如何使用单一责任原则

Question

I am trying to understand design principles in c++.

In database I have users.

users:
   id
   name
   age
   gender

I want to get my users in three ways.

• First: I want all my users.
• Second: I want all my users filtered by age.
• Third: I want all my users filtered by age and gender.

For example, if I use same class for getAllUsers and getFilteredByAge, it means that my class has two responsibility, it is responsible for getting users and also for filtering them. Am I right or not? And how Single-Responsibility Principle works in this example, should I split this three in different classes, or is there any better way ?

Answer 1

A good definition for SRP is:

A module should be responsible to one, and only one, actor.

(Clean Architecture)

This means that if the person telling you what these functions do is the same, then you can leave them in the same class/module.

If, let's say, getAllUsers() is requested by accounting and getUserAtLeastThisOld(int minimumAge) is requested by HR, then it might be sensible to have them in separate classes.

Answer 2

Following are answers to your question

Q] If I use same class for getAllUsers and getFilteredByAge, it means that my class has two responsibility?

A] No,because your class's job is to get users, rather these functions should be overloads and should not be in different classes.

Q] it is responsible for getting users and also for filtering them. Am I right or not?

A] I guess No!, filtering is not a different task, it is something that should be applied before retrieving objects.

Q] how Single-Responsibility Principle works in this example, should I split this three in different classes, or is there any better way ?

A]

In this case I suggest you to have only one class, which should have following functions overloads

1. GetUsers() - get all users
2. GetUsers(AgeFilter) - get users as per age filter
3. GetUsers(AgeFilter, genderFilter) - get users as per age filter and gender filter

Note : Now suppose in future you have want to add more functionality to this class for eg compute salary for user, or adding family details for users then in such case you can go for creating another class instead of putting burden on single class...