如何在 dart 中创建多个构造函数？

Question

I would like to create different objects by calling constructors that have different number of parameters. How can I achieve this in Dart?

class A{
  String b,c,d;

  A(this.b,this.c)
  A(this.b,this.c,this.d)

}

Answer 1

See Constructor section of Tour of Dart .

Basically Dart doesn't support methods/constructor overloading. However Dart allows named constructors and optional parameters.

In your case you could have:

class A{
  String b,c,d;

  /// with d optional
  A(this.b, this.c, [this.d]);

  /// named constructor with only b and c
  A.c1(this.b, this.c);
  /// named constructor with b c and d
  A.c2(this.b, this.c, this.d);
}

Answer 2

You can use the named constructor for clarity to implement multiple constructors for a class.

class A{
      String b,c,d;

      // A(); //default constructor no need to write it
      A(this.b,this.c); //constructor #1
    
      A.namedConstructor(this.b,this.c,this.d); //another constructor #2
    
    }

Answer 3

You can use factory constructors

class A{
  String b,c,d;

  A(this.b,this.c,this.d)
  
  factory A.fromBC(String b, String c) => A(b, c, "");
}