forked from boostorg/utility
176 lines
7.6 KiB
C++
176 lines
7.6 KiB
C++
// (C) Copyright Jonathan Turkanis 2004.
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// Permission to copy, use, modify, sell and distribute this software
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// is granted provided this copyright notice appears in all copies. This
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// software is provided "as is" without express or implied warranty, and
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// with no claim as to its suitability for any purpose.
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//
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// Intended as an alternative to type_traits::yes_type and type_traits::no_type.
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// Provides an arbitrary number of types (case_<0>, case_<1>, ...) for
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// determining the results of overload resultion using 'sizeof', plus a uniform
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// means of using the result. yes_type and no_type are typedefs for case_<1>
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// and case_<0>. A single case with negative argument, case_<-1>, is also
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// provided, for convenience.
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//
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// This header may be included any number of times, with
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// BOOST_SELECT_BY_SIZE_MAX_CASE defined to be the largest N such that case_<N>
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// is needed for a particular application. It defaults to 2.
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//
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// This header depends only on Boost.Config and Boost.Preprocessor. Dependence
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// on Type Traits or MPL was intentionally avoided, to leave open the
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// possibility that select_by_size could be used by these libraries.
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//
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// Example usage:
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//
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// #define BOOST_SELECT_BY_SIZE_MAX_CASE 7 // Needed for > 2 cases.
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// #include <boost/utility/select_by_size.hpp>
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//
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// using namespace boost::utility;
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//
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// case_<0> helper(bool); // could use 'case_<false>' or 'no_type'.
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// case_<1> helper(int); // could use 'case_<true>' or' yes_type'.
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// case_<2> helper(unsigned);
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// case_<3> helper(long);
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// case_<4> helper(unsigned long);
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// case_<5> helper(float);
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// case_<6> helper(double);
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// case_<7> helper(const char*);
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//
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// struct test {
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// static const int value =
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// select_by_size< sizeof(helper(9876UL)) >::value;
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// BOOST_STATIC_ASSERT(value == 4);
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// };
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//
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// For compilers with integral constant expression problems, e.g. Borland 5.x,
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// one can also write
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//
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// struct test {
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// BOOST_SELECT_BY_SIZE(int, value, helper(9876UL));
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// };
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//
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// to define a static integral constant 'value' equal to
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//
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// select_by_size< sizeof(helper(9876UL)) >::value.
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//
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// Include guards surround all contents of this header except for explicit
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// specializations of select_by_size for case_<N> with N > 2.
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#ifndef BOOST_UTILITY_SELECT_BY_SIZE_HPP_INCLUDED
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#define BOOST_UTILITY_SELECT_BY_SIZE_HPP_INCLUDED
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// The lowest N for which select_by_size< sizeof(case_<N>) > has not been
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// specialized.
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#define SELECT_BY_SIZE_MAX_SPECIALIZED 2
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#include <boost/config.hpp> // BOOST_STATIC_CONSTANT.
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#include <boost/preprocessor/arithmetic/inc.hpp>
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#include <boost/preprocessor/cat.hpp>
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#include <boost/preprocessor/repetition/repeat_from_to.hpp>
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#include <boost/preprocessor/repetition/repeat_from_to.hpp>
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/* Alternative implementation using max_align.
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#include <boost/type_traits/alignment_of.hpp>
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#include <boost/type_traits/type_with_alignment.hpp>
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namespace boost { namespace utility {
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template<int N>
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struct case_ { char c[(N + 1) * alignment_of<detail::max_align>::value]; };
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template<unsigned Size>
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struct select_by_size {
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BOOST_STATIC_CONSTANT(int, value =
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(Size / alignment_of<detail::max_align>::value - 1));
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};
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} } // End namespaces utility, boost.
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*/ // End alternate implementation.
