mirror of
https://github.com/boostorg/container_hash.git
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Daniel James
27e69dea72
Remove deprecated headers, move hash_fwd.hpp into hash subdirectory. And several minor internal changes. Mostly minor internal details. Merged revisions 51262-51263,51407-51409,51504-51505,51644-51646,51667 via svnmerge from https://svn.boost.org/svn/boost/trunk ........ r51262 | danieljames | 2009-02-15 19:32:04 +0000 (Sun, 15 Feb 2009) | 1 line Use the new 'boost:' links for the hash, unordered and quickbook documentation. ........ r51263 | danieljames | 2009-02-15 19:32:19 +0000 (Sun, 15 Feb 2009) | 2 lines Don't copy images for the standalone hash and unordered documentation, was only really required before the libraries were integrated into boost. ........ r51407 | danieljames | 2009-02-22 23:49:51 +0000 (Sun, 22 Feb 2009) | 1 line Fix the hash dirname. ........ r51408 | danieljames | 2009-02-22 23:50:04 +0000 (Sun, 22 Feb 2009) | 1 line Make copy_buckets and move_buckets member functions - so that calling them is a bit simpler. ........ r51409 | danieljames | 2009-02-22 23:50:20 +0000 (Sun, 22 Feb 2009) | 1 line Move some of the data structure classes out of hash table data. ........ r51504 | danieljames | 2009-03-01 14:15:09 +0000 (Sun, 01 Mar 2009) | 1 line Add missing return for operator=. ........ r51505 | danieljames | 2009-03-01 14:15:39 +0000 (Sun, 01 Mar 2009) | 3 lines Make the sort stable. Doesn't really matter, but it might as well be. ........ r51644 | danieljames | 2009-03-08 09:44:51 +0000 (Sun, 08 Mar 2009) | 1 line Detab. ........ r51645 | danieljames | 2009-03-08 09:45:11 +0000 (Sun, 08 Mar 2009) | 4 lines Move hash_fwd into the hash subdirectory. I should have done this in the last release. But now all of the hash implementation is in the hash subdirectory. ........ r51646 | danieljames | 2009-03-08 09:45:30 +0000 (Sun, 08 Mar 2009) | 3 lines Remove deprecated headers. Fixes #2412. ........ r51667 | danieljames | 2009-03-09 20:56:23 +0000 (Mon, 09 Mar 2009) | 1 line Update copyright dates in hash and unordered. ........ [SVN r51729]
106 lines
3.2 KiB
Plaintext
106 lines
3.2 KiB
Plaintext
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[/ Copyright 2005-2008 Daniel James.
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/ Distributed under the Boost Software License, Version 1.0. (See accompanying
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/ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) ]
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[section:portability Portability]
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[def __boost_hash__ [classref boost::hash]]
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__boost_hash__ is written to be as portable as possible, but unfortunately, several
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older compilers don't support argument dependent lookup (ADL) - the mechanism
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used for customisation. On those compilers custom overloads for `hash_value`
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needs to be declared in the boost namespace.
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On a strictly standards compliant compiler, an overload defined in the
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boost namespace won't be found when __boost_hash__ is instantiated,
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so for these compilers the overload should only be declared in the same
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namespace as the class.
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Let's say we have a simple custom type:
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namespace foo
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{
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template <class T>
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class custom_type
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{
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T value;
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public:
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custom_type(T x) : value(x) {}
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friend std::size_t hash_value(custom_type x)
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{
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__boost_hash__<int> hasher;
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return hasher(x.value);
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}
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};
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}
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On a compliant compiler, when `hash_value` is called for this type,
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it will look at the namespace inside the type and find `hash_value`
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but on a compiler which doesn't support ADL `hash_value` won't be found.
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To make things worse, some compilers which do support ADL won't find
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a friend class defined inside the class.
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So first move the member function out of the class:
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namespace foo
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{
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template <class T>
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class custom_type
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{
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T value;
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public:
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custom_type(T x) : value(x) {}
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std::size_t hash(custom_type x)
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{
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__boost_hash__<T> hasher;
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return hasher(value);
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}
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};
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template <class T>
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inline std::size_t hash_value(custom_type<T> x)
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{
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return x.hash();
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}
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}
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Unfortunately, I couldn't declare hash_value as a friend, as some compilers
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don't support template friends, so instead I declared a member function to
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calculate the hash, and called it from hash_value.
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For compilers which don't support ADL, hash_value needs to be defined in the
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boost namespace:
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#ifdef BOOST_NO_ARGUMENT_DEPENDENT_LOOKUP
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namespace boost
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#else
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namespace foo
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#endif
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{
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template <class T>
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std::size_t hash_value(foo::custom_type<T> x)
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{
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return x.hash();
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}
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}
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Full code for this example is at
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[@boost:/libs/functional/hash/examples/portable.cpp /libs/functional/hash/examples/portable.cpp].
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[h2 Other Issues]
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On Visual C++ versions 6.5 and 7.0, `hash_value` isn't overloaded for built in
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arrays. __boost_hash__, [funcref boost::hash_combine] and [funcref boost::hash_range] all use a workaround to
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support built in arrays so this shouldn't be a problem in most cases.
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On Visual C++ versions 6.5 and 7.0, function pointers aren't currently supported.
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When using GCC on Solaris, `boost::hash_value(long double)` treats
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`long double`s as `double`s - so the hash function doesn't take into account the
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full range of values.
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[endsect]
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