ClickHouse/src/Common/TypePromotion.h

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