1#pragma once
2
3#include <Common/typeid_cast.h>
4
5namespace DB
6{
7
8/* This base class adds public methods:
9 * - Derived * as<Derived>()
10 * - const Derived * as<Derived>() const
11 * - Derived & as<Derived &>()
12 * - const Derived & as<Derived &>() const
13 */
14
15template <class Base>
16class TypePromotion
17{
18private:
19 /// Need a helper-struct to fight the lack of the function-template partial specialization.
20 template <class T, bool is_const, bool is_ref = std::is_reference_v<T>>
21 struct CastHelper;
22
23 template <class T>
24 struct CastHelper<T, false, true>
25 {
26 auto & value(Base * ptr) { return typeid_cast<T>(*ptr); }
27 };
28
29 template <class T>
30 struct CastHelper<T, true, true>
31 {
32 auto & value(const Base * ptr) { return typeid_cast<std::add_lvalue_reference_t<std::add_const_t<std::remove_reference_t<T>>>>(*ptr); }
33 };
34
35 template <class T>
36 struct CastHelper<T, false, false>
37 {
38 auto * value(Base * ptr) { return typeid_cast<T *>(ptr); }
39 };
40
41 template <class T>
42 struct CastHelper<T, true, false>
43 {
44 auto * value(const Base * ptr) { return typeid_cast<std::add_const_t<T> *>(ptr); }
45 };
46
47public:
48 template <class Derived>
49 auto as() -> std::invoke_result_t<decltype(&CastHelper<Derived, false>::value), CastHelper<Derived, false>, Base *>
50 {
51 // TODO: if we do downcast to base type, then just return |this|.
52 return CastHelper<Derived, false>().value(static_cast<Base *>(this));
53 }
54
55 template <class Derived>
56 auto as() const -> std::invoke_result_t<decltype(&CastHelper<Derived, true>::value), CastHelper<Derived, true>, const Base *>
57 {
58 // TODO: if we do downcast to base type, then just return |this|.
59 return CastHelper<Derived, true>().value(static_cast<const Base *>(this));
60 }
61};
62
63}
64