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C++ Basics and Syntax Interview Questions for C++

25 C++ Basics and Syntax questions with answers for c++ roles — 13 easy, 10 medium, 2 hard.

About C++ Basics and Syntax

Start with how a C++ program is compiled and linked, then learn data types, operators, control flow, functions, overloading, namespaces, const and static. This topic also covers undefined behaviour and common pitfalls that interviewers use to test whether you really understand the language rather than just its syntax.

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01

What is C++ and how is it different from C?

C++ Basics and SyntaxEasy
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C++ is a compiled, statically typed, multi-paradigm language built on top of C. It adds classes, inheritance, templates, exceptions, namespaces, references, function overloading and a large Standard Library, while C is purely procedural. C++ keeps the low-level control and speed of C, which is why it is used in game engines, trading systems, browsers, databases, compilers and embedded firmware where performance and hardware access matter.
02

What is the difference between a declaration and a definition?

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A declaration tells the compiler that a name of a certain type exists, for example extern int count; or void f(int);. A definition actually allocates storage or provides the function body. You may declare a name many times but define it only once across the whole program, which is the One Definition Rule. Headers normally hold declarations and .cpp files hold definitions.
03

What are the fundamental data types in C++ and their typical sizes?

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The main types are bool, char, short, int, long, long long, float, double and long double. On a typical 64-bit Linux system char is 1 byte, short 2, int 4, long 8, long long 8, float 4 and double 8. The standard only guarantees minimum ranges, so check with sizeof, and use fixed-width types such as int32_t or uint64_t from <cstdint> when the exact size matters.
04

What is the difference between ++i and i++?

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++i increments the variable and returns the updated object, while i++ returns a copy of the old value and then increments. For an int the optimiser makes them equally fast. For iterators and class types i++ must create a temporary copy, so ++i is preferred in loops. Example: int a = 5; int b = a++; leaves b equal to 5 and a equal to 6.
05

A teammate added using namespace std; to a shared header file. What problems can this cause?

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Every file that includes the header now has all std names pulled into the global namespace. This can cause ambiguous calls or silently pick the wrong function when user code defines names like count, distance, min or data. The problem spreads to every dependent file and is hard to trace. I would remove it and use explicit std:: qualification or narrow using declarations inside .cpp files or function bodies.
06

What are default arguments and what rules apply to them?

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Default arguments supply values when a caller leaves out trailing arguments, for example void log(const std::string& msg, int level = 1);. Defaults must appear at the end of the parameter list and be given only once, usually in the header declaration. They are bound at compile time, so for a virtual function the default value comes from the static type of the pointer, not the dynamic type, which is a common trap.
07

What is the difference between struct and class in C++?

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The only language difference is default access. Members and base classes of a struct are public by default, while in a class they are private by default. Both can have constructors, member functions, inheritance and templates. By convention a struct is used for simple data aggregates such as a Point, and a class is used for types that protect invariants with private state.
08

Compare pass by value, pass by pointer and pass by reference.

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Pass by value copies the argument, so changes inside the function do not affect the caller. Pass by pointer passes an address, can be null and needs dereferencing. Pass by reference creates an alias that cannot be null and uses normal syntax. A good rule: pass small types such as int by value, large read-only objects by const reference, and use a non-const reference or pointer for output parameters.
09

Write a function to reverse a string in place without using std::reverse.

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#include <string> #include <utility> void reverseInPlace(std::string& s) { if (s.empty()) return; std::size_t i = 0, j = s.size() - 1; while (i < j) { std::swap(s[i], s[j]); ++i; --j; } } // Two pointers meet in the middle: O(n) time, O(1) extra space.
10

Write an efficient function to check whether a number is prime.

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bool isPrime(long long n) { if (n < 2) return false; if (n % 2 == 0) return n == 2; for (long long i = 3; i * i <= n; i += 2) { if (n % i == 0) return false; } return true; } // Only odd divisors up to sqrt(n) are checked: O(sqrt n) time.
11

Write a swap function using references and explain why a pass-by-value version fails.

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void swapByValue(int a, int b) { // works on copies, caller sees no change int t = a; a = b; b = t; } void swapByRef(int& a, int& b) { // a and b alias the caller's variables int t = a; a = b; b = t; } int x = 1, y = 2; swapByRef(x, y); // now x == 2, y == 1
12

Why did you choose C++ for your career instead of a language like Python or Java?

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Situation: in college I built a small game and a file compressor in Python and hit performance limits. Task: I wanted to understand what happens closer to the hardware. Action: I learned C++ properly, including memory management, STL and multithreading, and rewrote the compressor. Result: it ran about ten times faster and I understood systems much better. I enjoy work where that control matters, while I still use Python for scripting.
13

A program that prints a million lines with std::endl is very slow. Why, and how do you fix it?

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std::endl writes a newline and also flushes the stream buffer every time, forcing a system call per line. Replace it with the newline character so output is buffered and written in large chunks. For competitive programming or bulk output, also call std::ios::sync_with_stdio(false) and std::cin.tie(nullptr). Flush explicitly only when output must appear immediately, such as before a risky operation.
14

Explain the stages of compiling and running a C++ program.

