Module 0461: Types and Structures

• What is a type?
• What is a structure?
• An example
• Structure assignment
• Passing a structure as a parameter

What is a type?

A “type” in programming specifies a category of values. We have already encountered a few up to this point:

• int: short for integer.
• bool: short for Boolean. This is a category of values that can only be true or false.
• double: short for “IEEE Double Precision Floating Point”. This is a category of values that is like real numbers in mathematics.

All three are known as built-in types in C++. These are also called “scalar” types or “atomic” types because they are elemental and cannot be broken up into smaller pieces in C++.

What is a structure?

A structure is the packaging of multiple items into a logical unit. Each component of a structure is called a member. Members are distinguished from each other by their unique names.

The main purpose of using structures is organization. Without structures, programs can still be written, but such programs also become prone to errors.

Imagine that instead of a single page of a physical form, each field of the form is on a little slip of paper. This is an analogy of what happens without structures.

If we think of a structure as a form, then it is a form template. Think of this as the master copy of a form. To make an actual form for a person to fill in, we use a copier to make a photocopy of the master copy, and the photocopy is the one that we let people write over.

In C++ terminology, the template is called the structure, and the photocopy is called an object, or an instance of the structure.

Another way to think of it is that a structure is a cookie-cutter, and an object or structure instance is a cookie cut out from cookie dough using the cookie-cutter.

An example

struct StudentRecord {
  int id;
  double gpa;
  bool active;
};

This is the definition of a structure (the cookie cutter). The cookie cutter has a name of StudentRecord, and the structure has three members:

• id is an integer member.
• gpa is a real number member.
• active is a Boolean member.

A cookie cutter is fairly useless, let us expand the program as follows:

struct StudentRecord {
  int id;
  double gpa;
  bool active;
};

int main() {
  struct StudentRecord aRec;

  aRec.id = 1920416;
  aRec.gpa = 3.5;
  aRec.active = true;

  return 0;
}

The line that reads struct StudentRecord aRec; uses the structure as a user-defined type to create the object (structure instance, or cookie) called aRec. The subsequent lines assign specific values to each member of the structure instance/object/cookie. The execution of the previous program can be traced as follows:

	A	B	C	D	E
1	comments	line#
2		pre
3		11
4		12	aRec.id	aRec.aData.gpa	aRec.aData.active
5			?	?	?
6		14		3.5
7		15			true
8		16	1729384
9		18
10		post

The “dot notation” can be thought of as the apostrophe-s in English (possessive). This means that the expression aRec.gpa can be interpreted as “aRec’s GPA (member)” or “the GPA member of aRec”.

In a trace, a column is used for each member in a structure instance that is of a built-in type. This implies that structure definitions can nest. Consider the following:

struct Academic {
  double gpa;
  bool active;
};

struct StudentRec {
  int id;
  struct Academic aData;
};

int main() {
  struct StudentRec aRec;

  aRec.aData.gpa = 3.5;
  aRec.aData.active = true;
  aRec.id = 1729384;

  return 0;
}

The execution of this code is traced as follows:

	A	B	C	D	E
1	comments	line#
2		pre
3		11
4		12	aRec.id	aRec.aData.gpa	aRec.aData.active
5			?	?	?
6		14		3.5
7		15			true
8		16	1729384
9		18
10		post

Structure assignment

An single assignment statement can copy an entire structure instance to another.

struct Academic {
  double gpa;
  bool active;
};

struct StudentRec {
  int id;
  struct Academic aData;
};

int main() {
  struct StudentRec aRec;
  struct Academic acad;

  acad.gpa = 3.5;
  acad.active = true;
  aRec.aData = acad;
  aRec.id = 1729384;

  return 0;
}

The following is the trace of this code. Note how on row 10, both columns of member aData of the structure instance aRec are updated at the same time.

	A	B	C	D	E	F	G
1	comments	line#
2		pre
3		11
4		12	aRec.id	aRec.aData.gpa	aRec.aData.active
5			?	?	?
6		13				acad.gpa	acad.active
7						?	?
8		15				3.5
9		16					true
10		17		3.5	true
11		18	1729384
12		19
13		post

Passing a structure as a parameter

Unlike an array which can only be passed by reference, a structure can be passed by reference of by value.

struct Academic {
  double gpa;
  bool active;
};

struct StudentRec {
  int id;
  struct Academic aData;
};

void initAcad(struct Academic &acad, double gpa, bool active) {
  acad.gpa = gpa;
  acad.active = active;
}

int main() {
  struct StudentRec aRec;
  struct Academic acad;

  initAcad(acad, 3.5 true);
  aRec.aData = acad;
  aRec.id = 1729384;

  return 0;
}

When a structure is passed by reference, the parameter becomes a reference to the first column of the structure instance.

	A	B	C	D	E	F	G	H	I	J	K
1	comments	line#
2		pre
3		16
4		17	aRec	aRec	aRec
5			id	aData	aData
6				gpa	active
7			?	?	?
8		18				acad	acad
9						gpa	active
10						?	?
11		20						ret line #	acad	gpa	active
12								21	ref col F	3.5	true
13		11
14		12				3.5
15		13					true
16		14						ret line #	acad	gpa	active
17		21		3.5	true
18		22	1729384
19		24
20		post

This program can be simplied because variable acad was not really needed. A member that is a structure can be passed by reference itself:

struct Academic {
  double gpa;
  bool active;
};

struct StudentRec {
  int id;
  struct Academic aData;
};

void initAcad(struct Academic &acad, double gpa, bool active) {
  acad.gpa = gpa;
  acad.active = active;
}

int main() {
  struct StudentRec aRec;

  initAcad(aRec.aData, 3.5 true);
  aRec.id = 1729384;

  return 0;
}

	A	B	C	D	E	F	G	H	I
1	comments	line#
2		pre
3		16
4		17	aRec	aRec	aRec
5				aData	aData
6			id	gpa	active
7			?	?	?
8		19				ret line #	acad	gpa	active
9						20	ref col D	3.5	true
10		11
11		12		3.5
12		13			true
13		14				ret line #	acad	gpa	active
14		20	1729384
15		22
16		post

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