Fun with C11 _Generic()

codec.c

#include <stdio.h>
#include <string.h>

typedef struct {int v;} volume;
typedef struct {int s;} onoff;

struct settings {
	int a;
	char * b;
	volume v;
	onoff s;
};
/*---------------------------------------------------------------------------*/
void encode_int (int a, char ** string) {
	*string += sprintf(*string, "i32:%x;", a);
}

void encode_char_p(char * b, char ** string) {
	*string += sprintf(*string, "str:%s;", b);
}

void encode_volume(volume v, char ** string) {
	*string += sprintf(*string, "vol:%d;", v.v);
}

void encode_onoff(onoff s, char ** string) {
	*string += sprintf(*string, "swt:%d;", s.s);
}
/*---------------------------------------------------------------------------*/
void decode_int (int * a, char ** string) {
	sscanf(*string, "i32:%x;", a);
	*string = strchr(*string, ';') + 1;
}

void decode_char_p(char ** b, char ** string) {
	sscanf(*string, "str:%[^;];", *b);
	*string = strchr(*string, ';') + 1;
}

void decode_volume(volume * v, char ** string) {
	sscanf(*string, "vol:%d;", &(v->v));
	*string = strchr(*string, ';') + 1;
}

void decode_onoff(onoff * s, char ** string) {
	sscanf(*string, "swt:%d;", &(s->s));
	*string = strchr(*string, ';') + 1;
}
/*---------------------------------------------------------------------------*/
void print_int (int a, char * prefix) {
	printf("%s%d\n", prefix, a);
}

void print_char_p(char * b, char * prefix) {
	printf("%s%s\n", prefix, b);
}

void print_volume(volume v, char * prefix) {
	printf("%sVolume: %d\n", prefix, v.v);
}

void print_onoff(onoff s, char * prefix) {
	printf("%sSwitch: ", prefix);
	if(s.s)
		puts("on");
	else
		puts("off");
}
/*---------------------------------------------------------------------------*/
#define visit_settings(settings, func, data) \
do { \
	func(settings.a, data); \
	func(settings.b, data); \
	func(settings.v, data); \
	func(settings.s, data); \
} while (0)

#define encode(setting, string) _Generic((setting), \
	int: encode_int, \
	char *: encode_char_p, \
	volume: encode_volume, \
	onoff: encode_onoff \
	)(setting, string)

#define decode(setting, string) _Generic((setting), \
	int: decode_int, \
	char *: decode_char_p, \
	volume: decode_volume, \
	onoff: decode_onoff \
	)(&setting, string)

#define print(setting, prefix) _Generic((setting), \
	int: print_int, \
	char *: print_char_p, \
	volume: print_volume, \
	onoff: print_onoff \
	)(setting, prefix)
/*---------------------------------------------------------------------------*/
int main(void) {
	struct settings out = {
		.a = 9001,
		.b = "Generic text",
		.v = {11},
		.s = {0},
	};
	char buffer[255];
	char * index = buffer;

	visit_settings(out, print,  " <- ");
	visit_settings(out, encode, &index);

	puts(buffer);

	char b[15];
	struct settings in = {.b=b};
	index = buffer;

	visit_settings(in, decode, &index);
	visit_settings(in, print,  " -> ");
	return 0;
}