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There are four stages. The preprocessor expands #include directives and macros into a translation unit. The compiler turns each translation unit into assembly and then an object file. The linker combines object files and libraries and resolves symbols into an executable. At run time the loader maps it into memory. A missing function body therefore shows up as a linker error, not a compiler error.
15

What is the difference between const and constexpr?

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const means a value cannot be modified after initialisation, but it may still be computed at run time. constexpr means the variable or function can be evaluated at compile time when its inputs are constant. With constexpr int sq(int x) { return x * x; } you can write int arr[sq(4)];. Use constexpr for array sizes, template arguments and lookup tables, and consteval when evaluation must happen at compile time.
16

What is function overloading and how does the compiler choose an overload?

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Overloading means several functions share a name but differ in the number or types of parameters; return type alone is not enough. The compiler runs overload resolution: exact matches win, then promotions such as char to int, then standard conversions, then user-defined conversions. If two candidates are equally good the call is ambiguous and compilation fails. Name mangling gives every overload a unique linker symbol.
17

What does the static keyword mean in different contexts?

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Inside a function, a static local variable is initialised once and keeps its value between calls. At namespace scope, static gives internal linkage, so the name is visible only in that translation unit. Inside a class, a static data member belongs to the class rather than each object, and a static member function has no this pointer. Since C++17 inline static members can be defined directly in the header.
18

What is the difference between an inline function and a macro?

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A macro is text substitution done by the preprocessor with no type checking, so #define SQ(x) x*x gives a wrong result for SQ(a+1) and may evaluate arguments twice. An inline function is a real function with type checking, scope and debugger support, and the compiler may expand it at the call site. In modern C++ inline mainly allows a function to be defined in a header without breaking the One Definition Rule.
19

The linker reports undefined reference to a function that is declared in a header. What could be wrong?

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The declaration exists but the linker finds no definition. Common causes: the .cpp file with the body is not compiled or not linked, the signature differs slightly such as const, namespace or parameter type, a template is defined in a .cpp file instead of the header, a C function is called without extern C linkage, or a library is missing from the link command or listed in the wrong order.
20

Two third-party libraries both define a global function called init and your build fails with duplicate symbols. How do you fix it?

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The clean fix is to place each library's code in its own namespace so calls become liba::init and libb::init. If the source cannot change, I would check whether one function is only used internally and give it internal linkage, or wrap one library behind a shared library that hides its symbols using visibility settings. I avoid macro renaming hacks because they break silently on upgrades.
21

What are lvalues and rvalues?

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An lvalue has identity and a persistent address, such as a named variable x. An rvalue is a temporary without lasting identity, such as 5 or the result of a + b. C++11 splits these further into lvalue, xvalue and prvalue. The distinction matters because rvalue references, written T&&, bind to temporaries and enable move semantics, letting the program steal resources instead of making expensive copies.
22

Write a C++ program that counts how often each word appears in the input.

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#include <iostream> #include <map> #include <string> int main() { std::map<std::string, int> freq; std::string word; while (std::cin >> word) { ++freq[word]; } for (const auto& [w, c] : freq) { std::cout << w << ' ' << c << '\n'; } } // Use std::unordered_map for O(1) average lookups if sorted output is not needed.
23

Tell me about a time you found a difficult bug in your C++ code.

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Situation: a service crashed randomly about once a day in production. Task: find the root cause without blocking releases. Action: I enabled core dumps, analysed them in gdb and saw a corrupted vector inside a callback. A staging run with AddressSanitizer showed a use-after-free because a lambda captured a raw pointer to a destroyed object. I changed it to capture a weak_ptr. Result: no crashes for months, and sanitizer runs were added to CI.
24

What is undefined behaviour in C++? Give examples.

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Undefined behaviour means the standard places no requirement on the result, so the program may crash, print garbage or appear to work. Examples are signed integer overflow, reading an uninitialised variable, out-of-bounds array access, dereferencing a null or dangling pointer and data races. Optimisers assume UB never happens and remove checks based on that, so such bugs often appear only in optimised release builds.
25

Your program gives correct output in a debug build but wrong output in a release build. How do you investigate?

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This usually means undefined behaviour or an uninitialised variable that the optimiser exploits. I would compile with -Wall -Wextra and fix every warning, then run tests with -fsanitize=address,undefined to catch out-of-bounds access, overflow and use-after-free. ThreadSanitizer checks for data races. Building with -O2 -g lets me debug the optimised code, and bisecting functions or adding assertions narrows down the faulty spot.

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How important is C++ Basics and Syntax in C++ interviews?

Start with how a C++ program is compiled and linked, then learn data types, operators, control flow, functions, overloading, namespaces, const and static. This topic also covers undefined behaviour and common pitfalls that interviewers use to test whether you really understand the language rather than just its syntax.

How should I use the C++ Basics and Syntax sheet?

Go top to bottom — easy to hard. Answer before opening each answer, tick what you solved and save misses to Last-Minute Revision.

More question sheets

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