The constructor being called isn't a const constructor

Question

I am getting an error for creating a Column Widget saying 'The constructor being called isn't a const constructor.' Having a tough time creating a Column Widget itself,

    import 'package:flutter/material.dart';

    void main() {
      runApp(const MyApp());
    }

    class MyApp extends StatelessWidget {
      const MyApp({Key? key}) : super(key: key);
      @override
      Widget build(BuildContext context) {
        return MaterialApp(
            debugShowCheckedModeBanner: false,
            title: 'My First Flutter App',
            theme: ThemeData(
              scaffoldBackgroundColor: Colors.white,
            ),
            home: const WelcomeScreen());
      }
    }

    class WelcomeScreen extends StatelessWidget {
      const WelcomeScreen({Key? key}) : super(key: key);

      @override
      Widget build(BuildContext context) {
        return const Scaffold(

          body: Column(
              children: <Widget>[
                Text("Login"),
              ]
            )
          );
        }}

score 2 · Answer 1 · answered Dec 31 '21 at 17:03

The error is at the level of the scaffold widget.

Just remove the const before scaffold. Then add const keyword before the children of your column widget.

That is how your code is supposed to be

    import 'package:flutter/material.dart';

    void main() {
      runApp(const MyApp());
    }

    class MyApp extends StatelessWidget {
      const MyApp({Key? key}) : super(key: key);
      @override
      Widget build(BuildContext context) {
        return MaterialApp(
            debugShowCheckedModeBanner: false,
            title: 'My First Flutter App',
            theme: ThemeData(
              scaffoldBackgroundColor: Colors.white,
            ),
            home: const WelcomeScreen());
      }
    }

    class WelcomeScreen extends StatelessWidget {
      const WelcomeScreen({Key? key}) : super(key: key);

      @override
      Widget build(BuildContext context) {
        return Scaffold(

          body: Column(
              children: const <Widget>[
                Text("Login"),
                ...
              ]
            )
          );
        }}

You're welcome @SaumyaPrasad – Baimam Boukar Dec 31 '21 at 18:24 — Baimam Boukar, Dec 31 '21 at 18:24
I have a follow up question, why can't columns be constant? – Dhritiman Roy Mar 06 '22 at 15:45 — Dhritiman Roy, Mar 06 '22 at 15:45

score 1 · Answer 2 · answered Dec 31 '21 at 16:43

1

Just remove const from Scaffold widget

answered Dec 31 '21 at 16:43

zabaykal

1,134
1
8
21

1

and include on `Column(children: const [...]` – Md. Yeasin Sheikh Dec 31 '21 at 16:46
Remove the `const` in `WelcomeScreen `'s build method. And check your IDE config, it must mark it as an error at compiling time – Jesus Hernandez Barrios Dec 31 '21 at 16:56
Thanks for the help! I had tried removing **const** from `Scaffold Widget` previously but didn't seem to work. Thanks for the help! It's working now. – Saumya Prasad Dec 31 '21 at 16:59

score 1 · Answer 3 · answered Dec 31 '21 at 16:52

Column is a non-const constructor

 Column({
    Key? key,

You can use const where everything is available as compile time constant. In this case, Column breaks the condition. You can use const before children, it is possible with Text for now

class WelcomeScreen extends StatelessWidget {
  const WelcomeScreen({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(children: const <Widget>[
        Text("Login"),
      ]),
    );
  }
}

I will encourage you to lean more about const, you can check this question and on guides.

score 1 · Answer 4 · answered Dec 31 '21 at 17:21

Remove the const from the Scaffold widget and it would work fine. But there will be another warning telling that the Column widget does not have const.

To fix that you can either put const in front of <Widget> or put const in front of the children of columns

The constructor being called isn't a const constructor

4 Answers4

The error is at the level of the scaffold widget.