C++ Interview Questions & Tips
C++ Interview Stats
We've hosted over 100k interviews on our platform. C++ was the language of choice in those interviews 9% of the time, and engineers who did their interviews in C++ passed them 58% of the time.
Below is a distribution of programming languages and their popularity in technical interviews as well as success rates in interviews, by language.
C++ Idioms & Idiosyncrasies
As a powerful and complex language, C++ boasts a vast feature set and low-level access – but with great power comes great responsibility 🕸️. It’s a language that allows both procedural and object-oriented programming and supports generic and metaprogramming. Some amount of functional programming is even possible with the "newer" language features of lambdas and the std::function wrapper. It’s a language and culture with a fair amount of formality, especially regarding the type systems and memory management.
Candidates and interviewers may vary wildly in familiarity with C++’s class structure, templates, and other internals. The most important recommendation we can make is to make sure that if you’re using C++-specific terms to describe what you’re doing, you genuinely understand what they mean. It’s not required that you know all the details for an interview – and an understanding of all formal definitions is not obligatory – but you should be careful not to overreach, and you definitely shouldn’t attempt to gloss over your explanations, as C++ interviewers often appreciate precise and rigorous explanations.
With that in mind, here are some key, idiomatic concepts you'll want to be very comfortable with:
- Classes and Objects: C++ is an object-oriented language. Make sure you understand classes, objects, inheritance, polymorphism, and encapsulation.
- Templates: C++'s templates allow for code reuse and type safety and they are used in all modern production C++ codebases. Be comfortable with the syntax and usage of templates in classes and functions.
- STL (Standard Template Library): C++ provides a rich set of STL components like containers, algorithms, iterators, etc. Be familiar with common containers (like vector, set, map), algorithms (like sort, find), and iterators.
- Pointers and References: Pointers and references form a fundamental part of C++. Understand the usage, difference between pointers and references, and the concept of pointer arithmetic.
- Memory Management: Manual memory management is a significant part of C++. Understand dynamic memory allocation (new, delete) and the RAII (Resource Acquisition Is Initialization) idiom.
- Exception Handling: Understand how to use try, catch, and throw keywords for exception handling in C++. Know about the standard exception class hierarchy.
- Concurrency: Be familiar with the basics of multi-threading, synchronization primitives, and the C++11 threading library.
Common C++ Interview Mistakes
First and foremost, as with any language, you should NOT choose C++ as a way to show off if you don’t know it well. Make sure that you pick a language you are very comfortable with, have practiced interview questions with, and ideally have worked in recently (though not strictly required).
Outside of that, here are some things you should try to avoid when interviewing in C++ specifically.
Mismanaging Memory
C++ gives you manual control over memory. This can lead to problems if you don't manage memory correctly. For example, the following code results in a memory leak because memory allocated with new is never freed with delete:
int main() {
int* ptr = new int(5); // dynamically allocate integer
// do some stuff
return 0; // ptr goes out of scope and the memory is leaked
}
1
2int main() {
3 int* ptr = new int(5); // dynamically allocate integer
4 // do some stuff
5 return 0; // ptr goes out of scope and the memory is leaked
6}
7Not Leveraging the Standard Template Library (STL)
The STL provides a wide range of utilities such as containers and algorithms. Not using them or using them incorrectly can lead to inefficient code. For example, consider the following code to find a value in an unsorted std::vector:
int findInVector(const std::vector<int>& vec, int value) {
for (int i = 0; i < vec.size(); i++) {
if (vec[i] == value) {
return i;
}
}
return -1; // not found
}
1
2int findInVector(const std::vector<int>& vec, int value) {
3 for (int i = 0; i < vec.size(); i++) {
4 if (vec[i] == value) {
5 return i;
6 }
7 }
8 return -1; // not found
9}
10While it is a pride point in particular C++ developers to be able to build anything from scratch, you should show the interviewer you're a truly senior engineer that also knows when this isn't appropriate. It's tempting to write everything yourself, but in this case, this could be written much more simply and efficiently using std::find.
auto it = std::find(vec.begin(), vec.end(), value);
if (it != vec.end()) {
int index = std::distance(vec.begin(), it);
}
1
2auto it = std::find(vec.begin(), vec.end(), value);
3if (it != vec.end()) {
4 int index = std::distance(vec.begin(), it);
5}
6With the find method, while the implementation is similar to the code we could have written from scratch, we utilize iterators which reduce our choices of index-out-of bounds errors, we also follow best practices of code reuse rather than building something that wasn't necessary.
Overcomplicating Interviews With Advanced Features
While C++ offers many advanced features, such as metaprogramming and operator overloading, overuse can lead to unnecessarily complex and difficult-to-read code – especially for interviews! For example, unnecessary operator overloading:
class MySolution {
public:
MySolution(int value) : value(value) {}
MySolution operator+(MySolution other) {
// Unexpected behavior: adding 1 is not usually part of addition
return MySolution(value + other.value + 1);
}
private:
int value;
};
1
2class MySolution {
3public:
4 MySolution(int value) : value(value) {}
5 MySolution operator+(MySolution other) {
6 // Unexpected behavior: adding 1 is not usually part of addition
7 return MySolution(value + other.value + 1);
8 }
9private:
10 int value;
11};
12Ignoring Error Handling
C++ has robust error handling capabilities with exceptions, but ignoring them can lead to difficult-to-debug errors. Consider a function that fails to handle a potential division by zero error:
double divide(double numerator, double denominator) {
return numerator / denominator; // No error handling for division by zero
}
1
2double divide(double numerator, double denominator) {
3 return numerator / denominator; // No error handling for division by zero
4}
5It would be better to throw an exception when a division by zero occurs:
double divide(double numerator, double denominator) {
if (denominator == 0) {
throw std::invalid_argument("Denominator cannot be zero");
}
return numerator / denominator;
}
1
2double divide(double numerator, double denominator) {
3 if (denominator == 0) {
4 throw std::invalid_argument("Denominator cannot be zero");
5 }
6 return numerator / denominator;
7}
8How to Demonstrate C++ Expertise in Interviews
To impress your interviewer, delve deep into some of the core ideas behind the C++ type system, memory management, and STL.
More senior engineers who are already working in C++ daily and are involved in the community can demonstrate knowledge of the latest C++ standards and advanced features. Keep in mind, however, that this demonstration of mastery is only credible if it takes place on top of a strong foundation of fundamentals that you’ve already established earlier in the interview. Otherwise, you might come off as trying to obscure things – e.g. you can't code a binary search, but you're discussing how templates might be helpful in the problem? How does that make sense?
Feel free to discuss what you know of the language evolution and standards of C++. You can talk about how C++ has evolved over the years, and how the introduction of new standards (C++11, C++14, C++17, C++20) has brought significant improvements and features. Discuss how some of these features have influenced your coding practices or how they have resolved some of the common problems in C++ programming.
Lastly, above all, use C++ best practices – even though it is an interview! You should still be doing things like avoiding raw pointers, preferring pre-built STL algorithms over hand-written loops, writing exception-safe code, using const correctness, and following SOLID principles.
Remember, the ultimate goal is not really just to show that you know a lot about C++, but also that you can use the language effectively to write clean, efficient, and maintainable code.
C++ Interview Replays
Below you can find replays of mock interviews conducted on our platform in C++. The questions asked in these interviews tend to be language-agnostic (rather than asking about language-specific details and idiosyncrasies), but in these cases, the interviewee chose C++ as the language they would work in.
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