Opaque handles in C

The header declares an incomplete type, so callers cannot see the fields.

#include <stdlib.h>

typedef struct Counter Counter;

struct Counter {
    long value;
    long limit;
};

Counter *counter_new(long limit) {
    Counter *c = malloc(sizeof *c);
    if (c != NULL) {
        c->value = 0;
        c->limit = limit;
    }
    return c;
}

int counter_bump(Counter *c) {
    if (c->value >= c->limit) {
        return 0;
    }
    c->value++;
    return 1;
}

void counter_free(Counter *c) {
    free(c);
}

How it works

1. A forward-declared struct is an opaque type.
2. Only the implementation file has the definition.
3. Every operation goes through functions, so layout stays private.

Keywords and builtins used here

• Counter
• NULL
• counter_bump
• counter_free
• counter_new
• if
• int
• long
• return
• sizeof
• struct
• typedef
• void

The run, in numbers

Lines

29

Characters to type

368

Tokens

116

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 66 seconds.

Type this snippet

Step 1 of 3 in Encapsulation, step 5 of 11 in APIs, errors & testing.

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