src/lib/logging/VA_OPT.h

   1 //  Copyright (c) 2019 Will Wray https://keybase.io/willwray
   2 //
   3 //  Distributed under the Boost Software License, Version 1.0.
   4 //        http://www.boost.org/LICENSE_1_0.txt
   5 //
   6 //  Repo: https://github.com/willwray/VA_OPT
   7
   8 #pragma once
   9
  10 /*
  11   VA_OPT.hpp
  12   ==========
  13
  14   Preprocessor utilities for testing emptiness of macro arguments
  15   and for conditional expansion based on the emptiness of ARGS.
  16
  17   VA_OPT_SUPPORT(?)
  18       1 if __VA_OPT__ support is detected else 0 (see platform note).
  19
  20   C++20's __VA_OPT__ finally provides a reliable test for empty ARGS.
  21   The following macros use __VA_OPT__, if detected, otherwise they
  22   provide 'polyfill' fallbacks (though with some failing edge cases).
  23
  24   IS_EMPTY(...)
  25       1 if the ... ARGS is empty else 0.
  26
  27   IFN(...)
  28       If ... ARGS are not empty then a trailing 'paren expression' (X)
  29       is deparenthesized to X else the trailing (X) term is consumed.
  30       E.g. IFN()(N) vanishes, while IFN(NotEmpty)(N) -> N
  31
  32       In other words, IFN(ARGS) is like __VA_OPT__, but with explicit
  33       (ARGS) in place of an implicit __VA_ARGS__ check.
  34
  35   IFE(...)
  36       If ... ARGS is empty expand trailing 'paren expression' (X) to X
  37       else if ARGS are not empty consume the trailing paren expression.
  38       E.g. IFE(NotEmpty)(E) vanishes, while IFE()(E) -> E
  39
  40   IFNE(...)(N,E...)
  41       If ... ARGS are not empty expands to N else expands to E...
  42       E.g. IFNE(ARG)(X,()) is equivalent to IFN(ARG)(X)IFE(ARG)(())
  43       both put back a terminating () removed by the outer macro call.
  44
  45   Without VA_OPT_SUPPORT these 'emptiness' macros are not perfect;
  46   IS_EMPTY, IFN, IFE, IFNE may cause a compile error ('too few args')
  47   if the argument is a function-like macro name that expects ARG(s).
  48
  49   IBP(...)
  50       IS_BEGIN_PARENS macro to test if an argument is parenthesised:
  51       1 if ... ARGS begins with a 'paren expression' else 0.
  52
  53   Platform note: Current Sept 2019 __VA_OPT__ support:
  54   -------------
  55   Clang -std=c++2a enables it. GCC has it enabled without -std=c++2a
  56   but warns "__VA_OPT__ is not available until C++2a" if another -std
  57   flag is supplied along with -pedantic (dont know how to supress it).
  58   MSVC TBD
  59
  60   Credits
  61   -------
  62   Props to pre-pro pioneers, particularly Paul Mensonides.
  63   The 'emptiness' methods are adapted from BOOST_VMD_IS_EMPTY which,
  64 . in turn, depends on BOOST Preprocessor's BOOST_PP_IS_BEGIN_PARENS
  65   (adapted and exposed here as IBP 'Is Begin Parens'):
  66   www.boost.org/doc/libs/1_71_0/libs/vmd
  67   www.boost.org/doc/libs/1_71_0/libs/preprocessor
  68 */
  69
  70 #define VA_ARG1(A0,A1,...) A1
  71 // VA_EMPTY works only if __VA_OPT__ is supported, else always -> 1
  72 #define VA_EMPTY(...) VA_ARG1(__VA_OPT__(,)0,1,)
  73
  74 // VA_OPT_SUPPORT helper macro for __VA_OPT__ feature detection.
  75 // Adapted from https://stackoverflow.com/a/48045656/7443483
  76 // Use as #if VA_OPT_SUPPORT(?)
  77 #define VA_OPT_SUPPORT ! VA_EMPTY
  78
  79 #if VA_OPT_SUPPORT(?)
