Variables and Data Types in C++

# CHAPTER 4

Variables and Data Types #

1. Introduction #

2. Learning Objectives #

• Declare and initialize variables.

• Understand primitive data types (int, float, char, bool, double).

• Create constants using the const keyword.

• Use the modern C++11 auto keyword.

• Understand implicit and explicit type casting.

3. What is a Variable? #

Syntax: dataType variableName = value;

int age = 25;       // 'age' holds a whole number
float price = 19.99;// 'price' holds a decimal
char grade = 'A';   // 'grade' holds a single character
bool isReady = true;// 'isReady' holds true (1) or false (0)

4. Primitive Data Types in C++ #

*(Note: C++ also has modifiers like unsigned int which only allows positive numbers, effectively doubling the maximum positive value).*

5. Constants (const) #

const float PI = 3.14159f;
// PI = 3.14f; // COMPILER ERROR: assignment of read-only variable

6. The auto Keyword (C++11) #

auto score = 100;     // Compiler knows this is an int
auto price = 9.99;    // Compiler knows this is a double
auto initial = 'J';   // Compiler knows this is a char

*Best Practice:* Use auto when the type is obvious or when dealing with complex STL iterators (covered in Chapter 22).

7. Type Casting (Type Conversion) #

Implicit Conversion (Automatic): Smaller types are safely promoted to larger types.

int num = 10;
double decimal = num; // Automatically becomes 10.000000

Explicit Conversion (Manual): Forcing a conversion. C++ provides modern casting operators, but C-style casting is still widely used by beginners.

// C-Style Cast
double pi = 3.14159;
int rounded = (int)pi; // 'rounded' becomes 3 (truncates decimal)

// C++ Style Cast (Safer, preferred in Modern C++)
int rounded_cpp = static_cast<int>(pi);

8. Memory-Level Explanation #

1. 1. The compiler allocates 4 contiguous bytes of memory on the Stack.

1. 2. It notes that the memory address (e.g., 0x7ffee123) is labeled age.

1. 3. It converts 25 to binary (00011001) and stores it in those 4 bytes.

9. Common Mistakes #

• Using single quotes for strings: 'Hello' is wrong. Single quotes are ONLY for single characters ('H'). Use double quotes for strings ("Hello").

• Forgetting f on floats: float pi = 3.14; technically creates a double and downcasts it. Use float pi = 3.14f;.

• Uninitialized variables: int score; cout << score; prints a random "garbage" value left over in memory.

10. Exercises #

1. 1. Declare variables to store your age, height in meters (float), and blood type letter. Print them out.

1. 2. Create a constant for the speed of light (299792458) and print it.

1. 3. Use auto to define a variable and initialize it with true. Print it (it should print 1).

11. MCQ Quiz with Answers #

• Q: Why should you prefer staticcast over C-style casting (type)var in C++?

13. Summary #

14. Next Chapter Recommendation #