As we know now that the constructor is a special initialization member function of a class that is called automatically whenever an instance of a class is declared or created. The main function of the constructor is
(i) To allocate memory space to the object and
(ii) To initialize the data member of the class object

The definition of a class only creates a new user defined data type. The instances of the class type should be instantiated (created and initialized). Instantiating object is done using constructor. An array or a structure in c++ can be initialized during the time of their declaration. The initialization of class type object at the time of declaration similar to a structure or an array is not possible because the class members have their associated access specifiers (private or protected or public). Therefore Classes include special member functions called as constructors. The constructor function initializes the class object.

If an object is declared with in a function then it is called local object.
It cannot be accessed from outside the function.
# include < iostream >
# include < conio >
using namespace std
class add           //Global class
{
int a,b;
public:
int sum;
void getdata( )
{
a=5;
b=10;
sum=a+b;
}
} a1;                // global object
add a2;           // global object
int main( )
{
add a3;          //Local object for a global class
a1.getdata( );
a2.getdata( ):
a3.getdata( );
cout < < a1.sum;      //public data member accessed from outside the class
cout < < a2.sum;
cout < < a3.sum;
return 0;
}
Output:151515

Inside the class definition:
When a member function is defined inside a class, it behaves like inline functions. These are called Inline member functions.
Outside the class definition:
When Member function defined outside the class just like normal function definition (Function definitions you are familiar with) then it is be called as outline member function or non-inline member function.

A constructor having a reference to an already existing object of its own class is called copy constructor. In other words Copy Constructor is a type of constructor which is used to create a copy of an already existing object of a class type. It is usually of the form simple (simple &), where simple is the class name. The compiler provides a default Copy Constructor to all the classes.