C++ Constructors
Constructors
A constructor in C++ 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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Note: The constructor has the same name as the class, it is always public, and it does not have any return value.
Constructor 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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Just like functions, constructors can also be defined outside the class.
First, declare the constructor inside the class, and then define it outside of the
class by specifying the name of the class, followed by the scope resolution ::
operator, followed by the name of the constructor (which is the same as the
class):
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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