220 lines
8.6 KiB
C++
220 lines
8.6 KiB
C++
/** @file OObject.hpp
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*
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* @author Roland Conybeare, Dec 2025
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**/
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#pragma once
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#include "facet_implementation.hpp"
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#include "typeseq.hpp"
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#include <new>
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#include <cstring>
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namespace xo {
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namespace facet {
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template <typename OObject>
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consteval bool valid_object_traits()
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{
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static_assert(requires { typename OObject::AbstractInterface; },
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"OObject type must provide typename Object::AbstractInterface");
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static_assert(requires { typename OObject::ISpecific; },
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"OObject type must provide typename Object::ISpecific");
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static_assert(requires { typename OObject::DataType; },
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"OObject type must provide typename Object::DataType");
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static_assert(valid_facet_implementation<OObject::AbstractInterface, OObject::ISpecific>,
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"OObject::ISpecific must implement Object::AbstractInterface");
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static_assert(std::is_standard_layout_v<OObject>,
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"OObject must have standard layout, i.e. no virtual methods."
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" Virtual methods belong in OObject::AbstractInterface");
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static_assert(requires(const OObject & obj) {
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{ obj.iface() } -> std::convertible_to<const typename OObject::AbstractInterface*>; },
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"OObject must have non-virtual method iface()"
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" returning const OObject::AbstractInterface");
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static_assert(requires(const OObject & obj) {
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{ obj.data() } -> std::convertible_to<typename OObject::DataType*>; },
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"OObject must have non-virtual method data() returning OObject::DataType*");
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return true;
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}
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/** A "fat object pointer": combines two pointers:
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*
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* 1. behavior: an interface pointer
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* (implementation of @tparam AFacet a.k.a. vtable pointer)
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* for passive state DRepr.
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* Interface pointers are static globals.
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*
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* 2. state: a data pointer to instance of passive state DRepr
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* An OObject instance does not own its data pointer
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*
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* Performance note: when DRepr can be determined at compile time,
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* it's often feasible to optimize away the interface part.
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*
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* Runtime polymorphism when @tparam DRepr is @ref DVariantPlaceholder
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*
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* Application code should not use this directly.
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* Instead, inherit a facet-specific routing wrapper that automatically
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* injects @ref data as first argument to @ref iface_ methods.
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**/
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template <typename AFacet, typename DRepr = DVariantPlaceholder>
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struct OObject {
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using FacetType = AFacet;
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using ISpecific = FacetImplType<AFacet, DRepr>;
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using DataType = DRepr;
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using DataPtr = DRepr*;
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using Opaque = void *;
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explicit OObject() {}
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explicit OObject(DataPtr d) : data_{d} {}
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/** trivial: nothing to do for @ref iface_ and does not own @ref data_ **/
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~OObject() = default;
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/** OObject is truthy **/
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operator bool() const { return data_ != nullptr; }
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// ----- iface() for variant fat pointer -----
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/** interface pointer for variant OObject instances.
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* These instance support runtime polymorphism.
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**/
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const FacetType * iface() const
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requires std::is_same_v<DataType, DVariantPlaceholder>
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{
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/* std::launder:
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*
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* contents of iface_ at runtime will not match
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* compile-time datatype. This prohibits compiler de-virtualizing
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* calls to ISpecific methods, based on mistaken belief that
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* vtable pointer is known at compile time.
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*/
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return std::launder(&iface_);
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}
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/** non-const verison. Technically all interface methods are const.
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* But counterintuitive to have to mark interface methods const
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* that are dedicated to mutable data.
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**/
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FacetType * iface()
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requires std::is_same_v<DataType, DVariantPlaceholder>
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{
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return std::launder(&iface_);
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}
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// ----- iface() for typed fat pointer -----
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/** interface pointer for OObject instance with representation
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* known at compile time.
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*
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* Calls here should be straightforward to devirtualize
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**/
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const FacetType * iface() const
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requires (!std::is_same_v<DataType, DVariantPlaceholder>)
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{
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/* don't use std::launder: want compiler to devirtualize
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* calls to virtual @ref iface_ methods
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*/
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return &iface_;
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}
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/** non-const verison. Technically all interface methods are const.
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* But counterintuitive to have to mark interface methods const
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* that are dedicated to mutable data.
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**/
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FacetType * iface()
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requires(!std::is_same_v<DataType, DVariantPlaceholder>)
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{
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return &iface_;
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}
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DataPtr data() const { return data_; }
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Opaque opaque_data() const { return data_; }
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void reset() { data_ = nullptr; }
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void reset_opaque(Opaque data) { data_ = (DataPtr)data; }
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/**
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* We're either:
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* - assigning from pointer with compatible representation
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* - implementing the fat-object-pointer equivalent of
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* assigning a derived pointer to a base pointer.
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**/
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template <typename DOther>
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OObject & from_data(DOther * other) {
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static_assert(std::is_same_v<DRepr, DVariantPlaceholder>
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|| std::is_convertible_v<DOther*, DRepr*>);
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if constexpr (std::is_convertible_v<DOther*, DRepr*>) {
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/** assigning from data with same representation **/
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this->data_ = other;
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} else /*DRepr is DVariantPlaceholder*/ {
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/** assigning to variant **/
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/* acquire fat object pointer for (AFacet, DOther) */
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OObject<AFacet, DOther> oother(other);
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static_assert(sizeof(*this) == sizeof(oother));
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::memcpy((void*)this, (void*)&oother, sizeof(*this));
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}
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return *this;
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}
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/**
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* Downcast to pointer of type DOther*, if valid.
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* Provided when actual type of @ref data_ is not DRepr,
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* because DRepr is DVariantPlaceholder.
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*
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* We can't rely on dynamic_cast here, because DRepr's
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* don't need to be related as far as c++ type system is
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* concerned.
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**/
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template <typename DOther>
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DOther * downcast() const
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requires (std::is_same_v<DataType, DVariantPlaceholder>)
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{
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if (data_ && (typeseq::id<DOther>() == this->iface()->_typeseq())) {
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/* actual runtime type for data_ is DOther,
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* safe to reinterpret
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*/
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return reinterpret_cast<DOther*>(data_);
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} else {
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return nullptr;
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}
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}
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DRepr & operator*() { return *data_; }
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#ifdef NOT_IN_USE
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// not sure if this is a good idea. could just as well write
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// auto obj = ...;
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// *obj.data() == rhs
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/** assign contents of rhs in-place **/
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OObject & operator=(const DRect & rhs) {
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assert(data_);
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*(this->data_) = rhs;
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return *this;
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}
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#endif
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/** fetch data pointer. load-bearing for routing classes **/
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static bool _valid;
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/** runtime interface for this object **/
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ISpecific iface_;
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/** runtime state for this object **/
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DataPtr data_ = nullptr;
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};
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template <typename AFacet, typename DRepr>
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bool
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OObject<AFacet, DRepr>::_valid = valid_object_traits<OObject>();
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} /*namespace facet*/
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} /*namespace xo*/
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/* end OObject.hpp */
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