65816-llvm-mos/runtime/include/c++/etl/singleton_base.h

///\file

/******************************************************************************
The MIT License(MIT)

Embedded Template Library.
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https://www.etlcpp.com

Copyright(c) 2019 John Wellbelove
Copyright(c) 2024 BMW AG

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#ifndef ETL_SINGLETON_BASE_INCLUDED
#define ETL_SINGLETON_BASE_INCLUDED

///\defgroup singleton_base singleton_base
/// Templated version of the singleton pattern, implemented as base class
/// for singletons.
///\ingroup etl

#include "platform.h"
#include "error_handler.h"
#include "file_error_numbers.h"
#include "nullptr.h"

namespace etl
{
  //*************************************************************************
  /// Base singleton error exception.
  //*************************************************************************
  class singleton_base_exception : public etl::exception
  {
  public:

    singleton_base_exception(string_type reason_, string_type file_name_, numeric_type line_number_)
      : exception(reason_, file_name_, line_number_)
    {
    }
  };

  //*************************************************************************
  /// Singleton not created error exception.
  //*************************************************************************
  class singleton_base_not_created : public etl::singleton_base_exception
  {
  public:

    singleton_base_not_created(string_type file_name_, numeric_type line_number_)
      : singleton_base_exception(ETL_ERROR_TEXT("singleton_base:not created", ETL_SINGLETON_BASE_FILE_ID"A"), file_name_, line_number_)
    {
    }
  };

  //*************************************************************************
  /// Singleton instance already exists.
  //*************************************************************************
  class singleton_base_already_created : public etl::singleton_base_exception
  {
  public:

    singleton_base_already_created(string_type file_name_, numeric_type line_number_)
      : singleton_base_exception(ETL_ERROR_TEXT("singleton_base:already created", ETL_SINGLETON_BASE_FILE_ID"B"), file_name_, line_number_)
    {
    }
  };

  //***********************************************************************
  /// Base class for singletons.
  /// \tparam T   Any type that wants to expose the instance() interface.
  ///
  /// This class is designed to work as a generic base class for any class that
  /// wants to provide a singleton interface. It'll also work for classes that
  /// do not have a default constructor.
  ///
  /// Usage example:
  ///
  /// class Origin
  ///   : singleton<Origin>
  /// {
  /// public:
  ///     Origin(int x, int y)
  ///     :   singleton<Origin>(*this)
  ///     {}
  ///
  ///     int getX() const;
  /// } theOrigin(0, 0);
  ///
  /// int x = Origin::instance().getX();
  ///
  ///
  /// Note:
  ///
  /// It is important that a call to instance() will not create the instance of
  /// the class. It needs to be created by the user before calling instance().
  /// This way, the user has better control over the instance lifetime instead
  /// of e.g. lazy initialization.
  //***********************************************************************
  template <typename T>
  class singleton_base
  {
  public:

    //***********************************************************************
    // Returns a reference to the instance.
    //***********************************************************************
    static T& instance()
    {
      ETL_ASSERT(m_self != ETL_NULLPTR, ETL_ERROR(etl::singleton_base_not_created));

      return *m_self;
    }

    //***********************************************************************
    /// Returns whether an instance has been attached to singleton<T> or not.
    //***********************************************************************
    static bool is_valid()
    {
      return (m_self != ETL_NULLPTR);
    }

  protected:

    //***********************************************************************
    /// Constructs the instance of singleton.
    /// theInstance Reference to T, which will be returned when instance() is
    /// called.
    //***********************************************************************
    explicit singleton_base(T& theInstance)
    {
      ETL_ASSERT(m_self == ETL_NULLPTR, ETL_ERROR(etl::singleton_base_already_created));

      m_self = &theInstance;
    }

    //***********************************************************************
    /// Removes the internal reference to the instance passed in the
    /// constructor.
    //***********************************************************************
    ~singleton_base()
    {
      m_self = ETL_NULLPTR;
    }

  private:

    static T* m_self;
  };

  //***********************************************************************
  /// No violation of one definition rule as this is a class template
  //***********************************************************************
  template <class T>
  T* singleton_base<T>::m_self = ETL_NULLPTR;
} // namespace etl

#endif