Failed to assign to final instance variable in constructor

[DartBot]

This issue was originally filed by [email protected]

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I believe this should be legal:

class Object {
  Object(this._foo) {
    _bar = _foo.bar;
  }

  final _foo;
  final _bar;
}

However I get the error 'cannot assign value to final variable _bar'. I can't put the assignment in the initializer list as the assignment refers to _foo.

As it is, the only solution I've found is to make _bar non-final which is less than ideal.

[DartBot]

This comment was originally written by [email protected]

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You can put both the assignments into the initializer list, can't you?

class Object {
  Object(foo) : _foo = foo, _bar = foo.bar;

  final _foo;
  final _bar;
}

Slightly suboptimal (I love the shorthand syntax Obj(this._foo) in constructor arguments), but not exactly bad.

[DartBot]

This comment was originally written by [email protected]

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Apparently I can, yes. It's a shame there's an inconsistency there, but
it's a limitation I can live with :)

Thanks!

[DartBot]

This comment was originally written by [email protected]

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Removed Type-Defect label.
Added Type-Enhancement, Area-Language, Triaged labels.

[gbracha]

There's no inconsistency. We require final fields to be initialized in the initializer list, so we can ensure they get initialized. We do not allow access to 'this' on the right hand side of assignmenyts in the initializer list, so we don't have dependencies on virtual method calls and uninitialized fields etc. Hence the need to initialize _bar in the initializer list using a parameter rather than _foo.

The this.x initializing parameter sugar is for the common case where the only use of the parameter is to initialize a field. The parameter is not in scope in the constructor body or initializer list. In more complex cases, you live without the sugar.

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Set owner to @gbracha.
Added WontFix label.