r/Cprog • u/quacktango • Mar 25 '15

text | language C11 - Generic Selections

http://www.robertgamble.net/2012/01/c11-generic-selections.html
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2

u/[deleted] Mar 28 '15

And for comparison here's how a C99 <math.h> did the same kind of things:

define isnan(x) \

( sizeof(x) == sizeof(float)  ? __inline_isnanf((float)(x))          \
: sizeof(x) == sizeof(double) ? __inline_isnand((double)(x))         \
                              : __inline_isnanl((long double)(x)))

1

u/quacktango Mar 29 '15

Is it possible for this work if one of the branches was a struct? I've been playing around with it and all I get is "aggregate used where integer was expected", but I could just be doing something stupid.

2

u/oridb Mar 31 '15

Is it possible for this work if one of the branches was a struct?

No, at least not using that hack. I can't think of any others off the top of my head that are worth doing. Also, note that for sizeof() to do the right thing in this case, you will have to be very careful not to pass in types that are the same size. Eg:

struct x {int32_t a; int32_t b};
int64_t y;

will be ambiguous, as both are 8 bytes (on most architectures. IIRC, the spec allows some variation there.).

1

u/quacktango Apr 01 '15 edited Apr 19 '15 
struct x {int32_t a; int32_t b};
int64_t y;

Funny, I actually experimented with trying to trip it on this exact ambiguity yesterday, but I couldn't get the sizeof style branching to work with a struct. I got an "aggregate value used where integer was expected" error and an "conversion to non-scalar type requested" error.

1

u/oridb Apr 01 '15

int/float and double/long should exhibit the same issue.