1 files changed, 115 insertions, 0 deletions
diff --git a/indra/llcommon/apply.h b/indra/llcommon/apply.h
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+++ b/indra/llcommon/apply.h
@@ -0,0 +1,115 @@
+/**
+ * @file   apply.h
+ * @author Nat Goodspeed
+ * @date   2022-06-18
+ * @brief  C++14 version of std::apply()
+ * 
+ * $LicenseInfo:firstyear=2022&license=viewerlgpl$
+ * Copyright (c) 2022, Linden Research, Inc.
+ * $/LicenseInfo$
+ */
+
+#if ! defined(LL_APPLY_H)
+#define LL_APPLY_H
+
+#include <boost/type_traits/function_traits.hpp>
+#include <tuple>
+
+namespace LL
+{
+
+/**
+ * USAGE NOTE:
+ * https://stackoverflow.com/a/40523474/5533635
+ *
+ * If you're trying to pass apply() a variadic function, the compiler
+ * complains that it can't deduce the callable type, presumably because it
+ * doesn't know which arity to reify to pass.
+ *
+ * But it works to wrap the variadic function in a generic lambda, e.g.:
+ *
+ * @CODE
+ * LL::apply(
+ *     [](auto&&... args)
+ *     {
+ *         return variadic(std::forward<decltype(args)>(args)...);
+ *     },
+ *     args);
+ * @ENDCODE
+ *
+ * Presumably this is because there's only one instance of the generic lambda
+ * @em type, with a variadic <tt>operator()()</tt>.
+ *
+ * It's pointless to provide a wrapper @em function that implicitly supplies
+ * the generic lambda. You couldn't pass your variadic function to our wrapper
+ * function, for the same original reason!
+ *
+ * Instead we provide a wrapper @em macro. Sorry, Dr. Stroustrup.
+ */
+#define VAPPLY(FUNC, ARGS)                                          \
+    LL::apply(                                                      \
+        [](auto&&... args)                                          \
+        {                                                           \
+            return (FUNC)(std::forward<decltype(args)>(args)...);   \
+        },                                                          \
+        (ARGS))
+
+#if __cplusplus >= 201703L
+
+// C++17 implementation
+using std::apply;
+
+#else // C++14
+
+// Derived from https://stackoverflow.com/a/20441189
+// and https://en.cppreference.com/w/cpp/utility/apply
+template <typename CALLABLE, typename TUPLE, std::size_t... I>
+auto apply_impl(CALLABLE&& func, TUPLE&& args, std::index_sequence<I...>)
+{
+    // call func(unpacked args)
+    return std::forward<CALLABLE>(func)(std::move(std::get<I>(args))...);
+}
+
+template <typename CALLABLE, typename... ARGS>
+auto apply(CALLABLE&& func, const std::tuple<ARGS...>& args)
+{
+    // std::index_sequence_for is the magic sauce here, generating an argument
+    // pack of indexes for each entry in args. apply_impl() can then pass
+    // those to std::get() to unpack args into individual arguments.
+    return apply_impl(std::forward<CALLABLE>(func),
+                      args,
+                      std::index_sequence_for<ARGS...>{});
+}
+
+// per https://stackoverflow.com/a/57510428/5533635
+template <typename CALLABLE, typename T, size_t SIZE>
+auto apply(CALLABLE&& func, const std::array<T, SIZE>& args)
+{
+    return apply(std::forward<CALLABLE>(func), std::tuple_cat(args));
+}
+
+// per https://stackoverflow.com/a/28411055/5533635
+template <typename CALLABLE, typename T, std::size_t... I>
+auto apply_impl(CALLABLE&& func, const std::vector<T>& args, std::index_sequence<I...>)
+{
+    return apply_impl(std::forward<CALLABLE>(func),
+                      std::make_tuple(std::forward<T>(args[I])...),
+                      I...);
+}
+
+// this goes beyond C++17 std::apply()
+template <typename CALLABLE, typename T>
+auto apply(CALLABLE&& func, const std::vector<T>& args)
+{
+    constexpr auto arity = boost::function_traits<CALLABLE>::arity;
+    assert(args.size() == arity);
+    return apply_impl(std::forward<CALLABLE>(func),
+                      args,
+                      std::make_index_sequence<arity>());
+}
+
+#endif // C++14
+
+} // namespace LL
+
+#endif /* ! defined(LL_APPLY_H) */