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/**
* @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) */
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