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 data-type 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 iscin.get(); }