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/**
* Copyright (C) 2018-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
/**
* This header describes a mechanism for making "decorable" types.
*
* A decorable type is one to which various subsystems may attach subsystem-private data, so long as
* they declare what that data will be before any instances of the decorable type are created.
*
* For example, suppose you had a class Client, representing on a server a network connection to a
* client process. Suppose that your server has an authentication module, that attaches data to the
* client about authentication. If class Client looks something like this:
*
* class Client : public Decorable<Client>{
* ...
* };
*
* Then the authentication module, before the first client object is created, calls
*
* const auto authDataDescriptor = Client::declareDecoration<AuthenticationPrivateData>();
*
* And stores authDataDescriptor in a module-global variable,
*
* And later, when it has a Client object, client, and wants to get at the per-client
* AuthenticationPrivateData object, it calls
*
* authDataDescriptor(client)
*
* to get a reference to the AuthenticationPrivateData for that client object.
*
* With this approach, individual subsystems get to privately augment the client object via
* declarations local to the subsystem, rather than in the global client header.
*/
#pragma once
#include "mongo/util/decoration_container.h"
#include "mongo/util/decoration_registry.h"
#include <type_traits>
namespace mongo {
template <typename D>
class Decorable {
private:
struct DecorationConstructionTag {};
public:
template <typename T>
class Decoration {
public:
Decoration() = delete;
Decoration(DecorationConstructionTag ignored,
typename DecorationContainer<D>::template DecorationDescriptorWithType<T> raw)
: _raw(std::move(raw)) {}
T& operator()(D& d) const {
return static_cast<Decorable&>(d)._decorations.getDecoration(this->_raw);
}
T& operator()(D* const d) const {
return (*this)(*d);
}
const T& operator()(const D& d) const {
return static_cast<const Decorable&>(d)._decorations.getDecoration(this->_raw);
}
const T& operator()(const D* const d) const {
return (*this)(*d);
}
const D* owner(const T* const t) const {
return static_cast<const D*>(getOwnerImpl(t));
}
D* owner(T* const t) const {
return static_cast<D*>(getOwnerImpl(t));
}
const D& owner(const T& t) const {
return *owner(&t);
}
D& owner(T& t) const {
return *owner(&t);
}
private:
const Decorable* getOwnerImpl(const T* const t) const {
return *reinterpret_cast<const Decorable* const*>(
reinterpret_cast<const unsigned char* const>(t) - _raw._raw._index);
}
Decorable* getOwnerImpl(T* const t) const {
return const_cast<Decorable*>(getOwnerImpl(const_cast<const T*>(t)));
}
typename DecorationContainer<D>::template DecorationDescriptorWithType<T> _raw;
};
template <typename T>
static Decoration<T> declareDecoration() {
if constexpr (std::is_copy_constructible_v<D> || std::is_copy_assignable_v<D>) {
return {DecorationConstructionTag{},
getRegistry()->template declareDecorationCopyable<T>()};
} else {
return {DecorationConstructionTag{}, getRegistry()->template declareDecoration<T>()};
}
}
protected:
Decorable() : _decorations(this, getRegistry()) {}
~Decorable() = default;
Decorable(const Decorable& other) : _decorations(this, getRegistry(), other._decorations) {}
Decorable& operator=(const Decorable& rhs) {
getRegistry()->copyAssign(&_decorations, &rhs._decorations);
return *this;
}
private:
static DecorationRegistry<D>* getRegistry() {
static DecorationRegistry<D>* theRegistry = new DecorationRegistry<D>();
return theRegistry;
}
DecorationContainer<D> _decorations;
};
} // namespace mongo
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