audacity/src/AudacityException.h

#ifndef __AUDACITY_EXCEPTION__
#define __AUDACITY_EXCEPTION__

/**********************************************************************

 Audacity: A Digital Audio Editor

 AudacityException.h

 Paul Licameli
 
 Define the root of a hierarchy of classes that are thrown and caught
 by Audacity.
 
 Define some subclasses.  Not all subclasses need be defined here.

 **********************************************************************/

#include "MemoryX.h"
#include <wx/app.h>
#include <exception>

#include "Internat.h"

class wxString;

class AudacityException /* not final */
{
public:
   AudacityException() {}
   virtual ~AudacityException() = 0;

   // Action to do in the main thread at idle time of the event loop.
   virtual void DelayedHandlerAction() = 0;

protected:
   // Make this protected to prevent slicing copies
   AudacityException( AudacityException&& ) {}
   AudacityException( const AudacityException& ) = default;
   AudacityException &operator = (AudacityException &&) { return *this;}
   AudacityException &operator = ( const AudacityException & ) PROHIBITED;
};

/// \brief A subclass of AudacityException whose delayed handler action displays
/// a message box.  The message is specified by further subclasses.
/// Not more than one message box will be displayed for each pass through
/// the main event idle loop.
class MessageBoxException /* not final */ : public AudacityException
{
   // Do not allow subclasses to change this behavior further, except
   // by overriding ErrorMessage()
   using AudacityException::DelayedHandlerAction;
   void DelayedHandlerAction() override;

protected:
   // If default-constructed with empty caption, it makes no message box.
   explicit MessageBoxException( const wxString &caption = wxString{} );
   ~MessageBoxException() override;

   MessageBoxException( const MessageBoxException& );
   MessageBoxException &operator = ( MessageBoxException && );

   // Format a default, internationalized error message for this exception.
   virtual wxString ErrorMessage() const = 0;

private:
   wxString caption;
   mutable bool moved { false };
};

/// \brief A MessageBoxException that shows a given, unvarying string.
class SimpleMessageBoxException /* not final */ : public MessageBoxException
{
public:
   explicit SimpleMessageBoxException( const wxString &message_,
      const wxString &caption = _("Message") )
      : MessageBoxException{ caption }
      , message{ message_ }
   {}
   ~SimpleMessageBoxException() override;

   SimpleMessageBoxException( const SimpleMessageBoxException& ) = default;
   SimpleMessageBoxException &operator = (
      SimpleMessageBoxException && ) PROHIBITED;

   // Format a default, internationalized error message for this exception.
   virtual wxString ErrorMessage() const override;

private:
   wxString message;
};


/// \brief performs the delayed configured action, when invoked.
struct DefaultDelayedHandlerAction
{
   void operator () (AudacityException *pException) const
   {
      if ( pException )
         pException->DelayedHandlerAction();
   }
};

/// \brief SimpleGuard classes add the second argument of GuardedCall:
/// Frequently useful converter of all exceptions to some failure constant
template <typename R> struct SimpleGuard
{
   explicit SimpleGuard( R value ) : m_value{ value } {}
   R operator () ( AudacityException * ) const { return m_value; }
   const R m_value;
};

/// \brief SimpleGuard specialization that returns bool, and defines Default
template<> struct SimpleGuard<bool>
{
   explicit SimpleGuard( bool value ) : m_value{ value } {}
   bool operator () ( AudacityException * ) const { return m_value; }
   static SimpleGuard Default()
      { return SimpleGuard{ false }; }
   const bool m_value;
};

/// \brief SimpleGuard specialization that returns nothing, and defines Default
template<> struct SimpleGuard<void>
{
   SimpleGuard() {}
   void operator () ( AudacityException * ) const {}
   static SimpleGuard Default() { return {}; }
};

template < typename R >
SimpleGuard< R > MakeSimpleGuard( R value )
{ return SimpleGuard< R >{ value }; }

inline SimpleGuard< void > MakeSimpleGuard() { return {}; }

/***
  \brief GuardedCall performs a body action, and provided there is no 
  exception a handler action on completion.  If there is an exception, 
  it queues up the delayed handler action for execution later in 
  the idle event loop

  The template is rather configurable, and default behaviours can be 
  overridden.  GuardedCall makes use of SimpleGuard for the handler action.

  GuardedCall calls the body function (usually a lambda) inside a try block.
 
  The handler intercepts exceptions, and is passed nullptr if the
  exception is of a type not defined by Audacity.  It may return a value
  for the guarded call or throw the same or another exception.
  The handler executes in the same thread as the body.
 
  If the handler is passed non-null and does not throw, then delayedHandler
  executes later in the main thread, in idle time of the event loop.
 */
template <
   typename R = void, // return type

   typename F1, // function object with signature R()

   typename F2 = SimpleGuard< R >, // function object
      // with signature R( AudacityException * )

   typename F3 =
      DefaultDelayedHandlerAction // Any( AudacityException * ), ignore return
>
R GuardedCall
   ( const F1 &body,
     const F2 &handler = F2::Default(),
     const F3 &delayedHandler = {} )
{
   try { return body(); }
   catch ( AudacityException &e ) {

      auto end = finally([&]{
         // At this point, e is the "current" exception, but not "uncaught"
         // unless it was rethrown by handler.  handler might also throw some
         // other exception object.
         if (!std::uncaught_exception()) {
            auto pException = std::current_exception(); // This points to e
            wxTheApp->CallAfter( [=] { // capture pException by value
               try { std::rethrow_exception(pException); }
               catch( AudacityException &e )
                  { delayedHandler( &e ); }
            } );
         }
      });

      return handler( &e );
   }
   catch ( ... ) {
      return handler( nullptr );
   }
}

#endif