C++ For Interviews

Michael Sukkarieh

Introduction

• Michael Sukkarieh
• Currently SWE @ Google, Stadia Games & Entertainment team
• Previously McGill SE, part of GameDev McGill
• During degree have done tech interviews at:
• Ubisoft, EA, Activision, Riot Games, Autodesk, Microsoft, Google, Apple, NVIDIA,
• Bold == full time interview

Disclaimer

• Everything in this talk is from personal experience and is not affiliated/referring to how interviewing at Google works!
• I have never given an interview at Google, and am not interview trained (as of this talk)
• I will only be talking about C++ and C++ questions, you will also want to prepare for general algorithm type questions (esp for large companies)
• C++ questions like this are commonly only asked if the job will guaranteed to use C++ (ex. game dev)

Goals of the talk

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• Explain the major differences between C++ and C/Java
• Discuss aspects of C++ that are commonly asked during technical interviews
• Const correctness?
• Memory allocation (stack and heap)
• Bit twiddling
• Dynamic dispatching (i.e. virtual functions)
• Hopefully make C++ less intimidating and more interesting

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History

• First introduced in 1985, developed by Danish computer scientist Bjarne Stroustrup
• Developed to combine OO with the speed and power of C
• Was first called “C with classes”, but was later renamed to C++ as a play on words of the ++ increment operator in C
• Further updates to the language have been made in 2011 (C++11), 2014 (C++14), and 2017 (C++17), and now 2020 (C++20???)

Industries using C++

• Any industry requiring high performance, efficient code:
• Video games (Ubisoft, EA, Bethesda, Rockstar, etc)
• Backend Infrastructure (Google, Microsoft, Amazon, etc)
• Financial Applications (Bloomberg, etc)
• Embedded Systems
• and much more

Hello World

1 - #include preprocessor command (similar to C)

3 - using namespace declaration (discussed later)

5 - main method entry point (similar to C)

6 - cout is a stream which outputs to stdout (similar to C or printf). Can also just use printf

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Basic Similarities (C like)

• Core programming tools (if, while loop, for loop, variable declaration) are the similar to C/Java (C++ is strongly typed, unlike languages like python or js)

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C++ vs C

C++ is Object Oriented

• Unlike C, C++ allows for object-oriented programming techniques
• Classes, inheritance, polymorphism, etc
• Classes have constructors (called during creation, like Java) and destructors (called during deletion)
• Class methods/variables have accessibility restrictions (similar to Java)

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new/delete instead of malloc/free

• C++ introduces two new keywords for dynamic memory allocation: new and delete
• Instead of using malloc to allocate memory on the heap, you can simply call new to do it (similar to Java)
• However, malloc can still be used and is more powerful than new and may be needed in some cases (ex. custom memory manager)

Generic Programming with Templates

• Similarly to Java, C++ allows for generic programming using templates
• Generic programming allows a developer to generalize a class definition to work with different data types (which is specified during instance creation)

Namespaces

• Namespaces allow you to create a new scope which identifiers can be defined in
• This allows you to have multiple different classes/methods be named the same, as long as they are defined in different namespaces, there are no conflicts
• Example: the C++ standard library is defined under the std namespace, and this library uses very common identifiers(

C++ standard library

• One of the main things that make using C annoying is that lack of standard library, but this is (mostly) not the case in C++
• The C++ standard library provides common data structures and algorithms
• strings (no need to always use char* !), vectors, queue, etc
• sorting, binary search, etc

C++ vs Java

Manual Memory Management

• C++ does not have a garbage collector, and therefore the developer is required to deallocate all memory they are using throughout the execution of the program
• If the developer fails to do this, a memory leak will occur: memory which is allocated on the heap but the developer has lost a reference to it!

Instance Allocation on the Stack

• In Java, every instance is required to be initialized (using new) before being used
• In C++, you are able to have a instance be allocated on the heap (using a pointer and new), or have the instance be allocated on the stack

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Smart Pointers Prereq: RAII

• Resource Acquisition Is Initialization (RAII) is a programming idiom commonly related to objects which have a primary purpose of holding a resource (ex. a container class) and was first introduced in C++ by Stroustrup
• For a class that follows RAII, acquiring the resource (Resource Acquisition) is required to initialize the object (Is Initialization)

Smart Pointers

• Smart pointers simulate a normal pointer (now called raw pointer), but also provide for automatic memory management using the RAII idiom
• Two main smart pointers used in C++: unique_ptr and shared_ptr
• unique_ptr is the only owner of the pointer (this is enforced using some concepts covered later). In other words, a unique_ptr object cannot be copied
• shared_ptr may be one of many owners of the pointer (i.e. copying a shared_ptr is allowed)
• Smart pointers allow the developer to clearly indicate ownership of a resource and transfer of ownership

Smart Pointer Example

Operator Overloading

• C++ allows you to redefine (“overload”) what the common operators (+, =, etc) do when used with your user defined data type (class)
• Extremely useful, especially once we get into more memory management techniques!

