A tagged union in C

The C way to write a sum type: an enum saying which member is live.

typedef enum { VALUE_INT, VALUE_DOUBLE, VALUE_TEXT } Kind;

typedef struct {
    Kind kind;
    union {
        long as_int;
        double as_double;
        const char *as_text;
    } data;
} Value;

double as_number(const Value *v) {
    switch (v->kind) {
    case VALUE_INT:
        return (double) v->data.as_int;
    case VALUE_DOUBLE:
        return v->data.as_double;
    case VALUE_TEXT:
    default:
        return 0.0;
    }
}

How it works

1. The tag and the union travel together in one struct.
2. A switch on the tag is the only safe way to read it.
3. This is how interpreters represent values.

Keywords and builtins used here

• as_number
• case
• char
• const
• default
• double
• enum
• long
• return
• struct
• switch
• typedef
• union

The run, in numbers

Lines

22

Characters to type

354

Tokens

88

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 53 seconds.

Type this snippet

Step 2 of 5 in Unions & layout, step 2 of 13 in Unions, bitfields & undefined behavior.

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