chromium/base/functional/unretained_traits.h

// Copyright 2022 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_FUNCTIONAL_UNRETAINED_TRAITS_H_
#define BASE_FUNCTIONAL_UNRETAINED_TRAITS_H_

#include <concepts>
#include <type_traits>

#include "base/types/is_complete.h"
#include "build/build_config.h"

// Various opaque system types that should still be usable with the base
// callback system. Please keep sorted.
#define BASE_INTERNAL_LIST_OF_SAFE_FOR_UNRETAINED …

#define BASE_INTERNAL_SAFE_FOR_UNRETAINED …
BASE_INTERNAL_LIST_OF_SAFE_FOR_UNRETAINED
#undef BASE_INTERNAL_SAFE_FOR_UNRETAINED

namespace base::internal {

SameAsAny;

// Determining whether a type can be used with `Unretained()` requires that `T`
// be completely defined. Some system types have an intentionally opaque and
// incomplete representation, but should still be usable with `Unretained()`.
SafeIncompleteTypeForUnretained;

// Customization point. Specialize this to be `false` for types as needed. In
// general, you should not need this; types that do not support `Unretained()`
// should use `DISALLOW_UNRETAINED()`. However, this is necessary when
// disallowing `Unretained()` for types that do not (or cannot) use //base.
kCustomizeSupportsUnretained;

DisallowsUnretained;

template <typename T>
struct SupportsUnretainedImpl { … };

// Not meant for general use: merely checking this concept will
// `static_assert()` if the type does not support unretained. This is meant only
// for use inside the `Bind()` machinery, which wants that assertion.
//
// If we ever want to use this concept outside that machinery, we'll need to not
// only move the "allows unretained" assertion from above to the `Bind()` side,
// we'll also need to hoist or duplicate the incomplete type check there (and
// not assert in that case) so it does not fire multiple `static_assert()`s for
// incomplete types.
SupportsUnretained;

}  // namespace base::internal

#endif  // BASE_FUNCTIONAL_UNRETAINED_TRAITS_H_