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namespace boost { namespace utility {
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//--------------Definition of case_-------------------------------------------//
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template<int N> struct case_ { char c1; case_<N - 1> c2; };
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template<> struct case_<-1> { char c; };
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typedef case_<true> yes_type;
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typedef case_<false> no_type;
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//--------------Declaration of select_by_size---------------------------------//
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template<unsigned Size> struct select_by_size;
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} } // End namespaces utility, boost.
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//--------------Definition of SELECT_BY_SIZE_SPEC-----------------------------//
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// Sepecializes select_by_size for sizeof(case<n-1>). The decrement is used
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// here because the preprocessor library doesn't handle negative integers.
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#define SELECT_BY_SIZE_SPEC(n) \
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namespace boost { namespace utility { \
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namespace detail { \
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static const int BOOST_PP_CAT(sizeof_case_, n) = sizeof(case_<n - 1>); \
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} \
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template<> \
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struct select_by_size< detail::BOOST_PP_CAT(sizeof_case_, n) > { \
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struct type { BOOST_STATIC_CONSTANT(int, value = n - 1); }; \
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BOOST_STATIC_CONSTANT(int, value = type::value); \
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}; \
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} } \
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/**/
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//--------------Default specializations of select_by_size---------------------//
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SELECT_BY_SIZE_SPEC(0) // select_by_size< sizeof(case<-1>) >
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SELECT_BY_SIZE_SPEC(1) // select_by_size< sizeof(case<0>) >
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SELECT_BY_SIZE_SPEC(2) // select_by_size< sizeof(case<1>) >
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//--------------Definition of SELECT_BY_SIZE----------------------------------//
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#define BOOST_SELECT_BY_SIZE(type_, name, expr) \
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BOOST_STATIC_CONSTANT( \
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unsigned, \
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BOOST_PP_CAT(boost_select_by_size_temp_, name) = sizeof(expr) \
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); \
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BOOST_STATIC_CONSTANT( \
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type_, \
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name = \
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( boost::utility::select_by_size< \
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BOOST_PP_CAT(boost_select_by_size_temp_, name) \
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>::value ) \
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) \
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/**/
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#endif // #ifndef BOOST_UTILITY_SELECT_BY_SIZE_HPP_INCLUDED
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//----------Specializations of SELECT_BY_SIZE (outside main inclued guards)---//
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// Specialize select_by_size for sizeof(case_<N>) for each N less than
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// BOOST_SELECT_BY_SIZE_CASES for which this specialization has not already been
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// performed.
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#if !BOOST_PP_IS_ITERATING //-------------------------------------------------//
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#include <boost/preprocessor/iteration/iterate.hpp>
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#if !defined(BOOST_SELECT_BY_SIZE_MAX_CASE) || \
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(BOOST_SELECT_BY_SIZE_MAX_CASE < 2)
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#undef BOOST_SELECT_BY_SIZE_MAX_CASE
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#define BOOST_SELECT_BY_SIZE_MAX_CASE 2
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#endif
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#if (BOOST_SELECT_BY_SIZE_MAX_CASE > SELECT_BY_SIZE_MAX_SPECIALIZED)
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#define BOOST_PP_FILENAME_1 <boost/utility/select_by_size.hpp>
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#define BOOST_PP_ITERATION_LIMITS ( SELECT_BY_SIZE_MAX_SPECIALIZED, \
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BOOST_SELECT_BY_SIZE_MAX_CASE )
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#include BOOST_PP_ITERATE()
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#undef SELECT_BY_SIZE_MAX_SPECIALIZED
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#define SELECT_BY_SIZE_MAX_SPECIALIZED BOOST_SELECT_BY_SIZE_MAX_CASE
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#endif // #if (BOOST_SELECT_BY_SIZE_CASES > SELECT_BY_SIZE_MAX_SPECIALIZED)
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#undef BOOST_SELECT_BY_SIZE_MAX_CASE
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#else // #if !BOOST_PP_IS_ITERATING //----------------------------------------//
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SELECT_BY_SIZE_SPEC(BOOST_PP_INC(BOOST_PP_ITERATION()))
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#endif // #if !BOOST_PP_IS_ITERATING //---------------------------------------//
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