8 votes

Fortnightly Programming Q&A Thread

Automatically posted November 12, 2019

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Here is my schema:

```sql
CREATE TABLE article_to_warehouse (
  article_id   INTEGER
, warehouse_id INTEGER
)
;
```

How do I add a `UNIQUE` constraint?

1 comment

unknown user

November 22, 2019

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The answer is probably “No”, but still. Is there a way to get a “private structure” in plain C? The closest I've got is this: metres.h: typedef struct metres metres_t; int...

The answer is probably “No”, but still. Is there a way to get a “private structure” in plain C? The closest I've got is this:

metres.h:

typedef struct metres metres_t;

int       metres_get_value(metres_t *x);
metres_t *metres_add(metres_t *x, metres_t *y);
metres_t *metres_new(int value);
void      metres_delete(metres_t *x);

metres.c:

#include <stdlib.h>
// NULL
// free
// malloc

#include "metres.h"

struct metres {
	int value;
};

int metres_get_value(metres_t *x)
{
	if (x == NULL) {
		return 0;
	}

	return x->value;
}

metres_t *metres_add(metres_t *x, metres_t *y)
{
	if (x == NULL || y == NULL) {
		return NULL;
	}

	metres_t *z = malloc(sizeof(metres_t));
	if (z == NULL) {
		return NULL;
	}

	z->value = x->value + y->value;
	return z;
}

metres_t *metres_new(int value)
{
	metres_t *x = malloc(sizeof(metres_t));
	if (x == NULL) {
		return NULL;
	}

	x->value = value;
	return x;
}

void metres_delete(metres_t *x)
{
	free(x);
}

main.c

#include <stdio.h>
// printf

#include "metres.h"
// metres_add
// metres_delete
// metres_get_value
// metres_new
// metres_t

int main(void)
{
	metres_t *x = metres_new(5);
	metres_t *y = metres_new(4);
	metres_t *z = metres_add(x, y);

	printf("the result is: %dm\n", metres_get_value(z));

	metres_delete(x);
	metres_delete(y);
	metres_delete(z);

	return 0;
}

This works, but forces API users to use heap allocation at all times, because otherwise they get “incomplete type” errors when trying to use automatic allocation for metres_t.

(Inspired by this question on Russian StackOverflow.)

2 votes