Swift iOS interview questions and answers – Part 4 – Memory management

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In this article, I have answered the questions related to one of the main concepts of Swift iOS interview which is memory management. I will cover some core concepts of memory management such as ARC, retain cycle, circular dependency, reference cycle, capture list, and capture of self, etc. Memory management is an important aspect of software development and is crucial for building performant and reliable applications in iOS. As a result, it’s often a topic covered in both senior and junior iOS developer interviews.

For junior developers, memory management questions can help to assess their understanding of basic programming concepts and their ability to write efficient and stable code. Junior developers may be asked questions about reference counting, ARC, strong and weak references, and how to avoid strong reference cycles.

For senior developers, memory management questions can help to assess their experience and expertise in writing performant and scalable applications. Senior developers may be asked questions about performance optimization techniques, such as profiling and heap analysis, and how to handle more advanced memory management challenges, such as managing memory in large and complex applications. In both cases, questions about memory management can help to identify the candidate’s ability to write efficient and maintainable code and their understanding of the fundamental concepts and practices of iOS development.

How can iOS handle memory management?

iOS uses ARC or Automatic Reference Counting to manage memories. ARC increase its retain count by 1 when an object has a strong reference. If there is no strong reference to it means retain count is zero, ARC will deallocate the memory.

What is ARC ?

ARC/Automatic Reference Counting is a compile time feature for memory management in iOS. While using ARC, it only deallocate the memory for the objects when there are zero strong references to them.

What is strong reference?

In iOS, a strong reference is a reference to an object that prevents the object from being deallocated as long as at least one strong reference exists. A strong reference increases the reference count of an object, and when the reference count reaches zero, the object is deallocated.

Strong references are the most common type of reference in iOS, and they are created whenever an object is assigned to a variable or property. For example, if you have a property declared like this:

myObject: MyClass?

you are creating a strong reference to an instance of MyClass whenever you assign an instance to the property. If there are multiple strong references to an object, each of those references will keep the object alive as long as at least one strong reference exists. When all strong references to an object are removed, the object is deallocated and its memory is freed.

In general, it’s recommended to use weak references when you don’t need to keep an object alive, and strong references when you need to keep an object alive. Overuse of strong references can result in memory leaks and other performance issues, as objects can accumulate in memory even if they are no longer needed by the application.

What is circular dependencies ?

When two or more modules depend on each other directly or indirectly to work properly create circular dependency. These kind of dependencies are also called mutually recursive.

What is a retain cycles?

Retain Cycle is the situation when two objects keep a reference to each other and are retained, it creates a retain cycle since both objects try to retain each other, making it impossible to release. This mostly happens in classes and closures. Closures live in memory, so when you use “self” which is a reference, you need to make sure that you solve the retain cycle.

What is a memory leak?

Memory leak happens when an object remains in memory even after its lifecycle has finished.
A memory leak is a portion of memory that remains allocated, but never used. You should avoid a strong to strong relationship that creates a retain cycle which generates memory leak.

What is the difference between retain and copy?

In case of retaining an object, We share the same version with whoever passed the object to us. Calling retain on an object will increase its retain count by one. When the retain count of an object reaches zero, the object will be deallocated and released from memory.

On the other hand when we copy an object, we will not share the same version of the object, we will simply make a duplicate of that object. A copied object will have a retain count of one just like a newly initialised object.

Explain strong reference cycle in closures.

In order for a closure to execute later, it needs to retain any variables that it needs for it to run. A closure captures references as strong by default like a class. If you assign a closure to a property of a class instance, and the body of that closure captures the instance then strong reference cycle can occur.

The strong reference can occur because the closure’s body accesses a property/method of the instance(for example self.testProperty, self.testMethod(), In both case, these kind of accesses cause the closure to capture self and create a strong reference cycle.)

What is capture list?

A capture list is the procedure to handle the references captured in it. By default, Everything will be strongly referenced when you don’t use capture list. Capture lists are defined with a weak or unowned keyword prefix. We need to define a capture variable as weak if it could become nil and the closure won’t be deallocated before then. We need to define a captured reference as unowned if it will never become nil before the closure is deallocated.

What is the difference between garbage collector and ARC?

ARC is technically a form of garbage collection but a deterministic solution: it is predictable as to when it will happen. The languages such as java and C# use garbage collector which is non-deterministic. That means you can’t tell exactly when objects are being reclaimed because it’s being managed by the runtime by an external process.

The main advantage of ARC is that it’s deterministic behaviour and predictable deconstruction. The objects are deallocated immediately when they are no longer needed.

However, ARC can not deal with reference cycles without developer intervention. On the other hand, the main advantage of garbage collector is that it can detect reference cycles.

What is reference counting in iOS?

Reference counting is a system in iOS that keeps track of the number of references to an object and automatically deallocates an object when there are no references to it. Every object in iOS has a reference count, which is increased whenever a reference to the object is created and decreased when a reference is removed. When the reference count reaches zero, the object is deallocated.

The benefit of ARC in iOS

ARC stands for Automatic Reference Counting, and it’s a system in iOS that automates the process of reference counting. With ARC, developers do not have to manually keep track of reference counts and manage memory manually. Instead, the compiler inserts the necessary memory management calls for you.

How does ARC handle weak references in iOS?

A weak reference is a reference to an object that does not increase its reference count. This means that an object can be deallocated even if there is a weak reference to it. In ARC, weak references are implemented using a zeroing weak reference, which automatically sets the reference to nil when the object it references is deallocated. This helps to avoid strong reference cycles, where two objects have strong references to each other, leading to a memory leak.

What is the relation between strong reference cycle and memory leak?

A strong reference cycle is a situation where two or more objects have strong references to each other, leading to a memory leak. Because neither object’s reference count can reach zero, they cannot be deallocated, and their memory remains allocated even if they are no longer needed. To avoid strong reference cycles, it’s important to use weak references when appropriate and to understand how to break reference cycles in your code.

What is manual memory management in iOS?

Manual memory management is a system in which developers are responsible for managing the memory of objects in their applications. This involves keeping track of reference counts, releasing objects when they are no longer needed, and using techniques like retain and release to manage the lifetime of objects. Manual memory management was used in earlier versions of iOS, but has since been replaced by ARC.

Fellow iOS Developers, Please Keep In Mind

  • It’s important to keep in mind a few key points as you prepare for your interview. Firstly, it’s worth noting that there are over 1000 interview questions available in the interview section for you to review and prepare for. While reading the question, take the time to carefully consider your answer and think about the information that you want to convey. The answer provided here in this blog can be explained in a different way. You should also prepare your examples.
  • It’s also important to remember that these interview questions are not meant to be difficult. The interviewer is not looking to challenge you, but rather to start a conversation that will allow your abilities and interests to come to the forefront. They want to get to know you and your experience better.
  • Finally, it’s crucial to avoid simply answering questions with a “yes” or “no.” Interviewers are looking for more in-depth responses that include basic understanding, reasoning, explanation, and examples. So, make an effort to elaborate on your answers and provide specific, relevant information to support your response. This will demonstrate your thoughtfulness and show the interviewer that you are well-prepared for the interview.

Swift iOS interview questions and answers

Swift iOS interview questions and answers – Part 1

Swift iOS interview questions and answers – Part 2

Swift iOS interview questions and answers – Part 3

Swift iOS interview questions and answers – Part 4

Swift iOS interview questions and answers – Part 5

Planning to apply for iOS job? Check out this article to uplift your resume! Happy job hunting!

✍️ Written by Ishtiak Ahmed

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