Topic 43 of 91 8. Object-Oriented Programming in C++ Interactive Tutorial

Constructors in C++: Initialization Techniques and Constructor Types

7 mins read Last updated: 26 Sep 2026 20,391 views

On This Page0 sectionsQuick jump to subtopic

Direct Answer & Core Concept

C++ supports Default constructors, Parameterized constructors, Copy constructors, Move constructors, and Delegating constructors. Member Initializer Lists (: member(val)) should always be preferred because they initialize data members directly upon construction rather than default-constructing and then reassiging them, which is essential for const members and references.

Detailed Explanation & Mechanics

1. Constructor Taxonomy:

Default: ClassName()
Parameterized: ClassName(int a, double b)
Copy: ClassName(const ClassName& other)
Move: ClassName(ClassName&& other) noexcept
Delegating: ClassName(int a) : ClassName(a, 0.0) {}

main.cpp

Run Online

MEMBER INITIALIZER LIST vs BODY ASSIGNMENT:
Initializer List: [Allocate + Initialize with Value] (1 operation)
Body Assignment:  [Allocate + Default Init] -> [Overwrite with Assignment] (2 operations + redundant copy)

2. `explicit` Keyword:

Prevents the compiler from performing unintended single-argument implicit type conversions.

Runnable Code Example: Constructors in C++

C++20 / C++23

main.cpp

Run Online

#include <iostream>
#include <string>

class UserSession {
private:
    const std::string sessionId;
    int userId;
    bool isActive;

public:
    // Delegating & Initializer List Constructor
    explicit UserSession(int id) : UserSession(id, "SESS-DEFAULT-000") {}

    UserSession(int id, std::string token)
        : sessionId(std::move(token)), userId(id), isActive(true) {
        std::cout << "Session initialized for User ID: " << userId 
                  << " [Token: " << sessionId << "]\n";
    }

    void print() const {
        std::cout << "User: " << userId << " | Active: " << (isActive ? "YES" : "NO") << "\n";
    }
};

int main() {
    UserSession s1(1042);
    UserSession s2(2099, "SESS-SECURE-999");
    s1.print();
    s2.print();
    return 0;
}

Common C++ Technical Interview Questions & Answers

Interview Prep

Q: Why must const members and reference members be initialized in the member initializer list?

A: Const variables and references cannot be uninitialized or reassigned after creation. They must be bound directly during member initialization before entering the constructor body.

Q: What does the `explicit` keyword on a constructor prevent?

A: It prevents single-argument implicit conversions and copy-initialization syntax (`MyClass c = 5;`), requiring explicit direct initialization (`MyClass c(5);`).

Key Takeaways, Modern Best Practices & Idioms

C++ Best Practices & Production Rules

• Mark single-argument constructors explicit unless intentional implicit conversion is desired.
• Always use member initializer lists instead of assigning inside the constructor body.
• Initialize members in the exact order they are declared in the class definition to avoid subtle ordering bugs.

Previous TopicDestructors in C++: Object Cleanup, Rules, and Execution Timing

Next TopicAccess Control in C++: Public, Private, and Protected Members