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What's the difference between assignment operator and copy constructor?

Posted by: admin November 30, 2017 Leave a comment

Questions:

I don’t understand the difference between assignment constructor and copy constructor in C++. It is like this:

class A {
public:
    A() {
        cout << "A::A()" << endl;
    }
};

// The copy constructor
A a = b;

// The assignment constructor
A c;
c = a;

// Is it right?

I want to know how to allocate memory of the assignment constructor and copy constructor?

Answers:

A copy constructor is used to initialize a previously uninitialized object from some other object’s data.

A(const A& rhs) : data_(rhs.data_) {}

An assignment operator is used to replace the data of a previously initialized object with some other object’s data.

A& operator=(const A& rhs) {data_ = rhs.data_; return *this;}

You could replace copy construction by default construction plus assignment, but that would be less efficient.

(As a side note: My implementations above are exactly the ones the compiler grants you for free, so it would not make much sense to implement them manually. If you have one of these two, it’s likely that you are manually managing some resource. In that case, per The Rule of Three, you’ll very likely also need the other one plus a destructor.)

Questions:
Answers:

The first is copy initialization, the second is just assignment. There’s no such thing as assignment constructor.

A aa=bb;

uses the compiler-generated copy constructor.

A cc;
cc=aa;

uses the default constructor to construct cc, and then the *assignment operator** (operator =) on an already existing object.

I want know how to allocate memory of the assignment constructor and copy constructor?

IDK what you mean by allocate memory in this case, but if you want to see what happens, you can:

class A
{
public :
    A(){ cout<<"default constructor"<<endl;};
    A(const A& other){ cout<<"copy constructor"<<endl;};
    A& operator = (const A& other){cout <<"assignment operator"<<endl;}
};

I also recommend you take a look at:

Why is copy constructor called instead of conversion constructor?

What is The Rule of Three?

Questions:
Answers:

The difference between the copy constructor and the assignment operator causes a lot of confusion for new programmers, but it’s really not all that difficult. Summarizing:

  • If a new object has to be created before the copying can occur, the copy constructor is used.
  • If a new object does not have to be created before the copying can occur, the assignment operator is used.

Example for assignment operator:

Base obj1(5); //calls Base class constructor
Base obj2; //calls Base class default constructor
obj2 = obj1; //calls assignment operator

Example for copy constructor:

Base obj1(5);
Base obj2 = obj1; //calls copy constructor

Questions:
Answers:

the difference between a copy constructor and an assignment constructor is:

  1. In case of a copy constructor it creates a new object.(<classname> <o1>=<o2>)
  2. In case of an assignment constructor it will not create any object means it apply on already created objects(<o1>=<o2>).

And the basic functionalities in both are same, they will copy the data from o2 to o1 member-by-member.

Questions:
Answers:

What @Luchian Grigore Said is implemented like this

class A
{
public :
    int a;
    A(){ cout<<"default constructor"<<endl;};
    A(const A& other){ cout<<"copy constructor"<<endl;};
    A& operator = (const A& other){cout <<"assignment operator"<<endl;}
};

void main()
{
    A sampleObj; //Calls default constructor
    sampleObj.a = 10;

    A copyConsObj  = sampleObj; //Initializing calls copy constructor

    A assignOpObj; //Calls default constrcutor
    assignOpObj = sampleObj; //Object Created before so it calls assignment operator
}

OUTPUT


default constructor


copy constructor


default constructor


assignment operator


Questions:
Answers:

Something to add about copy constructor:

  • When passing an object by value, it will use copy constructor

  • When an object is returned from a function by value, it will use copy constructor

  • When initializing an object using the values of another object(as the example you give).

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I want to add one more point on this topic.
“The operator function of assignment operator should be written only as a member function of the class.” We can’t make it as friend function unlike other binary or unary operator.

Questions:
Answers:

In a simple words,

Copy constructor is called when a new object is created from an existing object, as a copy of the existing object.
And assignment operator is called when an already initialized object is assigned a new value from another existing object.

Example-

t2 = t1;  // calls assignment operator, same as "t2.operator=(t1);"
Test t3 = t1;  // calls copy constructor, same as "Test t3(t1);"