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<h3 class="pb-3 pt-3">Constructors in C++</h3>
<p>In this tutorial, we are going to learn about constructors.
</p>
<p><strong>Constructor</strong> is a special member function of a class, which is executed
whenever we create new objects of that class.</p>
<p>A constructor have a very different syntax as compared to other
member functions.</p>
<pre>class students {
public:
// create a constructor
students() {
// code
}
};</pre>
<p>You may have noticed in the above code that the constructor have
same name as its class i.e <strong>students</strong> .
</p>
<p>Also a constructor don’t have any return type, not even <strong>void</strong> .</p>
<img src="images/photo1__constructors.jpg" alt="photo1_constructors" height="400px">
<hr/>
<h4 class="pt-3 pb-3">Default constructor</h4>
<p>A default constructor do not contain any parameters, like the
constructor <strong>student()</strong> in the above example.</p>
<p>Following example shows how a default constructor invoked.</p>
<p><strong>Example:</strong></p>
<pre>#include <iostream>
using namespace std;
class students
{
public:
students()
{
cout<<"Default Constructor Invoked"<<endl;
}
};
int main()
{
students s1,s2;
return 0;
} </pre>
<p><strong>Output:</strong></p>
<pre>Default Constructor Invoked
Default Constructor Invoked
</pre>
<p><strong>Explanation:</strong>: in above example, we have a default constructor
<strong>students()</strong>, having a line of code in it. When we created two objects <strong>s1</strong>
and <strong>s2</strong> in main(), the constructor got automatically called and executed
two times without being called explicitly.</p>
<hr/>
<h4 class="pt-3 pb-3">Parameterized Constructor
</h4>
<p>In C++, we can also have parameters in a constructor known as
<strong>Parameterized constructor</strong>.</p>
<p>Look at the following example
</p>
<p><strong>Example:</strong></p>
<pre>#include <iostream>
using namespace std;
class students {
public:
string name;
int roll_no;
char grade;
students(string n,int r, char g)
{
name = n;
roll_no = r;
grade = g;
}
void display()
{
cout<< name <<" "<< roll_no<<" "<< grade<< endl;
}
};
int main(void) {
students s1 =students(Sonoo,89,'A');
students s2=students("Monoo",50,'B');
s1.display();
s2.display();
return 0;
} </pre>
<p><strong>Output:</strong></p>
<pre>Sonoo 89 A
Monoo 50 B</pre>
<p>in above example, the constructor <strong>students</strong> is now parameterised
hence we can pass values in it and get the result without calling them
explicitly, just by creating objects.</p>
<p><strong>Note:</strong>If we have not defined a constructor in our class, then the C++
compiler will automatically create a default constructor with an empty
code and no parameters.</p>
<hr/>
<h4 class="pt-3 pb-3">Copy Constructor
</h4>
<p>A copy constructor is a member function which initializes an object
using another object of same class.</p>
<p>The syntax for copy constructor is given below.</p>
<pre>class students{
... .. ...;
public:
students(students &obj){ //copy constructor
... .. ...;
}
};</pre>
<p><strong>Syntax:</strong></p>
<p><strong>className targetObject(sourceObject);</strong></p>
<p>or</p>
<p><strong>className targetObject=sourceObject;</strong></p>
<p>lets understand it with help of an example.</p>
<p><strong>Example:</strong></p>
<p>In this program, we have a parameterized constructor and a copy
constructor.</p>
<pre>class students{
int roll_no;
char grade;
public:
students(int a, char b){ //parameterized constructor
roll_no=a;
grade=b;
}
students(students &t){ //copy constructor
roll_no=t.roll_no;
grade=t.grade;
}
int display(){
cout<<"roll no : "<< roll_no << endl;
cout<<"grade : "<< grade<< endl;
return 0;
}
};
int main(){
cout<<"parameterized constructor"<< endl;
students s(12,'A');
//object created and value passed to parameterized constructor
s.display();
cout<<"copy constructor"<< endl;
students c(t);
//object created and value passed to copy constructor
c.display();
return 0;
}</pre>
<p>In this program , first we are passing values to parameterized
constructor by creating an object, <strong>S</strong>. then we are passing the same
object to a copy constructor by creating its object, <strong>C</strong>. Hence, values of
object <strong>S</strong> got copied in <strong>C</strong>.
</p>
<p><strong>Output:</strong></p>
<pre>parameterized constructor
roll no : 12
grade : A
copy constructor
roll no : 12
grade : A
</pre>
<p><strong>When we need copy constructor ?</strong></p>
<p>In C++, a Copy Constructor may be called in following cases:</p>
<ol>
<li>. When an object of the class is returned by value.</li>
<li>When an object of the class is passed (to a function) by value as an
argument.</li>
<li> When an object is constructed based on another object of the same
class.</li>
<li>When the compiler generates a temporary object.</li>
</ol>
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