5.3. Structures

Programmers often use the term "structure" to name two related concepts. They form structure specifications with the struct keyword and create structure objects with a variable definition or the new operator. We typically call both specifications and objects structures, relying on context to clarify the the specific usage.

Structures are an aggregate data type whose contained data items are called fields or members. Once specified, the structure name becomes a new data type or type specifier in the program. After the program creates or instantiates a structure object, it accesses the fields with one of the selection operators. Referring to the basket analogy suggested previously, a structure object is a basket, and the fields or members are the items in the basket. Although structures are more complex than the fundamental types, programmers use the same pattern when defining variables of either type.

Structure Specifications: Fields And Members

A program must specify or declare a structure before using it. The specification determines the number, the type, the name, and the order of the data items contained in the structure.

struct student
{
	int	id;
	string	name;
	double	gpa;
};

Structure Definitions And Objects

We can use a blueprint as another analogy for a structure. Blueprints show essential details like the number and size of bedrooms, the location of the kitchen, etc. Similarly, structure specifications show the structure fields' number, type, name, and order. Nevertheless, a blueprint isn't a house; you can't live in it. However, given the blueprint, we can make as many identical houses as we wish, and while each one looks like the others, it has a different, distinct set of occupants and a distinct street address. Similarly, each object created from a structure specification looks like the others, but each can store distinct values and has a distinct memory address.

Automatic Definition	Dynamic Allocation
student s1; student s2; student s3;	student* s4 = new student; student* s5 = new student; student* s6 = new student;
(a)	(b)
(c)
(d)

Initializing Structures And Fields

Figure 3 demonstrates that we are not required to initialize structure objects when we create them - perhaps the program will read the data from the console or a data file. However, we can initialize structures when convenient. C++ inherited an aggregate field initialization syntax from the C programming language, and the 2011 ANSI standard extends that syntax, making it conform to other major programming languages. More recently, the ANSI 2020 standard added a variety of default field initializations. The following figures illustrate a few of these options.

student s1 = { 123, "dilbert", 3.0 }; student s2 = { 456, "alice", 4.0 }; student s3 = { 789, "wally", 2.0 };	student s1 { 123, "dilbert", 3.0 }; student s2 { 456, "alice", 4.0 }; student s3 { 789, "wally", 2.0 };	student s3; . . . s3 = { 789, "wally", 2.0 };
(a) Inherited from C	(b) ANSI 2020	(c) ANSI 2015
(d)
id name gpa s1 123 dilbert 3.0 s2 456 alice 4.0 s3 789 wally 2.0
(e)

struct student
{
	int	id = 789;
	string	name = "wally";
	double	gpa = 2.0;
};

student s3 = { .id = 789, .name = "wally", .gpa = 2.0 };

student s3 = { .id = 789, .gpa = 2.0 };

student s3;
	...
s3 = { .id = 789, .name = "wally", .gpa = 2.0 };

student s3;
	...
s3 = { .id = 789, .gpa = 2.0 };

The next step is accessing the individual fields within a structure object.

Field Selection: The Dot And Arrow Operators

A basket is potentially beneficial, but so far, we have only seen how to put items into it, and then only en masse. To achieve its full potential, we need some way to put items into the basket one at a time, and there must be some way to retrieve them. C++ provides two selection operators that join a specific structure object with a named field. The combination of an object and a field form a variable whose type matches the field type in the structure specification.

The Dot Operator

The C++ dot operator looks and behaves just like Java's dot operator and performs the same tasks in both languages.

The Arrow Operator

Java only has one way of creating objects and only needs one operator to access its fields. C++ can create objects in two ways (see Figure 3 above) and needs an additional field access operator, the arrow: ->.

Choosing the correct selection operator

Locate where a selection operator is needed. Both selection operators are binary - they require a left- and right-hand operand. The left-hand operand is a structure (either an object or a pointer to an object), and the right-hand operand is a structure field name.

• Choose the arrow operator if the left-hand operand is a pointer.
• Otherwise, choose the dot operator.

Moving Structures Within A Program

A basket is a convenient way to carry and move many items by holding its handle. Similarly, a structure object is convenient for a program to "hold" and move many data items with a single (variable) name. Specifically, a program can assign one structure to another (as long as they are instances of the same structure specification), pass them as arguments to functions, and return them from functions as the function's return value.

Assignment

Assignment is a fundamental operation regardless of the kind of data involved. The C++ code to assign one structure to another is simple and should look familiar:

student s4; s4 = s1;

(a)

(b)

(c)

Function Argument

Once the program creates a structure object, passing it to a function looks and behaves like passing a fundamental-type variable. If we view the function argument as a new, empty structure object, we see that argument passing copies one structure object to another, behaving like an assignment operation.

void print(student temp) // function definition { cout << "ID: " << temp.id << endl; cout << "Name: " << temp.name << endl; cout << "GPA: " << temp.gpa << endl; } . . . print(s2); // function call

(a)

(b)

Function Return Value

Just as a program can pass a structure into a function as an argument, so it can return a structure as the function's return value, as demonstrated by the following example:

student read() { student temp; cout >> "Enter a student id: "; cin >> temp.id >> endl; cout >> "Enter a student name: "; cin >> temp.name >> endl; cout >> "Enter a student gpa: "; cin >> temp.gpa >> endl; return temp; } . . . student s3; s3 = read();

(a)

(a)