torch/include/c10/util/FunctionRef.h

//===- llvm/ADT/STLExtras.h - Useful STL related functions ------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file contains some templates that are useful if you are working with the
// STL at all.
//
// No library is required when using these functions.
//
//===----------------------------------------------------------------------===//

// c10: modified from llvm::function_ref
// c10: added more SFINAE to enable use in overloaded functions

#pragma once

#include <cstdint>
#include <type_traits>
#include <utility>

namespace c10 {

/// An efficient, type-erasing, non-owning reference to a callable. This is
/// intended for use as the type of a function parameter that is not used
/// after the function in question returns.
///
/// This class does not own the callable, so it is not in general safe to store
/// a function_ref.
template <typename Fn>
class function_ref;

template <typename Ret, typename... Params>
class function_ref<Ret(Params...)> {
  Ret (*callback)(intptr_t callable, Params... params) = nullptr;
  intptr_t callable;

  template <typename Callable>
  static Ret callback_fn(intptr_t callable, Params... params) {
    return (*reinterpret_cast<Callable*>(callable))(std::forward<Params>(
        params)...);
  }

 public:
  function_ref() = default;
  function_ref(std::nullptr_t) {}

  template <typename Callable>
  function_ref(
      Callable&& callable,
      typename std::enable_if<!std::is_same<
          typename std::remove_reference<Callable>::type,
          function_ref>::value>::type* = nullptr,
      typename std::enable_if<std::is_convertible<
          typename c10::invoke_result_t<Callable, Params...>,
          Ret>::value>::type* = nullptr)
      : callback(callback_fn<typename std::remove_reference<Callable>::type>),
        callable(reinterpret_cast<intptr_t>(&callable)) {}

  Ret operator()(Params... params) const {
    return callback(callable, std::forward<Params>(params)...);
  }

  operator bool() const {
    return callback;
  }
};

} // namespace c10