Structures in C++
Before discussing classes, this lesson will be an introduction to data
structures similar to classes. Structures are a way of storing many different
values in variables of potentially 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. Structs are generally useful whenever a lot of
data needs to be grouped together--for instance, they can be used to hold
records from a database or to store information about contacts in an address
book. In the contacts example, a struct could be used that would hold all of
the information about a single contact--name, address, phone number, and so
forth.
The format for defining a structure is
struct Tag {
Members
};
Where Tag is the name of the entire type of structure and Members are the variables within the struct. To actually create a single structure the syntax is
struct 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;
};
struct 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.
It's 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();
}
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