Pass null to named constructer parameter with default value-CodePudding

I made a class HelloWorld with a named constructer which accepts a single parameter named test which also has default value of Default Value. Here is my code:

import 'dart:core';

class HelloWorld {
  final String test;
  
  const HelloWorld({
    this.test = 'Default Value',
  });
}

void main() {
  const Map<String, dynamic> _json = {'hello': 'world'};
  const _helloWorld = HelloWorld(test: _json['test']);
  print(_helloWorld.test);
}

Then I made a map named _json which has only one key hello. Finally I tried to get the value of key test which obviously doesn't exist in _json which will return null. Despite having the default value for the test parameter in the named constructor, I still get this compilation error:

lib/main.dart:13:23:
Error: Constant evaluation error:
  const _helloWorld = HelloWorld(test: _json['test']);
                      ^
lib/main.dart:13:45:
Info: The method '[]' can't be invoked on '<String, dynamic>{"hello": "world"}' in a constant expression.
  const _helloWorld = HelloWorld(test: _json['test']);
                                            ^
lib/main.dart:13:9:
Info: While analyzing:
  const _helloWorld = HelloWorld(test: _json['test']);
        ^
Error: Compilation failed.

Dart version: Dart SDK version: 2.14.4 (stable) (Wed Oct 13 11:11:32 2021 0200) on "macos_x64"

CodePudding user response：

const means that the object's entire deep state can be determined entirely at compile time and that the object will be frozen and completely immutable.

const Map<String, dynamic> _json = {'hello': 'world'}; means this _json is compile-time constant.

But for _json['test'] it doesn't know what will be the value of it in compile time. It needs to go through the _json Map and find the value, and this will happen while running the code. For this reason _helloWorld can not be a const.

But if you do HelloWorld(test: "others"); or use default value HelloWorld(), the _helloWorld can be const type.


void main() {
  const Map<String, dynamic> _json = {'hello': 'world'};

  final _helloWorld = HelloWorld(test: _json['test']); // or can be use String
  print(_helloWorld.test);

  const _helloWorld1 = HelloWorld();
  print(_helloWorld1.test);
  const _helloWorld2 = HelloWorld(test: "on _helloWorld2");
  print(_helloWorld2.test);
}

Default value will be available only when you don't pass any value using test, it will be const _helloWorld1 = HelloWorld();.

But for handling null-Value from _json you can use HelloWorld(test: _json['test'] ?? "Got Null");. Passing Got Null because test require a non-nullable String. If you wish to have constructor default value in this case, check _json['x'] provide null or not, then assign on _helloWorld.

  final _helloWorld = _json['test'] == null
      ? HelloWorld()
      : HelloWorld(test: _json['test']);

I will recommend const and final SO answerto learn more it. Also, you can find on dart.dev

CodePudding user response：

I ended up using this way:

import 'dart:core';

class HelloWorld {
  final String test;
  
  const HelloWorld({
    String? test,
  }) : test = test ?? 'Default Value';
}

void main() {
  final Map<String, dynamic> _json = {'hello': 'world'};
  final _helloWorld = HelloWorld(test: _json['test']);
  print(_helloWorld.test);
}

CodePudding user response：

You simply need to change your test variable to this, to make it nullable:

final String? test;