How to use macro to construct a class?

Question

For example, i have a class

class A : public B
{
public:
    template<unsigned i> struct Element
    {
         // typedef void Type; // will specialize it.
    };

    A(Element<0>::Type v0 = initial0,
      Element<1>::Type v1 = initial1,
      Element<2>::Type v2 = initial2,
      Element<3>::Type v3 = initial3
     )
    : B(v0,v1,v2,v3)
    {}

};

And I want to use macro to replace the constructor, like

CONSTRUCT(A, initial0, initial1, initial2, initial3)

And the number of elements is variadic (need ... variadic macros).

Is it possible to do it? Thanks

Why do you want to do that? It sounds very unusual to me, because the situation is very usual in your case. — Nawaz, Jul 27 '13 at 17:07

user1899020 · Answer 1 · 2013-07-27T19:21:59.717

I figured it out.

// Construct with defaults.
FOO(x)                                  x

#define _1_DEFAULTS(N, def)             Element<N-1>::Type const& value0 = def
#define _2_DEFAULTS(N, def, ...)        Element<N-2>::Type const& value1 = def, FOO(_1_DEFAULTS(N, __VA_ARGS__))
#define _3_DEFAULTS(N, def, ...)        Element<N-3>::Type const& value2 = def, FOO(_2_DEFAULTS(N, __VA_ARGS__))
#define _4_DEFAULTS(N, def, ...)        Element<N-4>::Type const& value3 = def, FOO(_3_DEFAULTS(N, __VA_ARGS__))
#define _5_DEFAULTS(N, def, ...)        Element<N-5>::Type const& value4 = def, FOO(_4_DEFAULTS(N, __VA_ARGS__))
#define _6_DEFAULTS(N, def, ...)        Element<N-6>::Type const& value5 = def, FOO(_5_DEFAULTS(N, __VA_ARGS__))
#define _7_DEFAULTS(N, def, ...)        Element<N-7>::Type const& value6 = def, FOO(_6_DEFAULTS(N, __VA_ARGS__))
#define _8_DEFAULTS(N, def, ...)        Element<N-8>::Type const& value7 = def, FOO(_7_DEFAULTS(N, __VA_ARGS__))
#define _9_DEFAULTS(N, def, ...)        Element<N-9>::Type const& value8 = def, FOO(_8_DEFAULTS(N, __VA_ARGS__))
#define _10_DEFAULTS(N, def, ...)       Element<N-10>::Type const& value9 = def, FOO(_9_DEFAULTS(N, __VA_ARGS__))

#define _1_VALUES                       value0
#define _2_VALUES                       value1, _1_VALUES
#define _3_VALUES                       value2, _2_VALUES
#define _4_VALUES                       value3, _3_VALUES
#define _5_VALUES                       value4, _4_VALUES
#define _6_VALUES                       value5, _5_VALUES
#define _7_VALUES                       value6, _6_VALUES
#define _8_VALUES                       value7, _7_VALUES
#define _9_VALUES                       value8, _8_VALUES
#define _10_VALUES                      value9, _9_VALUES

#define _NUM_ARGS2(X,X64,X63,X62,X61,X60,X59,X58,X57,X56,X55,X54,X53,X52,X51,X50,X49,X48,X47,X46,X45,X44,X43,X42,X41,X40,X39,X38,X37,X36,X35,X34,X33,X32,X31,X30,X29,X28,X27,X26,X25,X24,X23,X22,X21,X20,X19,X18,X17,X16,X15,X14,X13,X12,X11,X10,X9,X8,X7,X6,X5,X4,X3,X2,X1,N,...) N
#define NUM_ARGS(...) FOO(_NUM_ARGS2(0, __VA_ARGS__ ,64,63,62,61,60,59,58,57,56,55,54,53,52,51,50,49,48,47,46,45,44,43,42,41,40,39,38,37,36,35,34,33,32,31,30,29,28,27,26,25,24,23,22,21,20,19,18,17,16,15,14,13,12,11,10,9,8,7,6,5,4,3,2,1,0))


#define _CONSTRUCT_V1(A, B, N, ...)     A(FOO(_##N##_DEFAULTS(N, __VA_ARGS__))) : B(_##N##_VALUES) {}
#define _CONSTRUCT_V2(A, B, N, ...)     FOO(_CONSTRUCT_V1(A, B, N, __VA_ARGS__))
#define CONSTRUCT(A, B, ...)            FOO(_CONSTRUCT_V2(A, B, NUM_ARGS(__VA_ARGS__), __VA_ARGS__))

How to use macro to construct a class?

1 Answers1