solution.md

Solution

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The solution demonstrates how traits enable polymorphic behavior through composition:

• Trait Implementation: VerbosityFilter implements the Logger trait, allowing it to be used anywhere a Logger is expected.
• Wrapper Pattern: The VerbosityFilter struct wraps an instance of StderrLogger. It intercepts the log call to apply filtering logic before delegating to the inner logger.
• Composition over Inheritance: Behavior is extended by wrapping one type in another, rather than through a class hierarchy.

Details

• Extensibility: This wrapper pattern (often called a decorator or middleware) allows adding functionality like filtering, buffering, or timestamping without modifying the underlying logger.
• Teaser for Generics: While VerbosityFilter is hardcoded to wrap StderrLogger, it could be made generic to wrap any type that implements Logger. This is covered in the next section.