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CXXD_VALID_VARIANTS — Tests for valid dual library variants.
// In header: <boost/cxx_dual/valid_variants.hpp>
CXXD_VALID_VARIANTS(...)The function-like macro tests for valid dual library variants. A variant consists of a series of CXXD-mod choices encoded as a variadic parameter. A CXXD-mod choice refers to whether the CXXD-mod chooses the C++ standard implementation or the Boost implementation.
If the CXXD-mod choices, based on the CXXD headers being included before the macro, is invoked match any one of the variants, the macro expands to 1, otherwise the macro expands to 0.
The macro provides a single invocation where the creator of a library or executable can test whether or not combinations of CXXD-mod choices match what the programmer wants to allow. In cases of header-only libraries or executables it should never be necessary to limit the combinations in any way, but in the case of a non-header only library the library implementor may well want to limit the acceptable combinations because supporting all possible variants, each with their own name and need to be built, might well prove onerous.
A variant is encoded by a series, known as a VMD sequence, of two-element Boost PP tuples. The first element is a particular CXXD-mod identifier, given in a following list, and the second element is 1 if the desired choice is the C++ standard implementation of that CXXD-mod or 0 if the desired choice is the Boost implementation of that CXXD-mod.
The VMD sequence of each choice makes up a combination of dual library choices, which denote a valid variant which the macro invoker says that his library will allow.
The list of CXXD-mod identifiers and their CXXD-mod is:
CXXD_ARRAY,array
CXXD_ATOMIC,atomic
CXXD_BIND,bind
CXXD_CHRONO,chrono
CXXD_CONDITION_VARIABLE,condition_variable
CXXD_ENABLE_SHARED_FROM_THIS,enable_shared_from_this
CXXD_FUNCTION,function
CXXD_HASH,hash
CXXD_MAKE_SHARED,make_shared
CXXD_MEM_FN,mem_fn
CXXD_MOVE,move
CXXD_MUTEX,mutex
CXXD_RANDOM,random
CXXD_RATIO,ratio
CXXD_REF,ref
CXXD_REGEX,regex
CXXD_SHARED_MUTEX,shared_mutex
CXXD_SHARED_PTR,shared_ptr
CXXD_SYSTEM_ERROR,system_error
CXXD_THREAD,thread
CXXD_TUPLE,tuple
CXXD_TYPE_INDEX,type_index
CXXD_TYPE_TRAITS,type_traits
CXXD_UNORDERED_MAP,unordered_map
CXXD_UNORDERED_MULTIMAP,unordered_multimap
CXXD_UNORDERED_MULTISET,unordered_multiset
CXXD_UNORDERED_SET,unorderd_set
CXXD_WEAK_PTR,weak_ptr
CXXD_MODS_ALL,all mods
As can be seen each CXXD-mod identifier is 'CXXD_' followed by the uppercase name of the CXXD-mod.
The CXXD_MODS_ALL identifier refers to all of the included CXXD headers choosing either the C++ standard implementation or the Boost implementation. Therefore if this identifier is used its Boost PP tuple should be the only one in the VMD sequence for that variant.
The macro invoker must pass at least one variant as a variadic parameter otherwise there is no point in using this macro, but may specify any number of further variants as variadic parameters.
A protoypical variant will look like: (CXXD_XXX,1 or 0)(CXXD_YYY,1 or 0)(CXXD_ZZZ,1 or 0)...
where CCXD_XXX, CXXD_YYY, and CXXD_ZZZ are one of the CXXD-mod identifiers listed above and the '1 or 0' denotes either the C++ standard implementation or Boost implementation as a choice for that CXXD-mod. There can be one or more Boost PP tuples in the VMD sequence which denote the variant. A variant is an 'AND' proposition where each Boost PP tuple in the VMD sequence must be true for the variant to match. Each variant as a variadic parameter is an 'OR' proposition where any variant must match for the macro to return 1. Otherwise the macro returns 0 if none of the variants match.