C++Builder Learn Definition of the Default Constructor Outside of a Class Definition

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Learn Definition of the Default Constructor Outside of a Class Definition
By Yilmaz Yoru June 13, 2021

The Constructor in C++ is a function, a method in the class, but it is a ‘special method’ that is automatically called when an object of a class is created. We don’t need to call this function. Whenever a new object of a class is created, the Constructor allows the class to initialize member variables or allocate storage. This is why the name Constructor is given to this special method.

class myclass
{         
  public:   
    myclass()
    {
       std::cout << "myclass is constructed!\n";
    };
};

There are different contractor types and the Default Constructor in classes is one of these. This method is not only used in classes but also used with struct and union data types A Default Constructor is a constructor type in Classes that is called when class is defined with no arguments, or it is defined with an empty parameter list, or with default arguments provided for every parameter. A type with a public default constructor is Default Constructible;

Do you want to learn how we define a Default Constructor Outside of a Class? In the definition of a default Constructor, class_name must name the current class or current instantiation of a class template. It must be a qualified class name when declared at namespace scope or in a friend declaration. Here are examples;

Definition of the Default Constructor Outside of a Class Definition​

We can define a default constructor outside of a class. Add a class name and ( ); inside the class also add class name and :: operator with class name and ( ) outside of that class as below,

Syntax,

class_name :: class_name()
{
// statement
}

An example with a class;

class myclass  // Class Definition
{         
  public:   
    myclass(); // Pre-Definition of the Default Constructor
};
 
myclass :: myclass() // Definition of the Default Constructor Outside of a Class Definition
{
 
}

Here is a full example about how a Constructor defined outside of a class. Here we just create a new test object, and it automatically sets x=100 and calls constructor method.

#include <iostream>
 
class myclass
{
  public:
 int x = 0;
 myclass();
};
 
myclass :: myclass()
{
 x = 100;
 std::cout << "I am constructed\n";
};
 
int main()
{
 class myclass test;
 
 std::cout << test.x << '\n';
 
 getchar();
 
 return 0;
}

Note that here x is defined in the first class and it is used in the Definition of the Default Constructor Outside of a Class Definition. The output will be as here,

I am constructed
100