Ruby Super Keyword

Syntax

The syntax for using the Ruby super keyword is straightforward. When calling a method using Ruby super, you simply write super followed by parentheses, like this:

Alternatively, if the method does not require any arguments, you can omit the parentheses:

The Ruby super keyword can be used within a subclass's instance method or a constructor (initialize method). By invoking super, you direct Ruby to look for the corresponding method in the superclass and execute it.

Parameters

When using super, you can pass arguments to the superclass method by providing them within the parentheses. For example:

In the above code, the Child class inherits from the Parent class. The Child class's initialize method calls super(name), passing the name argument to the Parent class's initialize method. This allows both the name and age instance variables to be properly initialized.

Return Value

The Ruby super keyword invokes the superclass's method and returns the value returned by that method. This behaviour allows the subclass to perform additional operations while leveraging the functionality provided by the superclass. Consider the following example:

Output:

In this code, the Child class overrides the greet method defined in the Parent class. Within the Child class's greet method, the super keyword invokes the greet method of the Parent class and stores its return value in the result variable. The subsequent line concatenates the additional greeting from the child class, resulting in the above mentioned output.

Exception

If a superclass method raises an exception, the exception will propagate up the inheritance chain. You should be aware of the exception-handling behaviour when using super to invoke a method. If the superclass method raises an exception, it will bypass the subclass's method and propagate the exception unless the subclass explicitly handles it.

Consider the following scenario:

Output:

The Parent class's foo method raises an exception in this example. The Child class's foo method calls super, which triggers the exception. However, the Child class's foo method also includes a rescue block to catch the exception and display an error message.

Super Without Arguments

When super is called without any arguments, it automatically passes the arguments received by the subclass method to the superclass method. This behaviour allows the subclass to forward the same arguments to the superclass, maintaining consistency in method invocation.

Consider the following example:

Output:

In this code, the Child class's bar method calls super without any arguments. The arguments passed to the bar method in the Child class are automatically forwarded to the bar method in the Parent class.

Super and Method Visibility

The super keyword respects method visibility in Ruby. When using super, the visibility of the method being invoked in the superclass determines whether the subclass can access it.

If the superclass method is private or protected, the super keyword can still be used within the subclass to invoke the method. However, attempting to directly call the superclass method from outside the class hierarchy will result in an error. This behaviour ensures encapsulation and prevents unauthorized access to methods with restricted visibility.

When to Use Super vs Super()

In Ruby, there are two forms of the super keyword: super and super(). The choice between them depends on whether or not you want to pass arguments to the superclass method.

When you call super without parentheses, it automatically forwards the arguments received by the subclass method to the superclass method. This form is useful when you want to maintain the same arguments and pass them along.

On the other hand, when you use super() with parentheses, you explicitly invoke the superclass method without passing any arguments. This form is useful when the superclass method does not expect any arguments or when you want to override the subclass's method entirely and call only the superclass's method.

Consider the following example:

In this code, the Child class overrides the foo and bar methods defined in the Parent class. In the foo method, super is called without arguments to maintain the same behaviour as the superclass. In the bar method, super() explicitly invokes the superclass method without passing any arguments.

Best Practices and Guidelines

When working with the super keyword, it's helpful to follow some best practices and guidelines to ensure code clarity and maintainability:

1. Document the Intended Behaviour:
   • When overriding a method in a subclass and using super, document the expected behaviour of the superclass method.
   • Clearly state any modifications, additions, or assumptions made in the subclass to help future developers understand the code's intent.
2. Avoid Complex Inheritance Hierarchies:
   • Inheritance can quickly become complex and difficult to manage.
   • Strive for a shallow inheritance hierarchy and favour composition or mixin-based approaches when appropriate.
   • This reduces the reliance on super and makes the codebase more modular and maintainable.
3. Keep Method Signatures Consistent:
   • To ensure code stability and ease of use, strive for consistency in method signatures across the inheritance hierarchy.
   • When overriding a method in a subclass and using super, ensure that the method signatures (i.e., method name, arguments, and return types) match the superclass method to avoid surprises and compatibility issues.
4. Prefer Composition over Inheritance:
   • Consider using composition over inheritance when appropriate.
   • Composition allows for greater flexibility and loose coupling between objects, reducing the need for extensive subclassing and the associated use of super.
   • You can achieve more modular and maintainable code by encapsulating behaviour in separate objects and leveraging interfaces or protocols.
5. Test Edge Cases and Inheritance Scenarios:
   • When working with inheritance and super, thoroughly test edge cases and various inheritance scenarios to ensure the desired behaviour.
   • Test both the superclass and subclass methods independently and in conjunction to identify any unexpected interactions or issues.

