Factory.hpp

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/*
 * Copyright (C) 2010-2026 The ESPResSo project
 * Copyright (C) 2002,2003,2004,2005,2006,2007,2008,2009,2010
 *   Max-Planck-Institute for Polymer Research, Theory Group
 *
 * This file is part of ESPResSo.
 *
 * ESPResSo is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * ESPResSo 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
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
#pragma once
 
#include <cassert>
#include <memory>
#include <stdexcept>
#include <string>
#include <typeindex>
#include <unordered_map>
 
namespace Utils {
 
/**
 * @brief Factory template.
 *
 * Can be used to construct registered instances of classes derived
 * from the base type (`Base`) by name.
 * Only one registry per base type (`Base`). To get a new one,
 * use a new type ( `struct Derived : public Base {};` ).
 * To add a new type it has to be given a name an a function of type
 * `%Factory<Base>::%builder_type` to create an instance has to be provided.
 * The class contains a default implementation for the creation
 * function (`%Factory<Base>::%builder<Derived>`) which just calls
 * new to create an instance. A user provided function could
 * be used to use a non-default constructor, or to allocate memory
 * for the instance in a specific way, e.g. by placing all new instances
 * in a vector.
 *
 * Example usage:
 * @code{.cpp}
 *     struct A {};
 *     struct B : public A {};
 *     struct C : public A {
 *       C(int c) : m_c(c) {}
 *       int m_c;
 *     };
 *
 *     // Register B as 'b' with default builder:
 *     Factory<A>::register_new<B>("b");
 *     // Register C as 'c' with user_defined builder:
 *     Factory<A>::register_new("c", []() -> typename Factory<A>::pointer_type {
 *         return new C(5); });
 *
 *     // Create a B
 *     auto b = Factory<A>::make("b");
 *     assert(dynamic_cast<B *>(b.get()));
 *
 *     // Create a C
 *     auto c = Factory<A>::make("c");
 *     assert(dynamic_cast<C *>(c.get())->m_c == 5);
 * @endcode
 */
template <class Base> class Factory {
public:
  /** The returned pointer type */
  using pointer_type = std::unique_ptr<Base>;
  /** Type of the constructor functions */
  using builder_type = pointer_type (*)();
 
public:
  /**
   * @brief Construct an instance by name.
   */
  pointer_type make(std::string const &name) const {
    try {
      auto builder = m_map.at(name);
      return assert(builder), builder();
    } catch (std::out_of_range const &) {
      throw std::domain_error("Class '" + name + "' not found.");
    }
  }
 
  /**
   * @brief Check if the factory knows how to make `name`.
   *
   * @param name Given name to check.
   * @return Whether we know how to make a `name`.
   */
  bool has_builder(std::string const &name) const {
    return m_map.contains(name);
  }
 
  /**
   * @brief Register a new type with the default construction function.
   *
   * @param name Given name for the type, must be unique.
   */
  template <typename Derived> void register_new(std::string const &name) {
    if (m_map.contains(name)) {
      throw std::runtime_error("Name '" + name + "' was already registered.");
    }
    m_map[name] = []() { return pointer_type(new Derived()); };
    m_type_map[typeid(Derived)] = name;
  }
 
  /**
   * @brief Look up name for type.
   *
   * For an object whose type can be created by
   * the factory this returns the name under which
   * it is registered.  This will consider the
   * dynamic type of polymorphic objects, e.g. it
   * will return the name of the most derived type.
   *
   * @param o Object whose type is to be considered.
   * @throw  std::out_of_range If the type is not registered.
   *
   * @return Name by which `Base` can be made.
   */
  auto const &type_name(Base const &o) const {
    return m_type_map.at(typeid(o));
  }
 
private:
  /** Maps names to construction functions. */
  std::unordered_map<std::string, builder_type> m_map;
  /** Maps types to names */
  std::unordered_map<std::type_index, std::string> m_type_map;
};
} /* namespace Utils */