C++ Variables

Memory is composed of cells that are addressed in bytes. A variable is an identifier assigned to an address (byte) or a group of addresses. When a variable is created, memory is allocated. Memory allocation refers to reserving space in memory for data or instructions during a program’s execution. Thus, when a variable is declared, a specific amount of memory, in bytes, is allocated and assigned an identifier for use in the program.

---

Identifiers

C++ identifiers follow these rules:

1. Can only consist of:

   • Letters ('a' - 'z' and 'A' - 'Z')
   • Digits ('0' - '9')
   • Underscore ('_')

2. Must begin with a letter or an underscore.

3. Cannot contain whitespace.

4. Cannot be a keyword.

Identifiers may not contain punctuation or other ASCII characters, as the compiler will reject them. Additionally, an identifier cannot start with a digit because the compiler would interpret the number as the beginning of a numeric value, causing a compilation error. Whitespace, including spaces, tabs, or newlines, is prohibited, as it would split the identifier into multiple identifiers. Keywords (reserved words) are predefined words in C++ with specific purposes and cannot be redefined, thus cannot be reused by the program as new identifiers. Some standard keywords include:

Keyword	Keyword	Keyword
alignas	alignof	and
and_eq	asm	auto
bitand	bitor	bool
break	case	catch
char	char8_t	char16_t
char32_t	class	compl
concept	const	const_cast
consteval	constexpr	constinit
continue	co_await	co_return
co_yield	decltype	default
delete	do	double
dynamic_cast	else	enum
explicit	export	extern
false	float	for
friend	goto	if
inline	int	long
mutable	namespace	new
noexcept	not	not_eq
nullptr	operator	or
or_eq	private	protected
public	register	reinterpret_cast
requires	return	short
signed	sizeof	static
static_assert	static_cast	struct
switch	template	this
thread_local	throw	true
try	typedef	typeid
typename	union	unsigned
using	virtual	void
volatile	wchar_t	while
xor	xor_eq

If an identifier includes an invalid character, whitespace, or a keyword (some keyword errors may vary), a compilation error occurs:

example % g++ test.cpp
test.cpp:7:8: error: expected ';' at end of declaration
  int n % ;
       ^
       ;
1 error generated.

If an identifier starts with a number, the following error is typical:

example % g++ test.cpp
test.cpp:7:7: error: expected unqualified-id
  int 1st;
      ^
1 error generated.

---

Declaring Variables

A variable must be declared to assign a name to a memory location. Variable declaration involves specifying the data type and identifier of a variable. The syntax is:

dataType identifier, identifier, … ;

The dataType can be any valid data type, determining the number of bytes the variable occupies and the type/range of values it can store. The identifier must adhere to the naming rules and cannot be reused within the same scope. Multiple variables of the same type can be declared in a single statement by separating them with commas.

To declare variables of fundamental data types:

#include <iostream>

using namespace std;

int main()
{
    int my_int; // Creates an integer variable
    long long my_long_long; // Creates a long long variable
    double my_double; // Creates a double variable
    float my_float; // Creates a float variable
    bool my_bool; // Creates a boolean variable
    char my_char; // Creates a character variable
    string my_string; // Creates a string variable
}

Each line defines a variable of the specified type.

Declaring Variables Before Use

Variables must be declared before they are used. Consider this example:

#include <iostream>

using namespace std;

int main()
{
    cout << my_int << endl;
    int my_int;
    return 0;
}

In this program, my_int is used in a cout statement before being declared. Since the compiler reads from top to bottom, the cout statement is encountered before my_int is recognized. This results in a compilation error:

example % g++ test.cpp
test.cpp:7:11: error: use of undeclared identifier 'my_int'
  cout << my_int << endl;
          ^
1 error generated.

