Another typedef usage example

bearophile bearophileHUGS at
Wed May 5 16:39:50 PDT 2010

Another usage example of the strong typedef of D1. This his D2 code, it shows how it can catch bugs in combineRectangles():

import std.algorithm: max, min;

typedef int Length;
typedef Length HorizPosition;
typedef Length VertPosition;

struct Point {
    HorizPosition x;
    VertPosition y;

struct Rectangle {
    Point upperLeft, lowerRight;

// Length is defined as a generic form of HorizPosition and VertPosition
// because of the typedefs. There is no problem when combining a
// generic type with a type lower on the hierarchy.

Length perimeter(const ref Rectangle r) {
    // Length len = 2 * (r.upperLeft.y - r.lowerRight.y); // problem
    Length len = 0;
    len += 2 * (r.upperLeft.y - r.lowerRight.y); // combining VertPosition with Length
    len += 2 * (r.lowerRight.x - r.upperLeft.x); // combining HorizPosition with Length
    return len;

/// To form one large rectangle that will just cover the rectangles r1 and r2.
Rectangle combineRectangles(const ref Rectangle r1, const ref Rectangle r2) {
    return Rectangle(Point(min(r1.upperLeft.x, r2.upperLeft.x),
                           max(r1.upperLeft.y, r2.upperLeft.y)),

                     // Here HorizPosition and VertPosition need to be
                     // treated as two separate and distinct types
                     Point(max(r1.lowerRight.y, r2.lowerRight.x), // Err
                           min(r1.lowerRight.x, r2.lowerRight.x))); // Err

void main() {}


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