C++ Constructors

In earlier examples you created an object and then set each attribute one line at a time. A constructor lets you do all of that setup in a single step, right when the object is born. This makes your code shorter and guarantees every object starts in a valid state.

Writing a Constructor

A constructor has the exact same name as the class and has no return type, not even void. It is called automatically, so you never call it by name yourself.

#include <iostream>
#include <string>
using namespace std;

class Car {
  public:
    string brand;
    int year;

    Car() {                 // constructor
      brand = "Unknown";
      year = 2000;
    }
};

int main() {
  Car myCar;               // constructor runs here
  cout << myCar.brand << " " << myCar.year;
  return 0;
}

Constructors With Parameters

A constructor can take parameters, letting you pass in the starting values when you create the object. This is the most common and most useful form.

#include <iostream>
#include <string>
using namespace std;

class Student {
  public:
    string name;
    int score;

    Student(string n, int s) {   // constructor with parameters
      name = n;
      score = s;
    }
};

int main() {
  Student a("Maya", 92);       // pass values on creation
  Student b("Leo", 78);
  cout << a.name << ": " << a.score << endl;
  cout << b.name << ": " << b.score << endl;
  return 0;
}

• A constructor's name must match the class name exactly.
• A constructor has no return type.
• It runs automatically when an object is created.
• You can pass starting values through the constructor's parameters.