Mahendra Dani

What are copy constructors?

oops

Aug 6, 2025 at 17:00 pm

There are three ways of initializing objects in C++:

  1. No-argument constructor
  2. Parameterized constructor
  3. Copy constructor

When an object is initialized using another object of the same class, copy constructors are used.

class Distance {
  private:
    int inches;
    float feet;
 
  public:
    // No argument constructor
    Distance() : inches(0), feet(0.0){}
 
    // Parameterized constructor
    Distance(int in, float ft) : inches(in), feet(ft){}
 
    // Copy constructor
    Distance(const Distance& other){
      this->inches = other.inches;
      this->feet = other.feet;
    }
}
int main(){
  Distance dist1; // Invokes no argument constructor
  Distance dist2(dist1); // Invokes parameterized constructor
  Distance dist3 = dist1; // Invokes copy constructor
}

In a copy constructor, the parameter is typically a constant reference to an object of the same class. This ensures that when an object is initialized using the copy constructor, a deep copy of the source object is created.

A deep copy involves allocating new memory for the copied object and then duplicating the values from the original object into this newly allocated space, rather than simply copying the reference. This way, the new object maintains its own independent copy of the data.

Question: If we overload the assignment operator in the above class, what will happen?

class Distance {
  private:
    int inches;
    float feet;
 
  public:
    // No argument constructor
    Distance() : inches(0), feet(0.0){}
 
    // Parameterized constructor
    Distance(int in, float ft) : inches(in), feet(ft){}
 
    // Copy constructor
    Distance(const Distance& other){
      this->inches = other.inches;
      this->feet = other.feet;
    }
 
    // overloading assignment operator
    Distance& operator=(Distance& other){
      this->inches = 1 + other.inches;
      this->feet = 1 + other.feet;
      return *this;
    }
}
int main(){
  Distance dist1;
  Distance dist2 = dist1; // Invokes copy constructor
  
  Distance dist3(3,1.5);
  dist2 = dist3; // Invokes overloaded assignment operator method
}

During initialization of an object using another object copy constructor will be invoked, though using assignment operator(=) is confusing. Once the object is initialized, then for every use of assignment operator, the corresponding overloaded operator function will be called.

You can also find the code snippets here.