类构造器与函数命名参数默认值：":"与"="的差异解析

Great question! Let's break down how these two syntaxes work, their differences, and why both exist using your Animal class example.

Functional Behavior #

In modern Dart versions (2.1 and later), both syntaxes behave exactly the same at runtime. Whether you use : or = to set default values for named constructor parameters, the end result is identical:

• If you create an Animal without passing any parameters, totalLegs will default to 4 and type will default to "cat".
• If you pass only one parameter (like Animal(type: 'dog')), the other will use its default value.

For example, both of your Animal class definitions will work the same way when you instantiate them:

var defaultAnimal = Animal(); // totalLegs: 4, type: 'cat'
var customDog = Animal(type: 'dog'); // totalLegs: 4, type: 'dog'
var bird = Animal(totalLegs: 2, type: 'bird'); // totalLegs: 2, type: 'bird'

Syntax & Version History #

The key difference lies in Dart's language evolution:

• The : syntax is the older, original approach to setting default values for named parameters. This was the only option in Dart versions prior to 2.1.
• The = syntax was introduced in Dart 2.1 to fix a syntax inconsistency. Before this update, regular function parameters used = for defaults (e.g., void printAnimal({int legs = 4})), but constructor named parameters used :—this mismatch was confusing for developers. The new = syntax makes parameter default notation consistent across all function and constructor types.

Why Both Exist? #

It all comes down to backward compatibility. Dart's development team prioritizes not breaking existing codebases. When they introduced the more intuitive = syntax, they kept the old : syntax functional so projects written in older Dart versions wouldn't fail immediately.

That said, the official Dart style guide now recommends using = for all parameter defaults (both constructor named parameters and regular function parameters) because it's more consistent and easier for new developers to learn.

内容的提问来源于stack exchange，提问作者f-person