126 lines
4.8 KiB
C++
126 lines
4.8 KiB
C++
/*
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BOOL-CHECKABLE.hpp - mixin template for defining a safe conversion to bool
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Copyright (C) Lumiera.org
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2009, Hermann Vosseler <Ichthyostega@web.de>
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This program is free software; you can redistribute it and/or
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modify it under the terms of the GNU General Public License as
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published by the Free Software Foundation; either version 2 of the
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License, or (at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, write to the Free Software
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Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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*/
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/** @file bool-checkable.hpp
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** When working with generic function objects and function pointers typed to
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** arbitrary signatures, often there is the necessity to hold onto such a functor
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** while hiding the actual signature behind an common interface ("type erasure").
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** The usual solution based on subclassing has the downside of requiring separate
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** storage for the concrete functor object, which might become problematic when
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** dealing with lots of functor objects.
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**
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** Especially when dealing with tr1::function objects, all of the type differences
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** are actually encoded into 3 internal pointers, thus yielding the same size for
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** all various types of functors. Building on this observation, we can create an
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** common container object to store the varying functors inline, while hiding the
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** actual signature.
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**
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** There remains the problem of re-accessing the concrete functor later on. As
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** C++ has only rudimental introspection capabilities, we can only rely on the
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** usage context to provide the correct function signature; only when using a
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** virtual function for the re-access, we can perform at least a runtime-check.
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**
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** Thus there are various flavours for actually implementing this idea, and the
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** picking a suitable implementation depends largely on the context. Thus we
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** provide a common and expect the client code to pick an implementation policy.
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**
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** @see control::Mutation usage example
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** @see function-erasure-test.cpp
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**
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*/
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#ifndef LIB_BOOL_CHECKABLE_H
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#define LIB_BOOL_CHECKABLE_H
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namespace lib {
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namespace imp {
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struct Dummy {};
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}
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/*
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* Provide an implicit conversion to "bool".
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* Inherit (mix-in) from this template by providing the
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* concrete subclass type ("CRTP"). Additionally, to implement
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* the specific logic of this bool check, the subclass using this
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* template must provide a public function \c isValid()
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*
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* \par safe bool conversion
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* Generally speaking, a direct automatic conversion to bool would be
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* a dangerous feature, because bool is considered an "integral type" in C.
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* Thus, you'd not only get the ability to write very expressive checks
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* like \c if(object) -- you would also get automatic conversions to int
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* where you'd never expect them. In C++ there is a well-known idiom to
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* circumvent this problem. Unfortunately, this idiom isn't easy to
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* grasp, involving the convoluted syntax of member pointers.
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* So we try to hide away these details into a mixin.
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*
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* The idea this implementation is based on is the fact that a
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* pointer-to-member is "somewhat a pointer" but not \em really a pointer.
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* Similar to a pointer, a member pointer can be \em unbound, and this
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* is the property we exploit here. But implementation-wise, a member pointer
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* is a type information plus an offset into this type, and thus can't be
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* converted to an integral type. Thus, depending on the result of the
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* \c isValid() function, the conversion operator returns either a
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* bound or unbound member pointer.
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*
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* @see bool-checkable-test.cpp
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* @see control::Mutation usage example
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*/
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template< class T ///< the target type implementing \c isValid()
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, class PAR = imp::Dummy ///< optional parent for inheritance chain
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>
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struct BoolCheckable
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: PAR
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{
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typedef bool (T::*ValidityCheck)() const;
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typedef ValidityCheck _unspecified_bool_type;
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/** implicit conversion to "bool" */
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operator _unspecified_bool_type() const ///< never throws
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{
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ValidityCheck isValid (&T::isValid);
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T const& obj = static_cast<T const&> (*this);
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return (obj.*isValid)()? isValid : 0;
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}
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bool operator! () const ///< never throws
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{
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ValidityCheck isValid (&T::isValid);
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T const& obj = static_cast<T const&> (*this);
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return !(obj.*isValid)();
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}
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};
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} // namespace lib
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#endif
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