ll_template_cast.h
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- /**
- * @file ll_template_cast.h
- * @author Nat Goodspeed
- * @date 2009-11-21
- * @brief Define ll_template_cast function
- *
- * $LicenseInfo:firstyear=2009&license=viewergpl$
- *
- * Copyright (c) 2009-2010, Linden Research, Inc.
- *
- * Second Life Viewer Source Code
- * The source code in this file ("Source Code") is provided by Linden Lab
- * to you under the terms of the GNU General Public License, version 2.0
- * ("GPL"), unless you have obtained a separate licensing agreement
- * ("Other License"), formally executed by you and Linden Lab. Terms of
- * the GPL can be found in doc/GPL-license.txt in this distribution, or
- * online at http://secondlifegrid.net/programs/open_source/licensing/gplv2
- *
- * There are special exceptions to the terms and conditions of the GPL as
- * it is applied to this Source Code. View the full text of the exception
- * in the file doc/FLOSS-exception.txt in this software distribution, or
- * online at
- * http://secondlifegrid.net/programs/open_source/licensing/flossexception
- *
- * By copying, modifying or distributing this software, you acknowledge
- * that you have read and understood your obligations described above,
- * and agree to abide by those obligations.
- *
- * ALL LINDEN LAB SOURCE CODE IS PROVIDED "AS IS." LINDEN LAB MAKES NO
- * WARRANTIES, EXPRESS, IMPLIED OR OTHERWISE, REGARDING ITS ACCURACY,
- * COMPLETENESS OR PERFORMANCE.
- * $/LicenseInfo$
- */
- #if ! defined(LL_LL_TEMPLATE_CAST_H)
- #define LL_LL_TEMPLATE_CAST_H
- /**
- * Implementation for ll_template_cast() (q.v.).
- *
- * Default implementation: trying to cast two completely unrelated types
- * returns 0. Typically you'd specify T and U as pointer types, but in fact T
- * can be any type that can be initialized with 0.
- */
- template <typename T, typename U>
- struct ll_template_cast_impl
- {
- T operator()(U)
- {
- return 0;
- }
- };
- /**
- * ll_template_cast<T>(some_value) is for use in a template function when
- * some_value might be of arbitrary type, but you want to recognize type T
- * specially.
- *
- * It's designed for use with pointer types. Example:
- * @code
- * struct SpecialClass
- * {
- * void someMethod(const std::string&) const;
- * };
- *
- * template <class REALCLASS>
- * void somefunc(const REALCLASS& instance)
- * {
- * const SpecialClass* ptr = ll_template_cast<const SpecialClass*>(&instance);
- * if (ptr)
- * {
- * ptr->someMethod("Call method only available on SpecialClass");
- * }
- * }
- * @endcode
- *
- * Why is this better than dynamic_cast<>? Because unless OtherClass is
- * polymorphic, the following won't even compile (gcc 4.0.1):
- * @code
- * OtherClass other;
- * SpecialClass* ptr = dynamic_cast<SpecialClass*>(&other);
- * @endcode
- * to say nothing of this:
- * @code
- * void function(int);
- * SpecialClass* ptr = dynamic_cast<SpecialClass*>(&function);
- * @endcode
- * ll_template_cast handles these kinds of cases by returning 0.
- */
- template <typename T, typename U>
- T ll_template_cast(U value)
- {
- return ll_template_cast_impl<T, U>()(value);
- }
- /**
- * Implementation for ll_template_cast() (q.v.).
- *
- * Implementation for identical types: return same value.
- */
- template <typename T>
- struct ll_template_cast_impl<T, T>
- {
- T operator()(T value)
- {
- return value;
- }
- };
- /**
- * LL_TEMPLATE_CONVERTIBLE(dest, source) asserts that, for a value @c s of
- * type @c source, <tt>ll_template_cast<dest>(s)</tt> will return @c s --
- * presuming that @c source can be converted to @c dest by the normal rules of
- * C++.
- *
- * By default, <tt>ll_template_cast<dest>(s)</tt> will return 0 unless @c s's
- * type is literally identical to @c dest. (This is because of the
- * straightforward application of template specialization rules.) That can
- * lead to surprising results, e.g.:
- *
- * @code
- * Foo myFoo;
- * const Foo* fooptr = ll_template_cast<const Foo*>(&myFoo);
- * @endcode
- *
- * Here @c fooptr will be 0 because <tt>&myFoo</tt> is of type <tt>Foo*</tt>
- * -- @em not <tt>const Foo*</tt>. (Declaring <tt>const Foo myFoo;</tt> would
- * force the compiler to do the right thing.)
- *
- * More disappointingly:
- * @code
- * struct Base {};
- * struct Subclass: public Base {};
- * Subclass object;
- * Base* ptr = ll_template_cast<Base*>(&object);
- * @endcode
- *
- * Here @c ptr will be 0 because <tt>&object</tt> is of type
- * <tt>Subclass*</tt> rather than <tt>Base*</tt>. We @em want this cast to
- * succeed, but without our help ll_template_cast can't recognize it.
- *
- * The following would suffice:
- * @code
- * LL_TEMPLATE_CONVERTIBLE(Base*, Subclass*);
- * ...
- * Base* ptr = ll_template_cast<Base*>(&object);
- * @endcode
- *
- * However, as noted earlier, this is easily fooled:
- * @code
- * const Base* ptr = ll_template_cast<const Base*>(&object);
- * @endcode
- * would still produce 0 because we haven't yet seen:
- * @code
- * LL_TEMPLATE_CONVERTIBLE(const Base*, Subclass*);
- * @endcode
- *
- * @TODO
- * This macro should use Boost type_traits facilities for stripping and
- * re-adding @c const and @c volatile qualifiers so that invoking
- * LL_TEMPLATE_CONVERTIBLE(dest, source) will automatically generate all
- * permitted permutations. It's really not fair to the coder to require
- * separate:
- * @code
- * LL_TEMPLATE_CONVERTIBLE(Base*, Subclass*);
- * LL_TEMPLATE_CONVERTIBLE(const Base*, Subclass*);
- * LL_TEMPLATE_CONVERTIBLE(const Base*, const Subclass*);
- * @endcode
- *
- * (Naturally we omit <tt>LL_TEMPLATE_CONVERTIBLE(Base*, const Subclass*)</tt>
- * because that's not permitted by normal C++ assignment anyway.)
- */
- #define LL_TEMPLATE_CONVERTIBLE(DEST, SOURCE)
- template <>
- struct ll_template_cast_impl<DEST, SOURCE>
- {
- DEST operator()(SOURCE wrapper)
- {
- return wrapper;
- }
- }
- #endif /* ! defined(LL_LL_TEMPLATE_CAST_H) */