Traits in C

test.c

#include "trait.h"

static inline int
cmp_int(int x, int y)
{
        if (x == y)
                return 0;
        return x < y ? -1 : 1;
}

#define TRAIT_INFO TYPE(int) IMPL(cmp, &cmp_int)
#include TRAIT_INC

static inline int
cmp_double(double x, double y)
{
        if (x == y)
                return 0;
        return x < y ? -1 : 1;
}

#define TRAIT_INFO TYPE(double) IMPL(cmp, &cmp_double)
#include TRAIT_INC

int foo(int x, int y)
{
        return TRAIT_DISPATCH(x, cmp)(x, y);
}

test.i

/* gcc -E test.c | grep -v '^#' | clang-format-4.0 --style=google */

static inline int cmp_int(int x, int y) {
  if (x == y) return 0;
  return x < y ? -1 : 1;
}

typedef int trait_type_0;

static const struct {
  typeof(&cmp_int) cmp;

} trait_struct_0 __attribute__((__unused__)) = {

    .cmp = &cmp_int,

};

static inline int cmp_double(double x, double y) {
  if (x == y) return 0;
  return x < y ? -1 : 1;
}

typedef double trait_type_1;

static const struct {
  typeof(&cmp_double) cmp;

} trait_struct_1 __attribute__((__unused__)) = {

    .cmp = &cmp_double,

};

int foo(int x, int y) {
  return (_Generic((x), trait_type_1
                   : trait_struct_1, trait_type_0
                   : trait_struct_0)
              .cmp)(x, y);
}

test.s

        .text
        .file   "test.c"
        .globl  foo                     # -- Begin function foo
        .p2align        4, 0x90
        .type   foo,@function
foo:                                    # @foo
        .cfi_startproc
# %bb.0:
        xorl    %ecx, %ecx
        xorl    %eax, %eax
        cmpl    %esi, %edi
        setge   %al
        leal    -1(%rax,%rax), %eax
        cmovel  %ecx, %eax
        retq
.Lfunc_end0:
        .size   foo, .Lfunc_end0-foo
        .cfi_endproc
                                        # -- End function

        .ident  "clang version 7.1.0-svn353565-1~exp1~20190407125230.69 (branches/release_70)"
        .section        ".note.GNU-stack","",@progbits
        .addrsig

trait.h

#pragma once

#define TRAIT_INC "trait_0.inc"

#define TRAIT_DISPATCH_LIST

#define TRAIT_DISPATCH(X, FIELD) (_Generic((X), TRAIT_DISPATCH_LIST).FIELD)

trait_0.inc

/* -*- mode: C; -*- */

#define TYPE(X) typedef X trait_type_0;
#define IMPL(X, Y)
TRAIT_INFO
#undef TYPE
#undef IMPL

static const struct {
#define TYPE(X)
#define IMPL(X, Y) typeof(Y) X;
        TRAIT_INFO
#undef IMPL
#undef TYPE
} trait_struct_0 __attribute__((__unused__)) = {
#define TYPE(X)
#define IMPL(X, Y) .X = Y,
        TRAIT_INFO
#undef IMPL
#undef TYPE
};

#define TRAIT_DISPATCH_LIST_0 trait_type_0 : trait_struct_0

#undef TRAIT_DISPATCH_LIST
#define TRAIT_DISPATCH_LIST TRAIT_DISPATCH_LIST_0

#undef TRAIT_INC
#define TRAIT_INC "trait_1.inc"

#undef TRAIT_INFO

trait_1.inc

/* -*- mode: C; -*- */

#define TYPE(X) typedef X trait_type_1;
#define IMPL(X, Y)
TRAIT_INFO
#undef TYPE
#undef IMPL

static const struct {
#define TYPE(X)
#define IMPL(X, Y) typeof(Y) X;
        TRAIT_INFO
#undef IMPL
#undef TYPE
} trait_struct_1 __attribute__((__unused__)) = {
#define TYPE(X)
#define IMPL(X, Y) .X = Y,
        TRAIT_INFO
#undef IMPL
#undef TYPE
};

#define TRAIT_DISPATCH_LIST_1 trait_type_1 : trait_struct_1

#undef TRAIT_DISPATCH_LIST
#define TRAIT_DISPATCH_LIST TRAIT_DISPATCH_LIST_1, TRAIT_DISPATCH_LIST_0

#undef TRAIT_INC
#define TRAIT_INC "trait_2.inc"

#undef TRAIT_INFO