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Swappable Engine Factory, I think a better variation to the Factory Pattern
#pragma once
/*
Swappable Engine Factory, I think a better variation to the
Factory Pattern
Usage:
using namespace car_factory;
auto car_1 =
assembly_line(
engine_tag::old );
car_1.start();
*/
#include <utility>
namespace car_factory {
// interface to swapable engines
enum class engine_tag : char { old = 'O', next = 'N' };
namespace engine_workshop { // hidden from clients
// interface to swapable engines
struct IEngine { virtual engine_tag start() = 0; };
// switchable engines
struct old : public IEngine { virtual engine_tag start() override { return engine_tag::old; } };
struct next : public IEngine { virtual engine_tag start() override { return engine_tag::next; } };
} // inner
// facade of the solution
// this is not ABC
class Automobile final {
using IEngine = typename engine_workshop::IEngine ;
// we use the simple native pointer
// thus swap idiom is very easy to implement
mutable IEngine * engine_{};
// this ctor is visible to the factory method
// we do not want this as a "converting constructor"
// ditto we declare it as explicit
explicit Automobile( IEngine * use_) : engine_(use_) {}
// no default ctor
// no nullptr engine
Automobile() = delete;
// the factory function declaration
friend Automobile assembly_line(engine_tag);
public:
// delegating to the engine
// inheritance is evil
engine_tag start() const { return engine_->start(); }
// it is easy to manage the pointer lifetime
~Automobile() { delete engine_; }
/*
required move semantics
NOTE: we can not allow for move or copy assignment
since this will effectively and quietly
change the engine "mid flight"
*/
// no copy
Automobile(const Automobile&) = delete;
Automobile& operator=(const Automobile&) = delete;
// move
Automobile(Automobile&& other_) noexcept { std::swap(this->engine_, other_.engine_); }
Automobile& operator=(Automobile&& other_) = delete; // noexcept { std::swap(this->engine_, other_.engine_); return *this; }
};
// factory inserts the engine on production line
// we do not return ABC so we enjoy the value semantics
inline Automobile assembly_line (engine_tag which_)
{
// I assume I do not need std::move in here
// I could also use std::enable_if and std::is_move_constructible
// in return value type to check the Automobile type movability
// or I could just leave it to the CL
using namespace engine_workshop;
if (engine_tag::next == which_ )
return Automobile(new next{});
if (engine_tag::old == which_)
return Automobile(new old{});
throw std::runtime_error{ __FUNCSIG__ " Unknown engine_tag ordered?"};
}
}
/*
Copyright 2018 by dbj@dbj.org
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
http ://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.
*/
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