August 2019

# Most frequent questions on programming job interviews

Thu
29
Aug 2019

There are different questions that appear on job interviews. Good preparation to such interviews is an art on its own, just like making a good CV, and there are whole books and trainings about it. A potential employer usually asks for details of what is in your CV - your past experience (especially previous 1 or 2 jobs) and why did you leave your previous job. He may ask you to solve some algorithmic puzzles to test your general “intelligence”. He may even ask some ridiculous questions like “Try to estimate how many litres of paint is used every year to renovate buildings in London?”, believing that your response and your way of thinking will be some estimate of whether you will be a good employee. But most of the interviewers also ask some real, technical questions, checking your knowledge from the domain you will work with. Out of these, I observed few questions occurring most frequently. Here they are, together with a draft of the correct response:

1. About C++

1.1. What does inline keyword do? Answer: It means the function should be inlined by the compiler in each place where it is called instead of leaving the function call in the compiled code. It’s only a hint, modern compilers decide which functions to inline on their own.

1.2. What does virtual keyword do? How does it work? Answer: It lets you implement polymorphism when using class inheritance. When an object of a derived class is passed by a pointer or a reference to a base class, calling such a function will call the version from the derived class - the one that the passed object “really is”. It works thanks to each object having an additional, hidden pointer to the virtual function table, which dispatches calls to such functions to the appropriate version.

1.3. What does volatile keyword do? Answer: It tells the compiler that a variable marked with this modifier can change value out of its control (e.g. by a different thread, operating system, or hardware) so it should not optimize its access by caching its value in CPU registers, assume it won’t change for some period etc., but use it in its original location every time. Side note: This keyword is rarely used. It probably appears more frequently on job interviews than in a real code :)

1.4. What does mutable keyword do? Answer: It allows to change a class member variable marked with this modifier even if the member function and the current object is const. It might be useful e.g. to perform lazy evaluation - object is passed as const& and appears to be unchanged, but its methods GetSomething() calculates and caches the value of “something” on first call. Side note: This keyword is rarely used. It probably appears more frequently on job interviews than in a real code :)

2. About general programming

2.1. What is the difference between process and thread? Answer: Process is launched from a particular executable file. It has its own address space (memory heap is common for the whole process), handles to open files, network sockets etc., separated from other processes in the operating system. It comprises a main thread and may have several additional threads. Each thread shares the same code and memory heap, but it has its own stack, instruction pointer (the place in the code currently executed), and values of CPU registers.

2.2. What is a mutex? Answer: It’s a synchronization object that allows multiple threads to access a common resource safely. If only one thread can access the resource, the code accessing it (called a critical section) has to be surrounded by locking the mutex above it and unlocking the mutex below it. Then only one thread will be able to execute that code in any moment. Other threads have to wait.

2.3. What is a deadlock? How to prevent it? Answer: Deadlock is an error in multithreaded code occurring when two or more threads wait for each other and will never make progress. To prevent it, always remember to unlock your mutexes (even when doing early return, break, throwing exception etc.) Also, when locking multiple mutexes, always lock them in the same order, never like: thread 1 locking A then B, thread 2 locking B then A.

3. About graphics programming

3.1. Describe graphics pipeline in modern GPUs. Answer: Depending on how detailed you want to describe it, you can tell that data is processed through following stages: vertex fetch / input assembler (fetching vertices/triangles) --> vertex shader --> optional tessellation (hull shader aka tessellation control shader --> fixed tessellator --> domain shader aka tessellation evaluation shader) --> optional geometry shader --> triangle clipping and culling (incl. viewport culling, backface culling etc.) --> rasterizer (converting triangles to pixels) --> pixel shader aka fragment shader --> depth-stencil test --> writing to render targets with blending.

3.2. What’s the difference between forward and deferred shading? Answer: In traditional forward shading, each object is rendered already shaded by each affecting light. In deferred shading, objects are rendered only once, with their parameters stored in intermediate render targets called G-buffer - like albedo color (taken straight from color texture), normal, depth (allows to reconstruct full position), material parameters (like roughness and metalness in case of PBR). Then separate screen-space passes use these data to apply shading from particular lights. One could say forward shading has complexity of O*L, where O is a number of objects and L is a number of lights, while deferred shading has O+L, which is better for a large number of lights. But deferred shading has its drawbacks: heavy G-buffers consume lots of bandwidth, the algorithm doesn’t work well with translucent objects, MSAA, or using different materials.

There are many more questions you may meet on programming job interview, whether from the topics described above (e.g. from advanced C++ - what is RAII, what is SFINAE), or any other topics important for a specific position, but from my experience those mentioned above are especially frequent, so preparing good answers to them may be the best investment of your time before the interview.

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