A Small, Metaprogramming Example in C

more_meta_programming.c

// Brian Chrzanowski
// 2021-11-30 00:06:44
//
// This is just a small demonstration that with the help of the C Preprocessor, and some macros, we
// can approach some meta programming things that get compile-time checked by the compiler. It isn't
// as elegant as something like Jai, but in theory, you could execute something like this at the
// very start of the program.
//
// If C supported more things out of the box to facilitate metaprogramming, this could could get a
// lot nicer, but that's what it is.
//
// OUTPUT:
//   metadata:
//   	size: 12
//   	fields: 3
//   
//   	name: i
//   		size:   4
//   		type:   int
//   		offset: 0
//   	name: j
//   		size:   4
//   		type:   int
//   		offset: 4
//   	name: k
//   		size:   4
//   		type:   int
//   		offset: 8

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

#define NameOf(x) (#x)
#define MemberSize(type, member) sizeof(((type *)0)->member)

// TestStruct : this is the actual struct you might have in the program as some "data".
struct TestStruct {
	int i, j, k;
};

// Field : defines a single field, within a structure
// NOTE (Brian): You could reasonably create a mapping between the text type and some enum
// for easier mangling in the future.
struct Field {
	char name[32];
	char type[64];
	int size;
	int offset;
};

// Structure : this is the metadata for a single structure
struct Structure {
	char *name;
	int size;
	struct Field fields[64];
	int cnt;
};

#define ADDMETA(m,s,t,y) AddMeta(m, NameOf(y), NameOf(t), MemberSize(s,y), offsetof(s,y))
void AddMeta(struct Structure *meta, char *name, char *type, int size, int offset)
{
	struct Field *field;

	if (meta && meta->cnt < 64) {
		field = meta->fields + meta->cnt++;

		strncpy(field->name, name, sizeof(field->name));
		strncpy(field->type, type, sizeof(field->type));
		field->size = size;
		field->offset = offset;
	}
}

int main(int argc, char **argv)
{
	struct Structure metadata;
	int i;

	memset(&metadata, 0, sizeof metadata);

	metadata.name = NameOf(struct Structure);
	metadata.size = 0;

	ADDMETA(&metadata, struct TestStruct, int, i);
	ADDMETA(&metadata, struct TestStruct, int, j);
	ADDMETA(&metadata, struct TestStruct, int, k);

	for (i = 0; i < metadata.cnt; i++) {
		metadata.size += metadata.fields[i].size;
	}

	printf("metadata:\n");
	printf("\tsize: %d\n", metadata.size);
	printf("\tfields: %d\n", metadata.cnt);
	printf("\n");

	for (i = 0; i < metadata.cnt; i++) {
		printf("\tname: %s\n", metadata.fields[i].name);
		printf("\t\tsize:   %d\n", metadata.fields[i].size);
		printf("\t\ttype:   %s\n", metadata.fields[i].type);
		printf("\t\toffset: %d\n", metadata.fields[i].offset);
	}

	return 0;
}