Implicit Arguments

In addition to explicitly passing arguments to the superclass method, Ruby also provides a mechanism for implicitly passing arguments. When calling super without parentheses or arguments, Ruby implicitly passes all the arguments received by the subclass method to the superclass method. This behaviour is particularly useful when the subclass and superclass methods have the same signature.

Consider the following example:

Output:

In this code, both the Parent class's greet method and the Child class's greet method accept a single argument name. By calling super without parentheses or arguments, the Child class implicitly passes the name argument to the greet method of the Parent class.

Super With Blocks

The super keyword can also be used in conjunction with blocks. When a method defined in the superclass accepts a block, the super keyword allows the subclass to invoke the superclass's method while passing the block to it.

Consider the following example:

In this code, both the Parent class's process method and the Child class's process method accept a block. By calling super with a block in the Child class's process method, the block is passed to the process method of the Parent class. This allows the code within the block to be executed within the context of the superclass's method.

Output:

Super With the Ancestors Chain

The super keyword is not limited to invoking methods defined in the immediate superclass. It traverses the entire ancestors chain, allowing you to call methods defined in higher-level superclasses.

Consider the following example:

Output:

In this code, the Child class inherits from the Parent class, which in turn inherits from the Grandparent class. Each class defines a foo method that calls super. As a result, when child.foo is called, the foo method defined in the Grandparent class is invoked via the ancestors chain.

Examples

Let us discuss some examples of the using the super keyword:

Example 1: Calling a Superclass Method With Modified Arguments

In this example, the Child class's greet method modifies the argument by adding a prefix before calling the superclass's greet method using super. This allows the subclass to customize the behaviour while leveraging the functionality provided by the superclass.

Example 2: Accessing and Modifying the Return Value of the Superclass Method

In this example, the Child class's calculate method calls the superclass's calculate method using super and then modifies the return value by adding 10. This allows the subclass to extend the behaviour of the superclass's method while utilizing its core logic.

Example 3: Accessing and Modifying Instance Variables in the Superclass

In this example, the Child class's initialize method calls the superclass's initialize method using super to initialize the instance variable @message with the value "Hello". Then, it appends " World!" to the @message variable. Finally, the display_message method is invoked, which prints the modified @message value.

Example 4: Using super with Additional Arguments

In this example, the Child class's greet method takes additional arguments (name and age) compared to the Parent class's greet method. Using super(name), the Child class invokes the greet method of the superclass (Parent) and passes the name argument. It then appends the age information to the result returned by the superclass method.

Example 5: Using Super in a Setter Method to Access Superclass Behaviour

In this example, the Child class overrides the setter method name= from the Parent class. It calls super(value) to invoke the superclass's setter method and capitalize the value. Then, it adds additional behaviour specific to the Child class by printing a message. Finally, it retrieves the modified name value using child.name and prints it.

Potential Pitfalls and Considerations

While the super keyword is a powerful tool for working with inheritance and method overriding, it's important to be aware of potential pitfalls and considerations when using it:

1. Incorrect Arguments or Mismatched Method Signatures:
   • When invoking a superclass method using super, it's crucial to ensure that the arguments passed to super match the expected parameters of the superclass method.
   • Failure to provide the correct arguments or using mismatched method signatures can lead to unexpected behaviour or errors.
2. Limitations of Method Visibility:
   • The visibility of a superclass method can affect the accessibility and usability of super in subclasses.
   • If a superclass method is private or protected, it can still be invoked using super within a subclass.
   • However, direct access to the superclass method from outside the class hierarchy is limited.
3. Maintaining Method Contracts:
   • When overriding a method in a subclass and using super to invoke the superclass method, it's important to uphold the method's contract and behaviour.
   • Modifying the behaviour too drastically in the subclass can break the expected behaviour defined in the superclass, leading to code that is harder to understand and maintain.
4. Overreliance on super for Code Flow:
   • Using super excessively throughout a class hierarchy can make the codebase more tightly coupled and harder to reason about.
   • It's important to strike a balance and consider alternative approaches, such as extracting common behaviour into separate methods or using composition instead of relying solely on inheritance.