Introduction
When working with object-oriented programming (OOP), the concept of member variables and their visibility is fundamental. Xbase++ follows these principles, offering various visibility levels for member variables in classes. This article intends to explore these concepts in the context of Xbase++, targeting coding practitioners in business application development. Our goal is to clarify when to use different visibility levels for member variables, enhancing your software design.
Member Variables in Xbase++
In Xbase++, as in many OOP languages, classes are blueprints for creating objects (instances). Each class can contain methods (functions) and member variables (properties or fields). Member variables hold the data that pertains to an object. For instance, in a `Person` class, member variables might include `name`, `age`, and `address`.
Visibility Levels
Xbase++ offers three primary visibility levels for member variables: hidden (private), protected, and exported (public).
- Hidden (Private): These variables are accessible only within the class they are declared in. They are crucial for encapsulation, ensuring internal details of the class are not exposed to the outside.
- Protected: Protected variables are accessible in the class they are declared in and any subclasses (derived classes). They are less restrictive than private variables but still offer a level of encapsulation.
- Exported (Public): Public variables are accessible from anywhere - inside the class, in subclasses, and from other external code. They form part of the class's interface with the external world.
Choosing the Right Visibility
Deciding which visibility level to use for a member variable is crucial. Here’s a simplified guide:
- Use hidden if the variable is an internal part of the class and its implementation, and exposing it would break encapsulation or behavior.
- Choose protected if the variable should be accessible to subclasses but not to the world outside.
- Opt for exported if the variable needs to be accessed from outside the class
The Decision Matrix
To help with this decision-making, here’s a decision matrix. Answer the questions for each member variable, and the majority of “Yes” answers in a column suggest the appropriate visibility level.
| Question | Hidden (Private) | Protected | Exported (Public) |
|---|
| Is it an internal implementation detail? | Yes | No | No |
| Should it be modifiable by subclasses only? | No | Yes | No |
| Is it part of the class's public interface? | No | No | Yes |
| Does changing its value require control logic? | Yes | Maybe | No |
| Is it tightly coupled with other private variables? | Yes | Maybe | No |
| Could exposing it lead to misuse or errors? | Yes | Maybe | No |
| Is it a constant or a configuration setting? | Maybe | Maybe | Yes |
| Is inheritance a key feature requiring access? | No | Yes | No |
| Is it part of a simple data structure? | No | No | Yes |
| Are there performance considerations for direct access? | No | No | Yes |
Notes:
Do not use access/assign methods to redirect member variable access to a method. This is a bad practice for several reasons. First, access/assign methods are called dynamically after the member variable access has been resolved, making an access/assign method call take twice as long as a direct method call. Second, when you use an access/assign method, you hide the fact that there are operations/workloads involved. So users of your interface can use the "member variable" inside inner loops and have no idea what real cost is involved in accessing that value. Finally, access/assign methods have been inherited from "Classy" in Xbase++ for compatibility reasons only.
Simplified Rules for Member Variable Visibility - Best Practice
- Use Set/Get Methods: Always utilize setter and getter methods for member variables instead of making them exported (public).
- Default to Protected: Set member variables as protected by default to balance accessibility and encapsulation.
- DTO Exception: For Data Transfer Objects (DTOs), use exported (public) member variables only.
These rules provide a straightforward approach to manage member variable visibility in object-oriented programming, emphasizing encapsulation and simplicity, while accommodating the special case of DTOs like Dataobjects for ease of data transfer between domain or even process boundaries.
Conclusion
Choosing the right visibility for member variables in Xbase++ is a critical aspect of software design. It impacts not only the security and integrity of your code but also its maintainability and flexibility. By following the guidelines and using the decision matrix, you can make informed decisions that suit your specific context, leading to robust and effective software solutions. In the event you do not want to give that much thought, just follow the simplified rules.
