llvm/llvm/include/llvm/ADT/FunctionExtras.h

//===- FunctionExtras.h - Function type erasure utilities -------*- 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
//
//===----------------------------------------------------------------------===//
/// \file
/// This file provides a collection of function (or more generally, callable)
/// type erasure utilities supplementing those provided by the standard library
/// in `<function>`.
///
/// It provides `unique_function`, which works like `std::function` but supports
/// move-only callable objects and const-qualification.
///
/// Future plans:
/// - Add a `function` that provides ref-qualified support, which doesn't work
///   with `std::function`.
/// - Provide support for specifying multiple signatures to type erase callable
///   objects with an overload set, such as those produced by generic lambdas.
/// - Expand to include a copyable utility that directly replaces std::function
///   but brings the above improvements.
///
/// Note that LLVM's utilities are greatly simplified by not supporting
/// allocators.
///
/// If the standard library ever begins to provide comparable facilities we can
/// consider switching to those.
///
//===----------------------------------------------------------------------===//

#ifndef LLVM_ADT_FUNCTIONEXTRAS_H
#define LLVM_ADT_FUNCTIONEXTRAS_H

#include "llvm/ADT/PointerIntPair.h"
#include "llvm/ADT/PointerUnion.h"
#include "llvm/ADT/STLForwardCompat.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/MemAlloc.h"
#include "llvm/Support/type_traits.h"
#include <cstring>
#include <memory>
#include <type_traits>

namespace llvm {

/// unique_function is a type-erasing functor similar to std::function.
///
/// It can hold move-only function objects, like lambdas capturing unique_ptrs.
/// Accordingly, it is movable but not copyable.
///
/// It supports const-qualification:
/// - unique_function<int() const> has a const operator().
///   It can only hold functions which themselves have a const operator().
/// - unique_function<int()> has a non-const operator().
///   It can hold functions with a non-const operator(), like mutable lambdas.
template <typename FunctionT> class unique_function;

namespace detail {

EnableIfTrivial;
EnableUnlessSameType;
EnableIfCallable;

template <typename ReturnT, typename... ParamTs> class UniqueFunctionBase { … };

template <typename R, typename... P>
template <typename CallableT, typename CalledAsT, typename Enable>
typename UniqueFunctionBase<R, P...>::NonTrivialCallbacks UniqueFunctionBase<
    R, P...>::CallbacksHolder<CallableT, CalledAsT, Enable>::Callbacks = …;

template <typename R, typename... P>
template <typename CallableT, typename CalledAsT>
typename UniqueFunctionBase<R, P...>::TrivialCallback
    UniqueFunctionBase<R, P...>::CallbacksHolder<
        CallableT, CalledAsT, EnableIfTrivial<CallableT>>::Callbacks{ … };

} // namespace detail

unique_function<R (P...)>;

unique_function<R (P...) const>;

} // end namespace llvm

#endif // LLVM_ADT_FUNCTIONEXTRAS_H