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Static data member and member function in c++ with Example

Updated on June 13, 2016

1. What is Static data member?

The static data member of the class is useful to store information that is common to the class definition and this member does not belong to individual objects created from the class template. Simply speaking, all instances of a class share the same static data member declared in the class. When we talk about static data member we should be aware of the member’s scope and its visibility.

Scope tells where can you access the variables. Visibility tells about the variable’s lifetime. We will explore that here.

2. Code Example for Static Data Member

The example code is given below. Go through it, I will explain the code in the next section.

// TestIt.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include "conio.h"

class CRectangle
{
public:
	//Sample 01: Constructor
	CRectangle(int L, int W)
	{
		m_lenght = L;
		m_width = W;
		count++;
	}

	//Sample 02: Static Method
	static int GetCount()
	{
		return count;
	}

private:
	//Sample 03: Static count declaration with other declarations
	static int count;
	int m_lenght;
	int m_width;

};

//Sample 04: Initializing static member
int CRectangle::count = 0;

int _tmain(int argc, _TCHAR* argv[])
{
	//Sample 05: Create two Objects
	CRectangle R1(3,3);
	CRectangle* R2 = new CRectangle(4,4);
	
	//Sample 06: Print the count using the object name
	printf("Total Objects Created %d\n", R2->GetCount());

	//Sample 07: Print the count using Class Name
	CRectangle(1,3);
	printf("Total Objects Created %d", CRectangle::GetCount());

	//Sample 08: If you try accessing the count directly,
	//you get error as the static member is still
	//respects the scope private.
	// CRectangle::count => Even though the count 
	// can be accessed by using ::, 
	// the private scope prohibits that
	// printf("Total Objects Created %d\n", CRectangle::count );

	getch();
}
private:
	//Sample 03: Static count declaration with other declarations
	static int count;
	int m_lenght;
	int m_width;

3. Static Data member Code Explanation

In the class CRectangle, we have three member variables declared under the private scope. In these declarations, we declared a static variable count also.


//Sample 01: Constructor
CRectangle(int L, int W)
{
	m_lenght = L;
	m_width = W;
	count++;
}

In the constructor, we initialise the normal member variables and increment the static variable “count”. As this count is a static variable, the count gets increased by one whenever the object is constructed.

	//Sample 02: Static Method
	static int GetCount()
	{
		return count;
	}

Then we placed a static method GetCount to return the count. Note that this method is in public scope and hence it can be accessed from outside the class. When we have the method as static, we can access only the static members. You know why? The static method can be accessed without creating the instance of the class object. This static method can also be accessed like CRectangle::GetCount. As the static function scope is public, it can be accessed anywhere (i.e.) inside the member or outside of class

When a static member is private, even though it can be accessed without creating the object, the restriction is that it can be accessed only by the member function of the class.

//Sample 04: Initializing static member
int CRectangle::count = 0;

Once the class declaration is over, the static member is initialized. This initialization happens when the program starts executing or I can say the initialization happen before the program enters the main method. This declaration is shown aside:

//Sample 05: Create two Objects
CRectangle R1(3,3);
CRectangle* R2 = new CRectangle(4,4);

In the main function, we are creating two CRectangle objects. So the constructor gets called twice (i.e.) once for each object that we created. We have two objects R1, and an Object pointed by R2. Code is aside:

Source

There are two copies of m_length and m_width. But at the same time, there is only one copy of the member count for both R1 and Object pointed by R2. Now remember, when we create the two objects, the constructor gets called for both the object. As there is only one copy of count that can be accessed by both the objects, the count incremented twice from zero.

//Sample 06: Print the count using the object name
printf("Total Objects Created %d\n", R2->GetCount());

The statement below prints two in the console output. Also, note that we are accessing the static method through a pointer to member ->. If we use the R1 object you can access the static method like R1.GetCount().

//Sample 07: Print the count using Class Name
CRectangle(1,3);
printf("Total Objects Created %d", CRectangle::GetCount());

In the below code, we called the static method through class name. The stuff we should note here is that the GetCount declared as static in the class helps us to call the method with the class name and scope resolution operator.

//Sample 08: If you try accessing the count directly,
//			 you get error as the static member is still
//			 respects the scope private.
// CRectangle::count => Even though the count can be accessed by using ::, 
// the private scope prohibits that
// printf("Total Objects Created %d\n", CRectangle::count );

When we try the below code in our example, we get a compiler error. Accessing the count through class name (CRectangle::count) is not possible in our example because the static count is under the private scope. You can uncomment the below code, move the static count variable to a public section in order to compile the code. So even though the static can be accessed through the class name without creating the object, the variable still respects the class scope private.

The output of running the Example is given below:

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