Constructor: Storing pre-processed params in helper variables passed to final fields-CodePudding

I'd like to provide an alternative constructor for my Circe class where the center and radius are calculated from 3 points. But I cannot find a way to store the calculated center to the temporal variable and pass it to both final fields. Calling _getCenter twice works, but this approach is suboptimal. Is there a more efficient way?

import 'dart:math';

class Circle {
  final Point center;
  final double radius;

  Circle(this.center, this.radius);

  Circle.fromPoints(Point p1, Point p2, Point p3)
      : center = _getCenter(p1, p2, p3),
        radius = _getCenter(p1, p2, p3).distanceTo(p1);

  static Point _getCenter(Point p1, Point p2, Point p3) {
    ...
  }

CodePudding user response：

You could make use of a factory constructor:

import 'dart:math';

class Circle {
  final Point center;
  final double radius;

  Circle(this.center, this.radius);

  factory Circle.fromPoints(Point p1, Point p2, Point p3) {
    final center = _getCenter(p1, p2, p3);
    final radius = center.distanceTo(p1);

    return Circle(center, radius);
  }

  static Point _getCenter(Point p1, Point p2, Point p3) {
    ...
  }
}

The point of such constructor is to behave like a constructor but does not create any object instance automatically. You must then return an object that is compatible with the class the factory constructor is part of.

CodePudding user response：

I'd probably use a factory constructor as suggested by julemand101's answer. Another approach is to use late members so that they can be initialized in a constructor body.

import 'dart:math';

class Circle {
  late final Point center;
  late final double radius;

  Circle(this.center, this.radius);

  Circle.fromPoints(Point p1, Point p2, Point p3) {
    center = _getCenter(p1, p2, p3);
    radius = center.distanceTo(p1);
  }

  static Point _getCenter(Point p1, Point p2, Point p3) {
    ...
  }

This would matter if you want to use the fromPoints constructor from a redirecting constructor or if you have some derived class that wants to invoke fromPoints as a base class constructor. However, late variables are potentially more expensive since they might add runtime checks.