In MQL5 (MetaQuotes Language 5), both classes and structures are used to define custom data types, but they serve different purposes and have distinct characteristics. Whether you should use a class or a structure depends on the specific requirements of your program. Let’s explore the reasons why you might choose one over the other:

1. Encapsulation:

• Classes support encapsulation, which means you can hide the internal implementation details of your data and provide public and private access to members (variables and functions). This helps in maintaining data integrity and preventing unintended modifications. Structures do not offer this level of encapsulation.

1. Object-Oriented Programming (OOP):

• If you want to follow object-oriented programming principles, classes are a better choice. You can define classes with properties (member variables) and methods (member functions) that operate on those properties, facilitating a more organized and modular code structure.

1. Inheritance:

• Classes support inheritance, allowing you to create new classes that inherit properties and methods from existing ones. This enables code reuse and the creation of hierarchies of related classes. In contrast, structures do not support inheritance.

1. Complex Data and Behavior:

• If your custom data type needs to have complex behavior, such as performing calculations, maintaining state, or interacting with other objects, classes are more suitable. You can define methods within classes to handle such behavior.

1. Object Instances:

• Classes allow you to create multiple instances (objects) of the same class. Each instance can have its own state and behavior, making it easier to manage multiple instances of a particular data type. Structures, on the other hand, are typically used for simple data storage and cannot have multiple instances with individual state.

1. Size and Copying:

• Structures are generally more memory-efficient than classes because they are value types and do not involve dynamic memory allocation. When you pass a structure to a function or assign it to another variable, a copy of the entire structure is made. Classes, being reference types, do not create copies by default; they work with references, which can be more efficient in certain scenarios.

1. Performance:

• For very lightweight data types, structures may offer better performance due to their simplicity and value-type nature. However, this performance gain is usually negligible in most MQL5 applications.

In summary, you should use a class in MQL5 when you need encapsulation, object-oriented features, complex behavior, and the ability to create multiple instances of a custom data type. Use a structure when you need a simple, lightweight data container with minimal overhead. Your choice between classes and structures should be based on the specific requirements of your trading algorithm or program.