Constructors

Constructors are member functions of a class or struct that initialize an object. The Core Guidelines define a constructor) as:

constructor: an operation that initializes (“constructs”) an object. Typically a constructor establishes an invariant and often acquires resources needed for an object to be used (which are then typically released by a destructor).

A constructor can take arguments, which can be used to assign values to member variables.

class Date {
 public:
  Date(int d, int m, int y) {  // This is a constructor.
    Day(d);
  }
  int Day() const { return day_; }
  void Day(int d) {
    if (d >= 1 && d <= 31) day_ = d;
  }
  int Month() const { return month_; }
  void Month(int m) {
    if (m >= 1 && m <= 12) month_ = m;
  }
  int Year() const { return year_; }
  void Year(int y) { year_ = y; }

 private:
  int day_{1};
  int month_{1};
  int year_{0};
};

As you can see, a constructor is also able to call other member functions of the object it is constructing. In the example above, Date(int d, int m, int y) assigns a member variable by calling Day(int d).

Default Constructor

A class object is always initialized by calling a constructor. That might lead you to wonder how it is possible to initialize a class or structure that does not define any constructor at all.

For example:

class Date { 
  int day{1};
  int month{1};
  int year{0};
};

We can initialize an object of this class, even though this class does not explicitly define a constructor.

This is possible because of the default constructor. The compiler will define a default constructor, which accepts no arguments, for any class or structure that does not contain an explicitly-defined constructor.