fixedSizePolymorphicHolder.h

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//
// Copyright 2025 Pixar
//
// Licensed under the terms set forth in the LICENSE.txt file available at
// https://openusd.org/license.
//
#ifndef PXR_EXEC_ESF_FIXED_SIZE_POLYMORPHIC_HOLDER_H
#define PXR_EXEC_ESF_FIXED_SIZE_POLYMORPHIC_HOLDER_H


#include "pxr/pxr.h"

#include "pxr/exec/esf/api.h"

#include "pxr/base/tf/diagnosticLite.h"
#include "pxr/base/tf/tf.h"

#include <cstddef>
#include <new>
#include <type_traits>
#include <utility>

PXR_NAMESPACE_OPEN_SCOPE

class ESF_API_TYPE EsfFixedSizePolymorphicBase
{
public:
    ESF_API virtual ~EsfFixedSizePolymorphicBase();

private:
    template <class Base, size_t BufferSize>
    friend class EsfFixedSizePolymorphicHolder;

    // Copies this object to the storage of another
    // EsfFixedSizePolymorphicHolder.
    //
    // This class provides a default implementation so that derived types can be
    // instantiated without having to override it. The proper overridden
    // implementation will be defined by EsfFixedSizePolymorphicHolder.
    //
    ESF_API virtual void _CopyTo(std::byte *storage) const;

    // Moves this object to the storage of another
    // EsfFixedSizePolymorphicHolder.
    //
    // This class provides a default implementation so that derived types can be
    // instantiated without having to override it. The proper overridden
    // implementation will be defined by EsfFixedSizePolymorphicHolder.
    //
    ESF_API virtual void _MoveTo(std::byte *storage);
};

template <class Base, size_t BufferSize = sizeof(Base)>
class EsfFixedSizePolymorphicHolder
{
    static_assert(std::is_base_of_v<EsfFixedSizePolymorphicBase, Base>);
    constexpr static size_t _REQUIRED_ALIGNMENT = alignof(Base);

public:
    using This = EsfFixedSizePolymorphicHolder<Base, BufferSize>;

    EsfFixedSizePolymorphicHolder() = delete;

    template <class Derived, class... Args>
    EsfFixedSizePolymorphicHolder(
        std::in_place_type_t<Derived> derivedTypeHint, Args &&...args);

    EsfFixedSizePolymorphicHolder(const This &other) {
        other->_CopyTo(_storage);
    }

    EsfFixedSizePolymorphicHolder(This &&other) {
        other->_MoveTo(_storage);
    }

    ~EsfFixedSizePolymorphicHolder() {
        Get()->~Base();
    }

    This &operator=(const This &other) {
        if (this != &other) {
            Get()->~Base();
            other->_CopyTo(_storage);
        }
        return *this;
    }

    This &operator=(This &&other) {
        if (this != &other) {
            Get()->~Base();
            other->_MoveTo(_storage);
        }
        return *this;
    }

    inline Base *Get() {
        return reinterpret_cast<Base *>(_storage);
    }

    inline const Base *Get() const {
        return reinterpret_cast<const Base *>(_storage);
    }

    inline Base *operator->() {
        return Get();
    }

    inline const Base *operator->() const {
        return Get();
    }

    inline Base &operator*() {
        return *Get();
    }

    inline const Base &operator*() const {
        return *Get();
    }

    struct Compatibility
    {
        template <class Derived>
        constexpr static bool FITS_IN_BUFFER = sizeof(Derived) <= BufferSize;
        
        template <class Derived>
        constexpr static bool DERIVES_FROM_BASE =
            std::is_base_of_v<Base, Derived>;
        
        template <class Derived>
        constexpr static bool HAS_ALIGNMENT =
            alignof(Derived) == _REQUIRED_ALIGNMENT;

        template <class Derived>
        static bool HasBaseAtSameAddress(const Derived &derived);
    };

private:
    // Implements the _CopyTo and _MoveTo methods for a derived type.
    template <class Derived>
    class _Holder : public Derived
    {
    public:
        // Constructs a Derived instance from the provided constructor Args.
        template <class... Args>
        _Holder(Args &&...args) : Derived(std::forward<Args>(args)...) {
            TF_AXIOM(Compatibility::HasBaseAtSameAddress(*this));
        }

    private:
        void _CopyTo(std::byte *storage) const final {
            const Derived *thisDerived = static_cast<const Derived *>(this);
            ::new (static_cast<void *>(storage)) _Holder<Derived>(*thisDerived);
        }

        void _MoveTo(std::byte *storage) final {
            Derived *thisDerived = static_cast<Derived *>(this);
            ::new (static_cast<void *>(storage)) _Holder<Derived>(
                std::move(*thisDerived));
        }
    };

    // Held instances are emplaced in this byte array.
    alignas(_REQUIRED_ALIGNMENT) std::byte _storage[BufferSize];
};

template <class Base, size_t BufferSize>
template <class Derived, class... Args>
EsfFixedSizePolymorphicHolder<Base, BufferSize>::EsfFixedSizePolymorphicHolder(
    std::in_place_type_t<Derived> derivedTypeHint, Args &&...args)
{
    TF_UNUSED(derivedTypeHint);

    static_assert(
        Compatibility::template FITS_IN_BUFFER<Derived>,
        "The size of the derived type is larger than the availble "
        "storage.");
    static_assert(
        Compatibility::template DERIVES_FROM_BASE<Derived>,
        "Derived is not a derived class of Base.");
    static_assert(
        Compatibility::template HAS_ALIGNMENT<Derived>,
        "The derived type has incompatible alignment.");

    ::new (static_cast<void *>(_storage)) _Holder<Derived>(
        std::forward<Args>(args)...);
}

template <class Base, size_t BufferSize>
template <class Derived>
bool
EsfFixedSizePolymorphicHolder<Base, BufferSize>::Compatibility::
    HasBaseAtSameAddress(const Derived &derived)
{
    if constexpr (!DERIVES_FROM_BASE<Derived>) {
        return false;
    }

    const Base *base = static_cast<const Base *>(&derived);
    const void *baseAddress = static_cast<const void *>(base);
    const void *derivedAddress = static_cast<const void *>(&derived);
    return baseAddress == derivedAddress;
}

PXR_NAMESPACE_CLOSE_SCOPE

#endif