Definition of static data members in c++ : 

It is known as a class data members and declared by using the Static keyword. it is a single copy of the variable created for all objects in C++.

Declaration :

Static Data_type Data_member_name

Static: Static is a keyword

Datatype : data_type in variable type in C++ like : int, float

Data Member Name: data member name is a user defined name of the variable

Example

static int sum;

Define the value of static data member

Also Read This :- function overloading in c++

it should be declared outside of class like this:

data_type class_name :: static_data_member_name = value;

Example of Static Data Member

#include<iostream>
#include<string.h>
using namespace std;

class Student {
  private:
  int rollNo;
  char name[10];
  int marks;
  public:
  static int objectCount;
  Student() {
    objectCount++;
}
void getdata() {
  cout << "Enter roll number: "<<endl;
  cin >> rollNo;
  cout << "Enter name: "<<endl;
  cin >> name;
  cout << "Enter marks: "<<endl;
  cin >> marks;
}
void putdata() {
  cout<<"Roll Number = "<< rollNo <<endl;
  cout<<"Name = "<< name <<endl;
  cout<<"Marks = "<< marks <<endl;
  cout<<endl;
}
};

int Student::objectCount = 0;

int main(void) {
 Student s1;
 s1.getdata();
 s1.putdata();
 Student s2;
 s2.getdata();
 s2.putdata();
 Student s3;
 s3.getdata();
 s3.putdata();
 cout << "Total created Object of Student Class = " << Student::objectCount << endl;

 return 0;
}

output

Enter roll number:
1
Enter name:
AAA
Enter marks:
80
Roll Number = 1
Name = AAA
Marks = 80
 
Enter roll number:
2
Enter name:
BBB
Enter marks:
65
Roll Number = 2
Name = BBB
Marks = 65
 
Enter roll number:
3
Enter name:
CCC
Enter marks:
90
Roll Number = 3
Name = CCC
Marks = 90
 
Total created Object of Student Class = 3