C++ Constructors
Constructors
A constructor is a special method that is automatically called when an object of a class is created.
To create a constructor, use the same name as the class, followed by
parentheses ():
Example
class MyClass { // The class
public:
// Access specifier
MyClass() {
// Constructor
cout << "Hello World!";
}
};
int main() {
MyClass myObj; // Create an object of MyClass (this will call
the constructor)
return 0;
}
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Constructor Rules
- The constructor has the same name as the class.
- It has no return type (not even
void). - It is usually declared public.
- It is automatically called when an object is created.
Constructor with Parameters
Constructors can also take parameters (just like regular functions), which can be useful for setting initial values for attributes.
The following class have brand, model and year attributes, and a constructor with
different parameters. Inside the constructor we set the attributes equal to the
constructor parameters (brand=x, etc).
When we call the constructor (by creating an object of the class), we pass parameters to the constructor, which will set the value of the corresponding attributes to the same:
Example
class Car { // The class
public: // Access specifier
string brand; // Attribute
string model; // Attribute
int year; // Attribute
Car(string x, string y, int z)
{ // Constructor with parameters
brand =
x;
model = y;
year = z;
}
};
int main() {
//
Create Car objects and call the constructor with different values
Car carObj1("BMW", "X5", 1999);
Car carObj2("Ford", "Mustang",
1969);
// Print values
cout << carObj1.brand << " "
<< carObj1.model << " " << carObj1.year << "\n";
cout <<
carObj2.brand << " " << carObj2.model << " " << carObj2.year << "\n";
return 0;
}
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Constructor Defined Outside the Class
You can also define the constructor outside the class using the scope resolution operator ::.
Example
class Car { // The class
public: // Access
specifier
string brand; // Attribute
string model; // Attribute
int year;
// Attribute
Car(string x, string y, int z); //
Constructor declaration
};
// Constructor definition outside the
class
Car::Car(string x, string y, int z) {
brand = x;
model = y;
year = z;
}
int main() {
// Create
Car objects and call the constructor with different values
Car
carObj1("BMW", "X5", 1999);
Car carObj2("Ford", "Mustang", 1969);
// Print values
cout << carObj1.brand << " " <<
carObj1.model << " " << carObj1.year << "\n";
cout <<
carObj2.brand << " " << carObj2.model << " " << carObj2.year << "\n";
return 0;
}
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Why Constructors Are Useful
Constructors run by themselves when you create an object. They set things up so everything is ready right away.
Think of it like this: When you order a pizza (object), the constructor is the chef who adds the sauce, cheese, and toppings before it gets to you - you don't have to do it yourself!
