intermediate changes

ref:cde9a383711a11544ce7e107a78147fb96cc4029

1 files changed, 76 insertions, 0 deletions

diff --git a/library/cpp/yt/misc/variant.h b/library/cpp/yt/misc/variant.h
new file mode 100644
index 00000000000..27c0a2bc086
--- /dev/null
+++ b/library/cpp/yt/misc/variant.h
@@ -0,0 +1,76 @@
+#pragma once
+
+#include <variant>
+
+namespace NYT {
+
+////////////////////////////////////////////////////////////////////////////////
+
+namespace NDetail {
+
+template <class T, class... Ts>
+struct TIndexOf;
+
+} // namespace NDetail
+
+template <class T, class V>
+struct TVariantIndex;
+
+template <class T, class... Ts>
+struct TVariantIndex<T, std::variant<Ts...>>
+    : std::integral_constant<size_t, NDetail::TIndexOf<T, Ts...>::Value>
+{ };
+
+template <class T, class V>
+constexpr size_t VariantIndexV = TVariantIndex<T, V>::value;
+
+////////////////////////////////////////////////////////////////////////////////
+
+class TStringBuilderBase;
+
+template <class... Ts>
+void FormatValue(TStringBuilderBase* builder, const std::variant<Ts...>& variant, TStringBuf spec);
+
+template <class... Ts>
+TString ToString(const std::variant<Ts...>& variant);
+
+////////////////////////////////////////////////////////////////////////////////
+
+//! A concise way of creating a functor with an overloaded operator().
+/*!
+ *  Very useful for std::visit-ing variants. For example:
+ *
+ *      std::visit(TOverloaded{
+ *          [] (int i)                { printf("The variant holds an int: %d!", i); },
+ *          [] (const std::string& s) { printf("The variant holds a string: '%s'!", s); }
+ *      }, variantVariable);
+ */
+template<class... Ts> struct TOverloaded : Ts... { using Ts::operator()...; };
+template<class... Ts> TOverloaded(Ts...) -> TOverloaded<Ts...>;
+
+////////////////////////////////////////////////////////////////////////////////
+
+//! An alternative to std::visit that takes its variant argument first.
+/*!
+ *  This deprives it of being able to visit a Cartesian product of variants but
+ *  in exchange allows to receive multiple visitor functors. All of operator()s
+ *  these functors have are used to visit the variant after a single unified
+ *  overload resolution. For example:
+ *
+ *      Visit(variantVariable,
+ *          [] (int i)                { printf("The variant holds an int: %d!", i); },
+ *          [] (const std::string& s) { printf("The variant holds a string: '%s'!", s); });
+ */
+template <class T, class... U>
+auto Visit(T&& variant, U&&... visitorOverloads)
+{
+    return std::visit(TOverloaded{std::forward<U>(visitorOverloads)...}, std::forward<T>(variant));
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+} // namespace NYT
+
+#define VARIANT_INL_H_
+#include "variant-inl.h"
+#undef VARIANT_INL_H_