chromium/third_party/abseil-cpp/absl/functional/function_ref.h

// Copyright 2019 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// -----------------------------------------------------------------------------
// File: function_ref.h
// -----------------------------------------------------------------------------
//
// This header file defines the `absl::FunctionRef` type for holding a
// non-owning reference to an object of any invocable type. This function
// reference is typically most useful as a type-erased argument type for
// accepting function types that neither take ownership nor copy the type; using
// the reference type in this case avoids a copy and an allocation. Best
// practices of other non-owning reference-like objects (such as
// `absl::string_view`) apply here.
//
//  An `absl::FunctionRef` is similar in usage to a `std::function` but has the
//  following differences:
//
//  * It doesn't own the underlying object.
//  * It doesn't have a null or empty state.
//  * It never performs deep copies or allocations.
//  * It's much faster and cheaper to construct.
//  * It's trivially copyable and destructable.
//
// Generally, `absl::FunctionRef` should not be used as a return value, data
// member, or to initialize a `std::function`. Such usages will often lead to
// problematic lifetime issues. Once you convert something to an
// `absl::FunctionRef` you cannot make a deep copy later.
//
// This class is suitable for use wherever a "const std::function<>&"
// would be used without making a copy. ForEach functions and other versions of
// the visitor pattern are a good example of when this class should be used.
//
// This class is trivial to copy and should be passed by value.
#ifndef ABSL_FUNCTIONAL_FUNCTION_REF_H_
#define ABSL_FUNCTIONAL_FUNCTION_REF_H_

#include <cassert>
#include <functional>
#include <type_traits>

#include "absl/base/attributes.h"
#include "absl/functional/internal/function_ref.h"
#include "absl/meta/type_traits.h"

namespace absl {
ABSL_NAMESPACE_BEGIN

// FunctionRef
//
// Dummy class declaration to allow the partial specialization based on function
// types below.
template <typename T>
class FunctionRef;

// FunctionRef
//
// An `absl::FunctionRef` is a lightweight wrapper to any invocable object with
// a compatible signature. Generally, an `absl::FunctionRef` should only be used
// as an argument type and should be preferred as an argument over a const
// reference to a `std::function`. `absl::FunctionRef` itself does not allocate,
// although the wrapped invocable may.
//
// Example:
//
//   // The following function takes a function callback by const reference
//   bool Visitor(const std::function<void(my_proto&,
//                                         absl::string_view)>& callback);
//
//   // Assuming that the function is not stored or otherwise copied, it can be
//   // replaced by an `absl::FunctionRef`:
//   bool Visitor(absl::FunctionRef<void(my_proto&, absl::string_view)>
//                  callback);
//
// Note: the assignment operator within an `absl::FunctionRef` is intentionally
// deleted to prevent misuse; because the `absl::FunctionRef` does not own the
// underlying type, assignment likely indicates misuse.
FunctionRef<R (Args...)>;

// Allow const qualified function signatures. Since FunctionRef requires
// constness anyway we can just make this a no-op.
FunctionRef<R (Args...) const>;

ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_FUNCTIONAL_FUNCTION_REF_H_