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64 lines
1.8 KiB
C++
64 lines
1.8 KiB
C++
#pragma once
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#include <Common/typeid_cast.h>
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namespace DB
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{
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/* This base class adds public methods:
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* - Derived * as<Derived>()
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* - const Derived * as<Derived>() const
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* - Derived & as<Derived &>()
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* - const Derived & as<Derived &>() const
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*/
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template <class Base>
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class TypePromotion
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{
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private:
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/// Need a helper-struct to fight the lack of the function-template partial specialization.
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template <class T, bool is_const, bool is_ref = std::is_reference_v<T>>
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struct CastHelper;
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template <class T>
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struct CastHelper<T, false, true>
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{
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auto & value(Base * ptr) { return typeid_cast<T>(*ptr); }
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};
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template <class T>
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struct CastHelper<T, true, true>
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{
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auto & value(const Base * ptr) { return typeid_cast<std::add_lvalue_reference_t<std::add_const_t<std::remove_reference_t<T>>>>(*ptr); }
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};
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template <class T>
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struct CastHelper<T, false, false>
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{
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auto * value(Base * ptr) { return typeid_cast<T *>(ptr); }
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};
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template <class T>
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struct CastHelper<T, true, false>
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{
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auto * value(const Base * ptr) { return typeid_cast<std::add_const_t<T> *>(ptr); }
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};
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public:
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template <class Derived>
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auto as() -> std::invoke_result_t<decltype(&CastHelper<Derived, false>::value), CastHelper<Derived, false>, Base *>
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{
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// TODO: if we do downcast to base type, then just return |this|.
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return CastHelper<Derived, false>().value(static_cast<Base *>(this));
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}
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template <class Derived>
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auto as() const -> std::invoke_result_t<decltype(&CastHelper<Derived, true>::value), CastHelper<Derived, true>, const Base *>
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{
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// TODO: if we do downcast to base type, then just return |this|.
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return CastHelper<Derived, true>().value(static_cast<const Base *>(this));
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}
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};
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}
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