cc17.h

/*
From PyTorch:

Copyright (c) 2016-     Facebook, Inc            (Adam Paszke)
Copyright (c) 2014-     Facebook, Inc            (Soumith Chintala)
Copyright (c) 2011-2014 Idiap Research Institute (Ronan Collobert)
Copyright (c) 2012-2014 Deepmind Technologies    (Koray Kavukcuoglu)
Copyright (c) 2011-2012 NEC Laboratories America (Koray Kavukcuoglu)
Copyright (c) 2011-2013 NYU                      (Clement Farabet)
Copyright (c) 2006-2010 NEC Laboratories America (Ronan Collobert, Leon Bottou, Iain Melvin, Jason Weston)
Copyright (c) 2006      Idiap Research Institute (Samy Bengio)
Copyright (c) 2001-2004 Idiap Research Institute (Ronan Collobert, Samy Bengio, Johnny Mariethoz)

From Caffe2:

Copyright (c) 2016-present, Facebook Inc. All rights reserved.

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Copyright (c) 2015 Google Inc.
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Copyright (c) 2015 Yangqing Jia
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All contributions from Caffe:
Copyright(c) 2013, 2014, 2015, the respective contributors
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All other contributions:
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*/
#pragma once
#include <type_traits>
#include <utility>

namespace tv {
  
#ifdef __cpp_lib_void_t

template <class T> using void_t = std::void_t<T>;

#else

// Implementation taken from http://en.cppreference.com/w/cpp/types/void_t
// (it takes CWG1558 into account and also works for older compilers)
template <typename... Ts> struct make_void { typedef void type; };
template <typename... Ts> using void_t = typename make_void<Ts...>::type;

#endif

namespace detail {
struct _identity final {
  template <class T> using type_identity = T;

  template <class T> decltype(auto) operator()(T &&arg) {
    return std::forward<T>(arg);
  }
};
template<class Func, class Enable = void>
struct function_takes_identity_argument : std::false_type {};
#if defined(_MSC_VER)
// For some weird reason, MSVC shows a compiler error when using guts::void_t instead of std::void_t.
// But we're only building on MSVC versions that have std::void_t, so let's just use that one.
template<class Func>
struct function_takes_identity_argument<Func, std::void_t<decltype(std::declval<Func>()(_identity()))>> : std::true_type {};
#else
template<class Func>
struct function_takes_identity_argument<Func, void_t<decltype(std::declval<Func>()(_identity()))>> : std::true_type {};
#endif

template<bool Condition>
struct _if_constexpr;

template<>
struct _if_constexpr<true> final {
  template<class ThenCallback, class ElseCallback, std::enable_if_t<function_takes_identity_argument<ThenCallback>::value, void*> = nullptr>
  static decltype(auto) call(ThenCallback&& thenCallback, ElseCallback&& /* elseCallback */) {
    // The _identity instance passed in can be used to delay evaluation of an expression,
    // because the compiler can't know that it's just the identity we're passing in.
    return thenCallback(_identity());
  }

  template<class ThenCallback, class ElseCallback, std::enable_if_t<!function_takes_identity_argument<ThenCallback>::value, void*> = nullptr>
  static decltype(auto) call(ThenCallback&& thenCallback, ElseCallback&& /* elseCallback */) {
    return thenCallback();
  }
};

template<>
struct _if_constexpr<false> final {
  template<class ThenCallback, class ElseCallback, std::enable_if_t<function_takes_identity_argument<ElseCallback>::value, void*> = nullptr>
  static decltype(auto) call(ThenCallback&& /* thenCallback */, ElseCallback&& elseCallback) {
    // The _identity instance passed in can be used to delay evaluation of an expression,
    // because the compiler can't know that it's just the identity we're passing in.
    return elseCallback(_identity());
  }

  template<class ThenCallback, class ElseCallback, std::enable_if_t<!function_takes_identity_argument<ElseCallback>::value, void*> = nullptr>
  static decltype(auto) call(ThenCallback&& /* thenCallback */, ElseCallback&& elseCallback) {
    return elseCallback();
  }
};
} // namespace detail

/*
 * Get something like C++17 if constexpr in C++14.
 *
 * Example 1: simple constexpr if/then/else
 *   template<int arg> int increment_absolute_value() {
 *     int result = arg;
 *     if_constexpr<(arg > 0)>(
 *       [&] { ++result; }  // then-case
 *       [&] { --result; }  // else-case
 *     );
 *     return result;
 *   }
 *
 * Example 2: without else case (i.e. conditionally prune code from assembly)
 *   template<int arg> int decrement_if_positive() {
 *     int result = arg;
 *     if_constexpr<(arg > 0)>(
 *       // This decrement operation is only present in the assembly for
 *       // template instances with arg > 0.
 *       [&] { --result; }
 *     );
 *     return result;
 *   }
 *
 * Example 3: branch based on type (i.e. replacement for SFINAE)
 *   struct MyClass1 {int value;};
 *   struct MyClass2 {int val};
 *   template <class T>
 *   int func(T t) {
 *     return if_constexpr<std::is_same<T, MyClass1>::value>(
 *       [&](auto _) { return _(t).value; }, // this code is invalid for T == MyClass2, so a regular non-constexpr if statement wouldn't compile
 *       [&](auto _) { return _(t).val; }    // this code is invalid for T == MyClass1
 *     );
 *   }
 *
 * Note: The _ argument passed in Example 3 is the identity function, i.e. it does nothing.
 *       It is used to force the compiler to delay type checking, because the compiler
 *       doesn't know what kind of _ is passed in. Without it, the compiler would fail
 *       when you try to access t.value but the member doesn't exist.
 *
 * Note: In Example 3, both branches return int, so func() returns int. This is not necessary.
 *       If func() had a return type of "auto", then both branches could return different
 *       types, say func<MyClass1>() could return int and func<MyClass2>() could return string.
 */
template<bool Condition, class ThenCallback, class ElseCallback>
decltype(auto) if_constexpr(ThenCallback&& thenCallback, ElseCallback&& elseCallback) {
#if defined(__cpp_if_constexpr)
  // If we have C++17, just use it's "if constexpr" feature instead of wrapping it.
  // This will give us better error messages.
  if constexpr(Condition) {
    if constexpr (detail::function_takes_identity_argument<ThenCallback>::value) {
      return std::forward<ThenCallback>(thenCallback)(detail::_identity());
    } else {
      return std::forward<ThenCallback>(thenCallback)();
    }
  } else {
    if constexpr (detail::function_takes_identity_argument<ElseCallback>::value) {
      return std::forward<ElseCallback>(elseCallback)(detail::_identity());
    } else {
      return std::forward<ElseCallback>(elseCallback)();
    }
  }
#else
  // C++14 implementation of if constexpr
  return detail::_if_constexpr<Condition>::call(std::forward<ThenCallback>(thenCallback),
                                                 std::forward<ElseCallback>(elseCallback));
#endif
}

template<bool Condition, class ThenCallback>
decltype(auto) if_constexpr(ThenCallback&& thenCallback) {
#if defined(__cpp_if_constexpr)
  // If we have C++17, just use it's "if constexpr" feature instead of wrapping it.
  // This will give us better error messages.
  if constexpr(Condition) {
    if constexpr (detail::function_takes_identity_argument<ThenCallback>::value) {
      return std::forward<ThenCallback>(thenCallback)(detail::_identity());
    } else {
      return std::forward<ThenCallback>(thenCallback)();
    }
  }
#else
  // C++14 implementation of if constexpr
  return if_constexpr<Condition>(std::forward<ThenCallback>(thenCallback), [] (auto) {});
#endif
}


}