in defining a function in Dart, how to set an argument's default to { }, ie, an empty Map?

Question

love how Dart treats function arguments, but cannot accomplish what should be a simple task:

  void func( String arg1, [ Map args = {} ] ) {
    ...
  }

get the error

  expression is not a valid compile-time constant

have tried new Map() for example, with same error.

Answer 1

You have to use the const keyword :

void func( String arg1, [ Map args = const {} ] ) {
  ...
}

Warning : if you try to modify the default args you will get :

Unsupported operation: Cannot set value in unmodifiable Map

Answer 2

The default value must be a compile time constant, so 'const {}' will keep the compiler happy, but possibly not your function.

If you want a new modifiable map for each call, you can't use a default value on the function parameter. That same value is used for every call to the function, so you can't get a new value for each call that way. To create a new object each time the function is called, you have to do it in the function itself. The typical way is:

void func(String arg1, [Map args]) {
  if (args == null) args = {};
  ...
}