C++ Constructors

🏗️ C++ Constructors

A constructor in C++ is a special class method that is automatically called when an object is created.
Its main purpose is to initialize the data members of a class.

🔹 1. Key Properties of Constructors

  • Name is same as class name

  • No return type (not even void)

  • Called automatically at object creation

  • Used to initialize objects

🔹 2. Default Constructor

A constructor with no parameters.

class Demo {
public:
Demo() {
cout << "Default Constructor Called";
}
};int main() {
Demo d;
}

🔹 3. Parameterized Constructor

A constructor that accepts parameters.

class Student {
public:
int id;
string name;Student(int i, string n) {
id = i;
name = n;
}
};

int main() {
Student s1(101, “Sanjit”);
}

🔹 4. Constructor Overloading

Multiple constructors in the same class with different parameters.

class Box {
public:
int length;Box() {
length = 0;
}

Box(int l) {
length = l;
}
};

🔹 5. Default Values Using Constructors

class Account {
public:
int balance;Account(int b = 0) {
balance = b;
}
};

🔹 6. Copy Constructor

Used to create an object from another object.

class Demo {
public:
int x;Demo(int val) {
x = val;
}

Demo(const Demo &d) {
x = d.x;
}
};

Called when:

Demo d1(10);
Demo d2 = d1; // copy constructor

🔹 7. Constructor with this Pointer

class Test {
public:
int x;Test(int x) {
this->x = x;
}
};

🔹 8. Constructor Initialization List (Best Practice)

class Point {
public:
int x, y;Point(int a, int b) : x(a), y(b) {}
};

✔ Faster
✔ Required for const members

🔹 9. Explicit Constructor

Prevents implicit conversions.

class Demo {
public:
explicit Demo(int x) {
cout << x;
}
};

🔹 10. When Constructor Is Not Written

The compiler provides a default constructor automatically if no constructor is defined.

❌ Common Mistakes

void Demo(); // ❌ not a constructor
Demo() { return; } // ❌ cannot return value

🔁 Constructor vs Destructor

ConstructorDestructor
Initializes objectCleans resources
Called at creationCalled at destruction
Can be overloadedCannot be overloaded

📌 Summary

  • Constructors initialize objects

  • Called automatically

  • Can be default, parameterized, copy

  • Support overloading

  • Initialization lists are recommended

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