C++ Structures
C++ Taking the Bull by the Horns – Part 14
Forward: In this part of the series, we shall look at another derived object type, which is called structure.
By: Chrysanthus Date Published: 22 Aug 2012
Introduction
Note: If you cannot see the code or if you think anything is missing (broken link, image absent), just contact me at forchatrans@yahoo.com. That is, contact me for the slightest problem you have about what you are reading.
As I said, I present C++ to you in this series the way the inventors see it. I do the presentation in simple terms. I believe that in this way you would understand C++ better. Remember, take things in this series as I give you. Do not try to add or subtract any idea in your mind to or from what I give you; that would be misleading. You can do any subtraction or addition after you complete the series.
Reasons for having the Structure
An array is a set of objects of the same type. There are times when you need a set of objects of different types. The credentials of a man form a set of objects of different types. The credentials of a man can consist of his name, his age, his salary and his highest qualification. His name e.g. “John Smith”, is a string. His age can be considered as an integer if you are not interested in fractional years. His salary (per week) is a float, because it may have some decimal digits, e.g. $1425.45. His highest qualification is a string. A structure is abbreviated as struct.
A struct Example
Imagine that John Smith mentioned above is my friend. The following code handles a struct for his credentials (the explanation is given below):
#include <iostream>
using namespace std;
int main()
{
struct {char *name; int age; float salary; char *HQ;} myFriend;
myFriend.name = "John Smith";
myFriend.age = 30;
myFriend.salary = 1425.45;
myFriend.HQ = "Msc";
cout << myFriend.name; cout << "\n";
cout << myFriend.age; cout << "\n";
cout << myFriend.salary; cout << "\n";
cout << myFriend.HQ; cout << "\n";
return 0;
}
You can try the above code (do not worry about any error message for now).
In the block, you have definitions of basic object types. In the case of an array, everything in the block would have been of the same object type. In our block above, the first object has the identifier, name. It is a pointer to a char. It is for my friend’s name. The second object has the identifier, age. It is of type int (we ignore fractions of years). It is for my friend’s age. The next object has the identifier, salary. It is of type, float. It is for my friend’s salary. The last object has the identifier, HQ for highest qualification. It is a pointer (memory address) to char. Remember, if a string in double quotes is assigned to a pointer to char, the pointer becomes a “string” pointer.
The next code segment in the code assigns values to those basic objects. To assign a value, you begin with the identifier of the struct, which in this case is myFriend, followed by a dot and then the identifier of the basic object in the struct. In the case of string it is the identifier of a pointer to char. The combination of the struct identifier, the dot and an internal struct object identifier refers to the internal struct object and would return the internal object content or have assigned an internal (basic) object value. If the internal object is a char pointer, then a char pointer will be returned from the combination. The next code segment displays the assigned values of the internal struct objects. Remember, the cout object would take as operand a string pointer and obtain the value (string) from the pointer and return it (send).
Note that for the assignment of the salary value, the currency ($) symbol was not included. This is because the value has to be float (made up of only digits and a decimal point) and $ is not a float digit.
Struct is an object type; precisely, a derived object type. It is an object type that you the programmer actually create. You can create a struct of credentials. Consider the following table, which gives the credentials of four employees in a company:
EmployeeID, Name, Age, Salary, HQ
Emp1 Mary Jones 35 $3856.42 Phd
Emp2 John Carlson 36 $2124.32 Msc
Emp3 James Taylor 32 $1523.42 Bsc
Emp4 Suzan White 33 $1424.32 Bsc
You can create a struct for these credentials. Here a credential refers to the name, age, salary, and highest qualification of an employee.
Creating a Structure
In simple terms, the syntax to create a struct is:
struct {Type IdentA; Type IdentB; Type IdentC; …} structIdent1, structIdent2, structIdent3,…;
You begin with the reserved word (keyword), struct. This is followed by a space and then a block. Inside the block you have statements. The statements in the block are separated by semicolons. Each statement is the definition of an object. The last statement in the block also has a semicolon. After the block you have identifiers for objects that will have the particular struct type. These identifiers are separated by commas. At the end of the struct creation statement, you still have a semicolon that marks the end of the struct statement. Each object in the block is called a member of the struct.
More than one Identifier for a struct
In the code below, a struct is created for the credentials for four employees in a company. Well, to save time and space, only the data for two of the employees are assigned; the age of one of the employees is displayed. The data for the code is taken from the above employee table.
#include <iostream>
using namespace std;
int main()
{
struct {char *name; int age; float salary; char *HQ;} Emp1, Emp2, Emp3, Emp4;
Emp1.name = "Mary Jones";
Emp1.age = 35;
Emp1.salary = 3856.42;
Emp1.HQ = "Phd";
Emp2.name = "John Carlson";
Emp2.age = 36;
Emp2.salary = 2124.32;
Emp2.HQ = "Msc";
cout << Emp2.age;
return 0;
}
In the line of the code where the struct is created, four identifiers are defined. The content type of what will be in each derived object of the four identifiers is what (the internal objects) is in the struct block. For the second code segment in the block of the main function, the values for the internal objects of the identifier, Emp1 are assigned. By the end of this code segment, the derived object identified by Emp1 has all its content. Its content is made of objects whose values have been assigned. That is, its content is made of objects that are not empty at the end of the code segment. Values can be assigned to other struct objects (Emp2, Emp3, Emp4) in the same way. To get the value of one of the objects of a struct object, type the identifier of the struct object first, then the dot and then the identifier of the corresponding internal object. That is what is done with the cout object.
Hey, you can make an object type from struct. We know fundamental objects types that are int, float, _Bool, char and void. We know derived object types that are enum, pointer, array and now struct. Note that you are the one who decides on what objects will be in the block of a struct. There are so many possibilities. Now that there are so many possibilities, possibilities full into sets, similar to the idea that you can integers as a set or floats as a set. So far as credentials are concerned (above), you can have credentials for employees, you can have credentials for friends, you can have credentials for family members, and so on.
Let us talk about fruits. If you go to a shop, you may see pears. To buy pears, you may need to know the weight and price. You may also see oranges. To buy oranges you may also need to know the weight and price. You can create your own object type, which you would call, fruits. Each fruit is a struct. The struct has the definition for weight and price. In simple terms the syntax to create an object type from struct is:
struct typeName {Type IdentA; Type IdentB; Type IdentC; …} structIdent1, structIdent2, structIdent3,…;
The syntax is the same as before except that you have the object type name in front of the block.
The following code would create the object type, fruits for you:
struct fruits {float weight; float price;} pear, orange;
You now have an object type, fruits. From it you can make the object, pear with its own weight and price; you can also make the object, orange with its own weight and price. Here, pear and orange are object identifiers, while fruits is object type you have created.
The following procedure will produce the same effects as the above statement:
struct fruits {float weight; float price;};
fruits pear;
fruits orange;
The last two statements clearly illustrate that you can create your own object types, from structs. The two statements are similar to
int myInt;
int hisInt;
Read and try the following code that illustrates this:
#include <iostream>
using namespace std;
int main()
{
struct fruits {float weight; float price;};
fruits pear;
fruits orange;
orange.weight = 1.2; //weight in kg
orange.price = 3.00; //price in $
pear.weight = 0.8; //weight in kg
pear.price = 3.25; //price in $
cout << pear.weight;
return 0;
}
You output should be 0.8.
Note: structs are actually called DATA structures.
Time to take a break. Let us stop here and continue in the next part of the series.
Chrys
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