C preprocessor directives operator ##

Question

I want to create a function void vec2_from_vec3(vec2 res, vec3 from) in pure C, using operator ## like so:

#define MAGIC_MACROS(n) \
void vec##(n-1)##_from_vec##n##(vec##(n-1) res, vec##n from);

but compiler woudn't let it. Is it possible?

Inspired by https://github.com/datenwolf/linmath.h/blob/master/linmath.h

Jean-François Fabre · Accepted Answer · 2017-03-28 19:25:56Z

1

The preprocessor won't evaluate/compute n-1, it will just expand it. Ex: 3-1, so string concatenation won't work

(a modern compiler does it but it's already too late)

You can always do that which isn't that bad already:

#define MAGIC_MACROS(n1,n2) \
void vec##n1##_from_vec##n2##(vec##n1 res, vec##n2 from)

and use as:

MAGIC_MACROS(2,3);

Note that you shouldn't end your macros with ;, so it's homogeneous with function calls and doesn't break editor auto-indentation.

answered Mar 28, 2017 at 19:25

Jean-François Fabre♦

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2 Comments

dizcza Over a year ago

Also, I want to create typedef int16_t vec3i[3] with # define INT_BITS 16 and then # define LINMATH_H_DEFINE_VECI(n,b) typedef int##b##_t vec##n##i[n]; and call LINMATH_H_DEFINE_VECI(3, INT_BITS). But it changes to typedef intINT_BITS_t vec3i[3]. It's not possible too, right?

Jean-François Fabre Over a year ago

no for same reason: preproc only replaces once. You have to pass (3, 16)

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