Initialize method.
     *  It's important that `Initialize` should not be virtual so derived classes can hide them with their own
     *  overload.
     */
    template <typename T, typename... Args>
    concept CSharedInitializeable = requires(T& object, Args&&... args)
    {
        object.Initialize(FWD(args)...);
    };
 
    /**
     *  @brief
     *  A wrapper around MakeShareable that automatically calls an initializer method Initialize on the
     *  instantiated object.
     *  
     *  This works around the annoyance of TSharedFromThis objects cannot use their shared pointers in their
     *  constructor, braking RAII in some cases. Of course this is only achievable if the object cooperates and
     *  implements an Initialize method.
     *  It's important that Initialize should not be virtual so derived classes can hide them with their own
     *  overload.
     */
    template <typename T, ESPMode Mode = ESPMode::ThreadSafe, typename... Args>
    requires CSharedInitializeable<T, Args...>
    TSharedRef<T, Mode> MakeShareableInit(T* newObject, Args&&... args)
    {
        TSharedRef<T, Mode> result = MakeShareable(newObject);
        result->Initialize(FWD(args)...);
        return result;
    }
 
    /**
     *  @brief  A combination of MakeShareable and Mcro::Construct::ConstructNew
     *  
     *  Usage:
     *  @code
     *  using namespace Mcro::SharedObjects;
     *  
     *  auto myObject = ConstructShared([](MyObject& _)
     *  {
     *      _.Foo = 42;
     *      _.Initialize();
     *      // etc...
     *  });
     *  static_assert(std::is_same_v<decltype(myObject), TSharedRef<MyObject>>);
     *  @endcode
     *
     *  @param    init  A lambda function with a single l-value reference parameter of the object type to initialize.
     *  @param    args  Arguments of the object constructor
     *  @return   A pointer to the object instance on heap.
     *  @remarks  The C++ 20 designated initializers with named arguments has annoying limitations, therefore this exists
     */
    template <
        CFunctorObject Initializer,
        typename... Args,
        ESPMode Mode = ESPMode::ThreadSafe,
        typename ResultArg = TFunction_Arg<Initializer, 0>,
        typename Result = std::decay_t<ResultArg>
    >
    requires std::is_lvalue_reference_v<ResultArg>
    TSharedRef<Result, Mode> ConstructShared(Initializer&& init, Args&&... args)
    {
        using namespace Mcro::Construct;
        return MakeShareable(
            ConstructNew(FWD(init), FWD(args)...)
        );
    }
 
    /**
     *  @brief  Create a shared pointer which takes in an object with in-place refcounting.
     *  
     *  Refcounted TSharedPtr/Ref doesn't take ownership of the object and when the last reference goes out of scope it
     *  simply decreases the refcount instead of deleting the object.
     */
    template <CRefCounted T, ESPMode Mode = ESPMode::ThreadSafe>
    TSharedRef<T, Mode> ShareRefCounted(T* object)
    {
        check(object);
        object->AddRef();
        return MakeShareable(object, [](T* object) { object->Release(); });
    }
 
    /**
     *  @brief
     *  Same as `SharedThis(this)` in `TSharedFromThis` but returning a weak pointer instead.
     *  
     *  Indeed, `TSharedFromThis` already has `AsWeak()` but that can only return the type which was originally set by
     *  `TSharedFromThis` making its usage slightly less convenient in derived classes.
     */
    template <
        CSharedFromThis T,
        ESPMode Mode = decltype(DeclVal<T const>().AsShared())::Mode
    >
    auto WeakSelf(const T* self) -> TWeakPtr<T const, Mode>
    {
        return StaticCastSharedRef<std::decay_t<T> const>(self->AsShared());
    }
 
    /**
     *  @brief
     *  Same as `SharedThis(this)` in `TSharedFromThis` but returning a weak pointer instead.
     *  
     *  Indeed, `TSharedFromThis` already has `AsWeak()` but that can only return the type which was originally set by
     *  `TSharedFromThis` making its usage slightly less convenient in derived classes.
    */
    template <
        CSharedFromThis T,
        ESPMode Mode = decltype(DeclVal<T>().AsShared())::Mode
    >
    auto WeakSelf(T* self) -> TWeakPtr<T, Mode>
    {
        return StaticCastSharedRef<std::decay_t<T>>(self->AsShared());
    }
 
    /** @brief Same as `SharedThis(this)` in `TSharedFromThis`. */
    template <
        CSharedFromThis T,
        ESPMode Mode = decltype(DeclVal<T const>().AsShared())::Mode
    >
    auto SharedSelf(const T* self) -> TSharedRef<T const, Mode>
    {
        return StaticCastSharedRef<std::decay_t<T> const>(self->AsShared());
    }
 
    /** @brief Same as `SharedThis(this)` in `TSharedFromThis`. */
    template <
        CSharedFromThis T,
        ESPMode Mode = decltype(DeclVal<T>().AsShared())::Mode
    >
    auto SharedSelf(T* self) -> TSharedRef<T, Mode>
    {
        return StaticCastSharedRef<std::decay_t<T>>(self->AsShared());
    }
 
    /**
     *  @brief
     *  A simple mutable shared storage of any value.
     *
     *  One particular use case for this may be delegates which may remove themselves once invoked. For example:
     *  @code
     *  auto once = MakeSharedDelegateHandle();
     *  once->Value = MyEvent.AddLambda([this, once]
     *  {
     *      // execute something only once.
     *      MyEvent.Remove(once->Value);
     *  });
     *  @endcode
     */
    template <CDefaultInitializable T>
    struct TSharedStorage
    {
        T Value;
    };
    
    template <CDefaultInitializable T>
    using TSharedStoragePtr = TSharedPtr<TSharedStorage<T>>;
    
    template <CDefaultInitializable T>
    using TSharedStorageRef = TSharedRef<TSharedStorage<T>>;
    
    template <CDefaultInitializable T>
    using TSharedStorageWeakPtr = TWeakPtr<TSharedStorage<T>>;
 
    /** @brief A simple convenience wrapper around making FDelegateHandles shared */
    FORCEINLINE TSharedStorageRef<FDelegateHandle> MakeSharedDelegateHandle()
    {
        return MakeShared<TSharedStorage<FDelegateHandle>>();
    }
}