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?
A copy constructor is used to initialize a previously uninitialized object from some other object's data.
A(const A& rhs) : data_(rhs.data_) {}
For example:
A aa;
A a = aa; //copy constructor
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;}
For example:
A aa;
A a;
a = aa; // assignment operator
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.)
=default;.
= default them where a default ctor is needed: simply implementing an empty body by ourselves still counts as a user-defined ctor and thus (on a Standardese level) isn't trivial and disqualifies the type from classifications that require a trivial ctor.
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:
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
vector <A> v3 and then v3 = v2 (where v2 is a previously declared and containing elements vector<A>) calls my explicit A's copy constructor instead the operator= ? I was expecting operator= to be called instead of the copy constructor because my v3 object was already declared at the time where I did the assignmentThe 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?
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);"
the difference between a copy constructor and an assignment constructor is:
<classname> <o1>=<o2>)<o1>=<o2>).And the basic functionalities in both are same, they will copy the data from o2 to o1 member-by-member.
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
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.
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).
