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Structures and Unions in C and C++ - Expalined with examples

Updated on November 20, 2016

1. Declaring a struct

A structure is a user-defined data type, which is nothing but a combination of predefined data types. A structure can be declared as follows:

struct dept
 int deptid;
 char deptname[20] ;
 float avgEmpSal;

2. What is structure in c++?

The structure name is specified after the struct keyword. Inside the structure, you can place the predefined data types and those together forms a user defined type. Usually, we call these predefined types inside the structure as data members of the structure.

The below statement creates a user-defined variable of type dept.

struct dept sales;

To access the members of the structure, use the period operator to the variable sales like as shown below:

sales.deptid = 101;

3. Structure in C++ example

Below is the example of using a structure:

#include "stdafx.h"

#include <stdio.h>
#include <conio.h>

struct dept
int deptid;
char deptname[20] ;
float avgEmpSal;

int _tmain(int argc, _TCHAR* argv[])
struct dept sales;
//struct dept purchase;
sales.deptid = 101;
sprintf(sales.deptname, "%s", "Sales Dept");
sales.avgEmpSal = 150000.45f;

printf("Printing the Structure Data");
printf("%s has spent %f$ for salary", sales.deptname, sales.avgEmpSal );

In this example, first, a structure called "dept" is declared with three data members of types int, char, and float. Note that the structure declaration ends with a semicolon.

In the main program, the structure is created using the statement "struct dept sales". Here dept denotes the user-defined type name and sales are the actual structure object that needs to be created.

In the remaining portion of the code, you can see how the structure members are accessed by using the period operator.

4. Structure Difference between C and C++

There some difference between C and C++ about how they treat structure and Union . In C a structure can contain only the data members. But in C++ they can have one or more Function(s) along with their data members.

In C++ when declaring the variable of type structure we no need to specify the keyword struct. Also, it is not required to have the typedef in the structure declaration. But, in C you have to specify the struct keyword before declaring the variable of a structure. Or, the Structure declaration should have the typedef.

struct st
	int memebr1;
	int memebr2;

int _tmain(int argc, _TCHAR* argv[])
	st var1;
	var1.memebr1 = 15;

The above code snippet is for C++. Note that there is no typedef for the structure. Also, note that there is no struct keyword when we are declaring the variable var1.

5. Unions in C++

Union is basic C data type. Like the structure, it also will contain data members. Then how it differs from the structure?

If a structure has 10 data members, space is allocated for all 10 data members. If a structure has 10 data members space is allocated for all ten data members. But, in the case of a union, the compiler examines for the largest data member in the union and allocates the space for it. The union members can be accessed in the same way how the structure member are accessed using the period operator. In union, the compiler looks for the largest data member in the union and designates the space for it. So, the reading and writing from individual data member can be done only one at a time.

6. Declaring a Union

Below is the declaration for the sample union data type:

union Department
  int deptid;
  long numberofEmp;
  float total_salary_expense;

In the above declaration, the union is named as Department. And it has three data members. So the size allocated for the union is a size of the float as it depletes more bits for its storage when compared to other members.

So, if union allocates storage for the largest member, Is it possible to assign and retrieve a value for all the member of the union at the same time? No. That is the answer. Because the values are overwritten as the allocated space is shared by all the members.

7. Union C++ Example

The Example program code is shown below:

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

union Department
	int deptid;
	long numberofEmp;
	float total_salary_expense;

int _tmain(int argc, _TCHAR* argv[])
	union Department dept;
	dept.deptid = 100;
	printf("Dept id = %d \n", dept.deptid );

	dept.numberofEmp = 15000;
	printf("Total Employee = %ld \n", dept.numberofEmp );
	printf("Try to Print Dept Id = %d", dept.deptid );
	return 0;

The output of the code shown in the previous section is shown in below screen shot:

Fig.1 Code Output
Fig.1 Code Output | Source

8. What is the difference between structure and union?

Some of the important difference between the structure and union are listed in the below Table:

Allocates different memory location to the individual members of it
All the members share a single allocation given to it
Since, all the members have their own allocation, the size depends on all the members of it.
the memory is allocated to largest member of the union and all other members shares this location. Hence the size of the union depends on the largest member present in it
only one member can be used one at a time
In structure one can use all the members at the same time

9. Summary

Even though we accessed the DeptId we got 15000 as the value in the last line of the output. So as there is only a single storage (The storage required for the largest among the member) and it shared by all three, we got 15000 for dept id in the last line.

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