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C++ Interview Topics

Main Topics

• Memory allocation and management
• Working with bits (bit twiddling)
• Dynamic Dispatching

Memory Allocation and Management

• Understanding how memory allocation and proper management works is important, and is a common topic for interviews
• Things like:
• Stack vs Heap (including pointers)
• Working with bytes
• Virtual memory potentially

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Program Memory Layout

• Basic program memory layout (non-virtual configuration)
• Stack is usually allocated all at once for a function call, since we know the variables that will be allocated
• Heap is free memory which we can request memory from the OS

Program Memory Layout

• Example question: What is faster, allocating on the stack or on the heap?

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Program Memory Layout

• Example question: What is faster, allocating on the stack or on the heap?
• Answer: STACK. Allocation on the heap requires the OS to find a page (memory block), while allocation on the stack is well defined when entering a new call stack

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Program Memory Layout

• Example question: What is faster, allocating on the stack or on the heap?
• Answer: STACK. Allocation on the heap requires the OS to find a page (memory block), while allocation on the stack is well defined when entering a new call stack

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Working on the bit level

• Advanced techniques may require you to work with memory on a bit level (ex. masks, serialization, etc)
• Example: If you’re given a list of unsinged int’s (16 bits) but you want a list of unsigned char’s (8 bits), how would you do it?

Working on the bit level

• Advanced techniques may require you to work with memory on a bit level (ex. masks, serialization, etc)
• Example: If you’re given a list of unsigned int’s (16 bits) but you want a list of unsigned char’s (8 bits), how would you do it?
• Answer: Use a mask to extract info from the long int’s:

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Working on the bit level

• Advanced techniques may require you to work with memory on a bit level (ex. masks, serialization, etc)
• Example: If you’re given a list of unsigned int’s (16 bits) but you want a list of unsigned char’s (8 bits), how would you do it?
• Answer: Use a mask to extract info from the long int’s:

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Virtual Methods and Dynamic Dispatch in C++

• In Java, we can override superclass methods in the subclass and they will always be called if we have a instance of the subclass (even if it contained within a superclass reference!). In other words, java always adopts dynamic dispatching
• This is not the case with C++, by default static dispatching is used but we can force dynamic dispatching using the virtual keyword

Example

Virtual Method Example and Explanation

source: https://www.learncpp.com/cpp-tutorial/125-the-virtual-table/

• To implement dynamic dispatching, most C++ compilers will use a virtual table (vtable)
• Each class which has a virtual method, or is derived from a class that does, has a vtable and each instance of that class has a pointer to its vtable
• Therefore, whenever a virtual function is called we are able to index the vtable to find the proper method to call

Possible Questions

• Why would we want to set a destructor to be virtual?

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Possible Questions

• Why would we want to set a destructor to be virtual?
• Answer: If we have a deriving class which allocates it’s own memory that is freed in the destructor, the destructor of the parent must be virtual to ensure the proper memory is freed

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Possible Questions

• Why would we want to set a destructor to be virtual?
• Answer: If we have a deriving class which allocates its own memory that is freed in the destructor, the destructor of the parent must be virtual to ensure the proper memory is freed
• What is the overhead/cost of calling a virtual function?

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Possible Questions

• Why would we want to set a destructor to be virtual?
• Answer: If we have a deriving class which allocates its own memory that is freed in the destructor, the destructor of the parent must be virtual to ensure the proper memory is freed
• What is the overhead/cost of calling a virtual function?
• Answer: we incur an extra pointer dereference, because we need to lookup the function pointer in the vtable

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Misc C++ Concepts

References

• References can be used to create an alias for a variable (declared using the & symbol, similar to how pointers are declared using *)
• They are very powerful because they allow us to pass values by reference into functions, as well as prevent unnecessary copies of data

Passing by reference

Reference vs Pointer

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• References and pointers both allow us to pass objects in as reference and prevent unnecessary copies
• However, the main benefit of references is that they cannot be NULL (we don’t need to worry about dereferences a null pointer)
• The variable they are referring to also cannot be changed (unlike regular pointers)
• Usually, pointer parameters are used to indicate that the object passed in will be changed by the function, while const references are used simply for efficiency reasons

Const and Const Correctness

• const is a sometimes confusing, but very powerful and commonly used keyword in C++
• Anything declared as const is unchangeable (i.e. is constant)
• However, class methods can also be declared const, which “enforces” that the method does not change the internal state of the instance
• A class method must be declared const in order to be called by a const instance

Const Class Methods Example

Extra topics that I wish I could cover

• Moving vs Copying (move semantics, rvalue references)
• Move/Copy constructors
• default/delete
• Macros
• Smart pointers

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Resources

Questions?

Thank you!