To fix this, always declare variables before using them:

#include <iostream>

using namespace std;

int main()
{
    int my_int;
    cout << my_int << endl;
    return 0;
}

---

Initializing Variables

If a variable is used before being explicitly assigned a value, such as in:

#include <iostream>

using namespace std;

int main()
{
    int my_int;
    cout << my_int << endl;
    return 0;
}

It will be noticed that my_int has a seemingly random value each time the program runs. This is because memory assigned to the variable was previously used by another program, and its state remains unchanged until explicitly overwritten.

example % g++ test.cpp
985495345
example % g++ test.cpp
24654876

To avoid unpredictable values, always initialize, or give a value for the first time, variables with an explicit value (explained here):

int my_int = 0;

This ensures the variable starts with a known value rather than an arbitrary one.

---

Terms

1. Variable - A name (identifier) given to an address (byte) or grouping of addresses in memory. When a variable is created, memory is allocated.

2. Memory Allocation - The process of reserving memory space for data or instructions during a program’s execution.

3. Identifier - A user-defined name assigned to a variable, function, or other entities in C++.

4. Keywords (Reserved Words) - Predefined words in C++ that have a specific meaning and cannot be redefined by the programmer.

5. Whitespace - Spaces, tabs, and line breaks.

6. Variable Declaration - The process of defining a variable by specifying its data type and identifier.

---

Questions

General Questions

1. What is a variable in C++?

2. What happens when a variable is created in C++?

3. What is memory allocation in the context of variables?

4. Why is memory allocation important when declaring variables?

Identifiers

5. What is an identifier in C++?

6. What characters can be used in a C++ identifier?

7. What are the rules for naming identifiers in C++?

8. Why can't an identifier start with a digit?

9. Why can't an identifier contain whitespace?

10. What happens if an identifier contains an invalid character?

11. What error occurs if an identifier starts with a number?

Keywords

12. What are keywords in C++?

13. Why can't keywords be used as identifiers?

14. Can keywords be redefined by the programmer?

15. What are some examples of C++ keywords?

16. What kind of error occurs if a keyword is mistakenly used as an identifier?

Variable Declaration

17. What does it mean to declare a variable in C++?

18. What is the syntax for declaring a variable?

19. What are the components of a variable declaration?

20. How does the dataType affect a variable?

21. Can multiple variables be declared in a single statement? If so, how?

Declaring Variables Before Use

22. Why must a variable be declared before it is used?

23. What happens if a variable is used before being declared?

24. What kind of error occurs when an undeclared variable is used?

25. How can you ensure a variable is properly declared before use?

Variable Initialization

26. What does it mean to initialize a variable?

27. Why is it important to initialize variables?

28. What happens if a variable is used before it is initialized?

29. Why do uninitialized variables sometimes display random values?

30. How can you ensure a variable always starts with a known value?

Code Examples and Behavior

31. What does the following code do?

    int my_int;
    cout << my_int << endl;

32. Why does the output of an uninitialized variable change each time the program runs?

33. What does the following error mean?

    test.cpp:7:11: error: use of undeclared identifier 'my_int'

34. How can you modify a program to prevent the use of an undeclared variable?

35. What will happen if you declare multiple variables of the same type on a single line?

36. Why is it important to understand how memory is assigned to variables?

37. What error will occur in the following code, and why?

    int main() {
        int 1variable;
        return 0;
    }

    How can it be fixed?

38. What error will be produced by the following code?

    int main() {
        int my var;
        return 0;
    }

    How can it be fixed?

39. What compilation error will result from this incorrect variable declaration?

    int main() {
        double myDouble
        return 0;
    }

    How can it be fixed?

40. What happens if a variable is declared twice in the same scope?

    int main() {
        int num;
        int num;
        return 0;
    }

    What error message will the compiler generate? How can it be fixed?

41. What error will occur if a variable is used before being declared?

    int main() {
        cout << myNumber << endl;
        int myNumber;
        return 0;
    }

    Why does this happen? How can it be fixed?

42. What error does the following code cause?

    int main() {
        int num;
        cout << num << endl;
        return 0;
    }

    Why does the output vary each time the program is executed? How can it be fixed?

43. What will happen if a variable is declared without a type?

    int main() {
        myVariable;
        return 0;
    }

    How can this be corrected?

44. What error will occur in the following code due to using a keyword as an identifier?

    int main() {
        int return;
        return 0;
    }

    How does the compiler handle this situation?