CODEWITHSUNDEEP
c++cvisual-studio-2012macrosvariadic-macros
ysap
ysap

Reputation: 8115

How to define recursive variadic macros?

I am trying to convert boolean values, which represent bits of a signal bus, into an integer. I am using the following construct:

#define B_TO_UINT1(b00)      (((uint32_t) b00 << 0))
#define B_TO_UINT2(b01, ...) (((uint32_t) b01 << 1) | B_TO_UINT1(__VA_ARGS__))
#define B_TO_UINT3(b02, ...) (((uint32_t) b02 << 2) | B_TO_UINT2(__VA_ARGS__))
#define B_TO_UINT4(b03, ...) (((uint32_t) b03 << 3) | B_TO_UINT3(__VA_ARGS__))
// ...

When using the macro cascade to convert a 1-, 2- and 3-bit buses, the first 2 are OK, but the 3-bit conversion gives an error:

cmd = B_TO_UINT1(1);          // line_1
cmd = B_TO_UINT2(1, 0);       // line_2
cmd = B_TO_UINT3(0, 1, 1);    // line_3

Build errors on line_3:

warning C4003: not enough actual parameters for macro 'B_TO_UINT1'

error C2059: syntax error : '<<'

So, looks like the __VA_ARGS__ part is not expanding correctly with the recursion. Is this really the case? If so, is there a workaround?

Upvotes: 0

Views: 1451

Answers (1)

ysap
ysap

Reputation: 8115

Apparently, MS VC++ behaves differently than expected with the recursive expansion of __VA_ARGS__ that includes multiple comma-separated arguments. When expanded in a nested macro, the the compiler treats the whole arg list as one argument.

Examples can also be seen here and here.

However, @Ise Wisteria suggested the EXPAND(x) macro as a workaround. Using this tip, I changed my code to:

#define EXPAND( x ) x
#define B_TO_UINT1(b00)      (((uint32_t) b00 << 0))
#define B_TO_UINT2(b01, ...) (((uint32_t) b01 << 1) | EXPAND( B_TO_UINT1(__VA_ARGS__)))
#define B_TO_UINT3(b02, ...) (((uint32_t) b02 << 2) | EXPAND( B_TO_UINT2(__VA_ARGS__)))
#define B_TO_UINT4(b03, ...) (((uint32_t) b03 << 3) | EXPAND( B_TO_UINT3(__VA_ARGS__)))
// ...

Now the build errors are eliminated.

Note: I am not explicitly saying "a bug", b/c there may be a place for understanding the standard the MS way, as described here.

Upvotes: 1

Related Questions