X-macros in C

One list, several expansions: the table and the code stay in step by construction.

#include <stdio.h>

#define STATES(X) \
    X(IDLE) \
    X(RUNNING) \
    X(DONE)

#define AS_ENUM(name) STATE_##name,
#define AS_STRING(name) #name,

typedef enum { STATES(AS_ENUM) STATE_COUNT } State;

static const char *names[] = {STATES(AS_STRING)};

void list_states(void) {
    for (int i = 0; i < STATE_COUNT; i++) {
        printf("%d = %s\n", i, names[i]);
    }
}

How it works

1. The list is a macro that calls a per-item macro.
2. Redefining that macro re-uses the list for a new purpose.
3. Adding an item updates the enum and the names together.

Keywords and builtins used here

• char
• const
• enum
• for
• int
• list_states
• static
• typedef
• void

The run, in numbers

Lines

19

Characters to type

346

Tokens

77

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 46 seconds.

Type this snippet

Step 3 of 3 in Text manipulation, step 7 of 12 in Preprocessor & headers.

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