Hack Insight: It Security Magazine (2013)

Lesson 7: Structures

Before discussing classes, this lesson will be an introduction to data structures similar to classes. Structures are a way of storing many different variables of different types under the same name. This makes it a more modular program, which is easier to modify because its design makes things more compact. It is also useful for databases.

The format for declaring a structure (in C++, it is different in C) is:
struct Tag { Members };
Where Tag is the name of the entire type of structure. To actually create a single structure the syntax is Tag name_of_single_structure;
To access a variable of the structure it goes:
name_of_single_structure.name_of_variable;
For example:

struct example { int x;
};
example an_example; //Treating it like a normal variable type
an_example.x = 33; //How to access it's members

Here is an example program: struct database {
int id_number;

int age;
float salary;
};
int main()
{
database employee; //There is now an employee variable that has modifiable // variables inside it. employee.age = 22; employee.id_number = 1; employee.salary = 12000.21;
}

The struct database declares that database has three variables in it, age, id_number, and salary. You can use database like a variable type like int. You can create an employee with the database type as I did above. Then, to modify it you call everything with the 'employee.' in front of it. You can also return structures from functions by defining their return type as a structure type. For instance:

database fn();

I will talk only a little bit about unions as well. Unions are like structures except that all the variables share the same memory. When a union is declared the compiler allocates enough memory for the largest datatype in the union. Its like a giant storage chest where you can store one large item, or a small item, but never the both at the same time.

The '.' operator is used to access different variables inside a union also.

As a final note, if you wish to have a pointer to a structure, to actually access the information stored inside the structure that is pointed to, you use the -> operator in place of the . operator. All points about pointers still apply.

A quick example:

#include <iostream> using namespace std; struct xampl {

int x; }; int main() {

xampl structure; xampl *ptr; structure.x = 12; ptr = &structure; // Yes, you need the & when dealing with structures

// and using pointers to them cout<< ptr->x; // The -> acts somewhat like the * when used with pointers
// It says, get whatever is at that memory address
// Not "get what that memory address is" cin.get(); }