Classes in C++
Class:-
- Class is a collection of a characteristics and a behaviors.
- Characteristics means data members defined in the class and behaviors means a member functions/methods which are defined in the class and this methods are applied on to the characteristics of the class.
e.g.
class demo
{
public:
int i,j;
void fun()
{
cout<<”Inside the fun of demo”;
cout<<i;
cout<<j;
}
};
- In the above example, demo is our class which contains two characteristics as I and j. and both are integers.
- Behaviors of the demo class is a function fun() which applied on the data types i and j.
- To use above class we have to create an instance of that class means we have to create an object of that class.
e.g.
demo d;
or
class demo d;
- In the above syntax, we create an object of the class demo named as d.
- In the C++, writing a keyword class, struct and union while creating an object is optional but in C it is Compulsory.
Struct demo
{
int i,j;
};
Struct demo d; // This is the syntax which is used in C.
Demo d; // This is the syntax which is used in C++.
Memory allocation for the class:-
Class demo
{
public:
int i,j,k;
float f;
int fun()
{
cout<<”Inside the fun of demo”;
}
int gun()
{
cout<<”Inside the gun of demo”;
}
};
int main()
{
cout<<sizeof(demo); // 16
demo d;
cout<<sizeof(d); // 16
return 0;
}
- In this example, memory allocated for our class is 16 bytes, because, this class contains 3 integers and 1 float data type.
- This class also contains two functions namely as fun() and gun().
- When we create an multiple objects of same class then memory allocated for each and every object is 16 bytes.
- But there is no memory allocation for functions of a class, because, function definitions do not vary across the object but the data members of an objects vary across the object.
- Important things about the memory allocation of an object is that,
1) Memory allocation for data types of an object is in sequential manner that is in our example, first 4 bytes contains i, next 4 bytes contains j and so on.
2) Memory allocated for an object is only for non static data members of an object.
- We can call member function of the class by using “.” like,
d.fun();
- But if we have a pointer of that class then we can call the member function using arrow operator like,
e.g.
demo *p = new demo();
p->fun();
- In this case, p is a pointer which is of the type of a class demo.
- We allocate a memory dynamically for object of the class demo and base address of that dynamically allocated memory is accepted inside a pointer p and from that pointer p, we can access the member functions as well as data members.
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