Freshers C++ Interview Questions

What is the difference between std::string and a char array in C++?

#include <cstring>
#include <iostream>
#include <string>

int main() {
    std::string s = "Hello";
    s += " World";
    std::cout << s << ' ' << s.size() << '\n';

    char c[] = "Hello";
    std::cout << sizeof(c) << ' ' << std::strlen(c) << '\n';      // 6 bytes (with \0), 5 characters
}

See C++ Strings.

What is the STL in C++?

The STL has three main parts:

• Containers: vector, list, map, set, unordered_map, stack, queue.
• Iterators: objects which point to the items of a container.
• Algorithms: sort, find, count, transform, accumulate and many more.

#include <algorithm>
#include <iostream>
#include <vector>

int main() {
    std::vector<int> v = {4, 1, 3};
    std::sort(v.begin(), v.end());
    for (int x : v) std::cout << x << ' ';
    std::cout << '\n';
}

See STL Containers and Algorithms.

What is the auto keyword in C++?

#include <iostream>
#include <map>
#include <string>
#include <vector>

int main() {
    auto n = 5;                                  // int
    auto d = 2.5;                                // double
    auto name = std::string("C++");
    std::vector<std::map<std::string, int>> data = {{{"a", 1}}};
    for (const auto& m : data) {                 // no long type name
        std::cout << m.size() << '\n';
    }
    std::cout << n << ' ' << d << ' ' << name << '\n';
}

• auto needs an initial value.
• auto removes const and references: use const auto& when you need them.

See C++ Variables.

What is the difference between const and constexpr in C++?

#include <array>
#include <iostream>

constexpr int square(int n) { return n * n; }

int main() {
    constexpr int size = square(3);               // calculated while compiling
    std::array<int, size> a{};
    const int input = 5;                          // const, set at the start
    std::cout << a.size() << ' ' << input << '\n';
}

See C++ Variables.

What is the static keyword in C++?

• Static local variable: keeps its value between calls.
• Static data member: one copy for the whole class.
• Static member function: can be called without an object, and has no this.
• Static at file level: the name is visible only in that file.

#include <iostream>

int next_id() {
    static int id = 0;           // set only once
    return ++id;
}

struct Counter {
    static inline int total = 0;
    Counter() { total++; }
};

int main() {
    std::cout << next_id() << next_id() << next_id() << '\n';
    Counter a, b;
    std::cout << Counter::total << '\n';
}

See Static Members.

What are access specifiers in C++?

class A {
private:
    int secret = 1;
public:
    int open = 2;
};

int main() {
    A a;
    int x = a.open;
    int y = a.secret;
}

See Access Specifiers and Encapsulation.

What is exception handling in C++?

Use throw to report an error and try ... catch to handle it.

#include <iostream>
#include <stdexcept>

int main() {
    try {
        throw std::runtime_error("something went wrong");
    } catch (const std::exception& e) {
        std::cout << "Caught: " << e.what() << '\n';
    }
}

See C++ Exceptions.

How to compile and run a C++ program?

#include <iostream>

int main() {
    std::cout << "Hello, C++!\n";
}

The steps inside are: preprocessing, compiling, assembling and linking. See C++ Installation and Setup.