Static In C++

Importance And Use Of Static In C++ With Examples.

In our previous topics on storage classes, we were introduced to the word static. We learned about static variables that are declared in a C++ program. We know that static variables are initialized only once and they retain the value throughout the program.

Similar to the static variables, in this tutorial, we will extend the use of a static keyword to:

• Static member variables in a class
• Static class objects
• Static methods class

=> Check The In-Depth C++ Training Tutorials Here.

Static Member Variables In A Class

A static variable is never allocated on a stack. They are allocated space on different static storage. This means that when we declare a static variable in a class, this variable is shared by all the objects of that class.

As static variables are initialized only once and are shared by all objects of a class, the static variables are never initialized by a constructor. Instead, the static variable should be explicitly initialized outside the class only once using the scope resolution operator (::).

When the first object is created, all static data of primitive type is initialized to zero when no other initialization is present.

Check the following example that demonstrates the static variable in a class.

As shown in the below code, we have the static variable count as a member of the class sample. Note that we have initialized this variable explicitly outside the class with the initial value = 0;

Then we increment this static variable in the constructor of the class.

Let us look at an example program.

#include <iostream>
#include <string>
using namespace std;
class sample{
   int var;
   static int count;
 
   public:
   sample(int var):var(var){
      cout<<"Count = "<<count<<endl;
      count++;
   }
};
int sample::count = 0;                 //static variable initialisation
int main()
{
   sample s(2), s1(10),s3(1);
}

Output:

Count = 0
Count = 1
Count = 2

In the main function, we create three different objects. In the output, we see that the value of the static variable is maintained between the object creations and not reset with every object creation. This for the first object, count = 0. Then it’s incremented to 1. For the next object the count = 1 and so on.

If the count was any ordinary variable, then the output would have been:

Count = 0
Count = 0
Count = 0

Static Class Objects

Just like static member variables of class, we can declare class objects as static. Static class objects are also initialized only once and remain active throughout the program. As the object is a user-defined type, a static class object is initialized similarly to the ordinary objects using a constructor.

Let us take a programming Example to better understand static class objects.

#include <iostream>
using namespace std;
class xyz
{
   int i;
   public:
   xyz() {
      i=0;
      cout << "Constructor::xyz"<<endl;
   }
   ~xyz() {
      cout << "Destructor::xyz"<<endl;
   }
};
int main()
{
   int x=0;
   if(x==0){
      static xyz obj;
   }
   cout << "End Main"<<endl;
}

In this program, we have a class xyz with a constructor and a destructor. In the main function, we declare a variable x = 0; If x is equal to zero, we create a static object of class xyz.

The program gives the following output.

Output: 

Constructor::xyz
End Main
Destructor::xyz

Normally the output should have been

Constructor::xyz
Destructor::xyz
End Main

But as we create a static object, this object has a scope until the end of the program and not when the object goes out of the scope (end of if statement). This is the reason, for which the destructor for object obj executes only after the end of the main function is reached.

Static Methods In A Class

We can also have static methods in a class. Just like static objects and static member variables, static member functions also have scope until the program execution ends.

When a class method is declared static, it can only access static members’ i.e. static variables and static functions of the class. It cannot access ordinary members of the class.

Also, there is no “this” pointer available for static class methods.

We are allowed to use the object and the dot operator to access the static methods of a class but it’s recommended to use the class name and the scope resolution operator to access these methods.

Below is an example of using a static method in a class.

In this example, we defined two static member variables A and B, and a static method printValues. The variables A and B are initialized to values 10 and 20 respectively. In the static method printValues, values of A and B undergo post Increment and pre Increment respectively. After that, the values are printed.

In the main method, we directly call the static method printValues using the class name as we do not need any object to invoke the static functions.

#include <iostream>
using namespace std;
class Sample
{
   static int A;
   static int B;
   public:
   static void printValues(){
      A++;
      ++B;
      cout <<"Value of A: " << A << endl;
      cout <<"Value of B: " << B << endl;
   }
};
int Sample :: A =10;
int Sample :: B =20;
int main(){
   Sample::printValues();
  
   return 0;
}

Output:

Value of A: 11

Value of B: 21

The screenshot of the same output is given below.

So in the output, we see the values of both the static variables are changed as per the operations performed on them.

Purpose Of Static Functions

Having seen the various uses of keyword static in this tutorial, a question remains as to what is the purpose of static functions.

Purpose of static functions can be summarized as below:

• We use static functions when that function does not depend on the object for invoking and working.
• Yet another purpose of using static function is to limit its use. Unlike global functions, access to static functions is limited to the file they are placed in. Thus in order to limit the access to function, we make it static.
• Apart from the above two reasons, we use static functions when we do not want to create an object of a class just to execute a function that is not referring to any class members.

Conclusion

To conclude this topic, we can say that static keyword in C++ can be used in various ways to declare variables, member variables, class objects, methods, etc.

Static member functions and variables need not be accessed with the object, rather they can directly be accessed using the class name. Also, the scope of static entities remains throughout the execution of the program. Hence static keyword can also be used to control the access of a particular entity.

In our upcoming tutorials, we will learn more about several other OOP topics in C++.

=> Check Here To See A-Z Of C++ Training Tutorials Here.