Structures in C++ is a collection of variables. Structures in C++ can be declared even without the keyword "struct". By default all the members of a structure are "public", even "private" members can also be declared in a function.

   struct struct-type-name{
       type name1: length;
       type name2: length;
       .
       .
       type nameN : length;
       }variable_list;

   #include <iostream.h>
   struct Emp
     {
       int empno;
       int empsal;
     };
   void main( )
     {
       Emp emp1= { 23, 12000};
       cout << "Employee Number::" << 
                         emp1.empno << '\n';
       cout << "Employee Salary:: "<< emp1.empsal;
     }

    Employee Number:: 23
    Employee Salary:: 12000

In the above example, the structure "Emp" is used initialize the integers, that are referenced in the "main()" function.

Unions in C++ is a user defined data type that uses the same memory as other objects from a list of objects. At an instance it contains only a single object.

   union union-type-name{
       type member-name;
       type member-name;
   
     }union-variables;

#include <iostream.h>
union Emp
  {                      
    int  num;       
    double sal;     
  };
int main()
  {
     Emp value;
     value.num = 2;
     cout << "Employee Number::" << value.num 
           << "\nSalary is:: " << value.sal << endl;
     value.sal = 2000.0;
     cout  << "Employee Number::" << value.num 
        << "\nSalary is:: " << value.sal << endl;
     return 0;
  }

    Employee number is::2
    Salary is::2.122e-314
    Employee number is::0
    Salary is::2000

In the above example, only "value.num" is assigned, but still the "val.sal" gets a value automatically, since the memory locations are same.