  80
  81 #  define IS_EMPTY(...) VA_EMPTY(__VA_ARGS__)
  82 #  define IFN(...) VA_EAT __VA_OPT__(()VA_IDENT)
  83 #  define IFE(...) VA_IDENT __VA_OPT__(()VA_EAT)
  84 #  define IFNE(...) VA_ARGTAIL __VA_OPT__((,)VA_ARG0)
  85
  86 #else
  87
  88 #  define IS_EMPTY(...) IFP(IBP(__VA_ARGS__))(IE_GEN_0,IE_IBP)(__VA_ARGS__)
  89 #  define IFN(...) IFP(IBP(__VA_ARGS__))(GEN_IDENT,EAT_OR_IDENT)(__VA_ARGS__)
  90 #  define IFE(...) IFP(IBP(__VA_ARGS__))(GEN_EAT,IDENT_OR_EAT)(__VA_ARGS__)
  91 #  define IFNE(...) IFP(IBP(__VA_ARGS__))(GEN_ARGTAIL,ARG0_OR_TAIL)(__VA_ARGS__)
  92
  93 #endif
  94
  95 #define VA_EAT(...)
  96 #define VA_IDENT(...) __VA_ARGS__
  97 #define VA_ARG0_(A0,...) A0
  98 #define VA_ARG0(...) VA_ARG0_(__VA_ARGS__)
  99 #define VA_ARGTAIL_(A0,...) __VA_ARGS__
 100 #define VA_ARGTAIL(...) VA_ARGTAIL_(__VA_ARGS__)
 101
 102 // IFP helper macros to test IBP for IFN and IS_EMPTY
 103 #define IFP_0(T,...) __VA_ARGS__
 104 #define IFP_1(T,...) T
 105
 106 #define IFP_CAT(A,...) A##__VA_ARGS__
 107 #define IFP(BP) IFP_CAT(IFP_,BP)
 108
 109 // IS_BEGIN_PAREN helper macros adapted from BOOST VMD
 110 #define IBP_CAT_(A,...) A##__VA_ARGS__
 111 #define IBP_CAT(A,...) IBP_CAT_(A,__VA_ARGS__)
 112
 113 #define IBP_ARG0_(A,...) A
 114 #define IBP_ARG0(...) IBP_ARG0_(__VA_ARGS__)
 115
 116 #define IBP_IS_ARGS(...) 1
 117
 118 #define IBP_1 1,
 119 #define IBP_IBP_IS_ARGS 0,
 120
 121 // IBP IS_BEGIN_PAREN returns 1 or 0 if ... ARGS is parenthesised
 122 #define IBP(...) IBP_ARG0(IBP_CAT(IBP_, IBP_IS_ARGS __VA_ARGS__))
 123
 124 // IFN, IFE, IFNE and IF_EMPTY helpers without __VA_OPT__ support
 125 #if ! VA_OPT_SUPPORT(?)
 126
 127 #  define IBP_(T,...) IBP_ARG0(IBP_CAT(IF##T##_, IBP_IS_ARGS __VA_ARGS__))
 128
 129    // IS_EMPTY helper macros, depend on IBP
 130 #  define IE_REDUCE_IBP(...) ()
 131 #  define IE_GEN_0(...) 0
 132 #  define IE_IBP(...) IBP(IE_REDUCE_IBP __VA_ARGS__ ())
 133
 134 #  define GEN_IDENT(...) VA_IDENT
 135 #  define GEN_EAT(...) VA_EAT
 136 #  define GEN_ARGTAIL(...) VA_ARGTAIL
 137 #  define GEN_ARG0(...) VA_ARG0
 138
 139    // IFN, IFE, IFNE helper macros
 140 #  define EAT_OR_IDENT(...) IBP_(N,IE_REDUCE_IBP __VA_ARGS__ ())
 141 #  define IFN_1 VA_EAT,
 142 #  define IFN_IBP_IS_ARGS VA_IDENT,
 143
 144 #  define IDENT_OR_EAT(...) IBP_(E,IE_REDUCE_IBP __VA_ARGS__ ())
 145 #  define IFE_1 VA_IDENT,
 146 #  define IFE_IBP_IS_ARGS VA_EAT,
 147
 148 #  define ARG0_OR_TAIL(...) IBP_(NE,IE_REDUCE_IBP __VA_ARGS__ ())
 149 #  define IFNE_1 VA_ARGTAIL,
 150 #  define IFNE_IBP_IS_ARGS VA_ARG0,
 151
 152 #endif // IFN and IF_EMPTY defs