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In C and C++, a tagged union can be created from untagged unions using a strict access discipline where the tag is always checked:

enum ShapeKind { Square, Rectangle, Circle };

struct Shape {
    int centerx;
    int centery;
    enum ShapeKind kind;
    union {
        struct { int side; };           /* Square */
        struct { int length, height; }; /* Rectangle */
        struct { int radius; };         /* Circle */
    };
};

int getSquareSide(struct Shape* s) {
    assert(s->kind == Square);
    return s->side;
}

void setSquareSide(struct Shape* s, int side) {
    s->kind = Square;
    s->side = side;
}

/* and so on */