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Learn how to resolve issues related to assigning constant default values in Dart constructors by understanding compile-time constants and the use of `static const`.
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How to Assign a Constant Value as Default Value in Constructor Using Dart

When working with Dart, you may encounter situations where you want to assign a constant value as a default parameter in your constructor. This can often lead to confusion, especially when receiving compile-time errors stating that the default value of an optional parameter must be a constant. In this guide, we’ll explore a common scenario leading to this error and provide an in-depth solution to ensure you can effectively use const values as defaults in your Dart classes.

The Issue at Hand

Consider the following class definition:

[[See Video to Reveal this Text or Code Snippet]]

In the above code snippet, we attempt to assign a default value for the parameter x by referencing defValue, which is defined as a getter. However, we encounter a compile-time error, which states: The default value of an optional parameter must be a constant. At first glance, it might seem that defValue should suffice, as it is conceptually a constant that returns a value of 10. But Dart has specific rules regarding constant expressions that we must adhere to.

Understanding the Problem

What’s Wrong with the Getter?

The core of the problem lies in the nature of the defValue getter. While it appears to return a constant value, Dart evaluates the getter when it is called rather than treating its value as a compile-time constant directly. Here are the reasons that lead to the error:

Getters Require Execution: The getter needs to be executed to retrieve its value. This is not allowed in a constant context because constant evaluation can only handle compile-time constants and cannot include method or getter execution.

Compile-Time Constant Definition: For a value to be considered as a default parameter, it must be defined as a compile-time constant, which means it has to be declared in a way that Dart recognizes during compilation as a constant.

The Solution

To resolve this issue, we need to change the way we define defValue. Instead of using a getter, we should declare it as a static const variable. This declaration guarantees that the value is treated as a compile-time constant. Here’s how to do it:

Step 1: Update the defValue Declaration

Modify the defValue definition to the following:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Class Implementation

Your updated Foo class should now look like this:

[[See Video to Reveal this Text or Code Snippet]]

Clarifying Common Misconceptions

While some documentation might suggest that you can use a static method as a compile-time constant, it is essential to understand the distinction:

Static Method vs. Constant Reference: You can reference a static method itself as a constant, but you cannot call the method to return a value. For example, the following is valid:

[[See Video to Reveal this Text or Code Snippet]]

Conversely, since a getter involves an execution to access its value, it cannot be treated similarly to a method reference in this context.

Conclusion

By understanding how Dart handles compile-time constants, you can effectively assign constant values as default parameters in your constructors without running into compile-time errors. Always remember to use static const for such variables to avoid execution issues. Happy coding in Dart!