What are data types?

Data type sets what a variable stores.

It tells compiler memory size and allowed ops.

Int stores whole numbers. Float stores decimals. Char stores one letter.

Think container for a specific item type.

Why do we need data types?

• Defines value type a variable holds
• Helps compiler allocate right memory
• Prevents invalid operations
• Improves performance and memory use
• Makes programs easier to maintain

Classification of data types

                    Data Types
                        │
     ┌──────────────────┼──────────────────┐
     │                  │                  │
     ▼                  ▼                  ▼
 Fundamental      Derived Data Types   User-Defined
  Data Types                          Data Types
     │                  │                  │
     ▼                  ▼                  ▼
 int               Arrays            class
 char              Pointers          struct
 float             References        union
 double            Functions         enum
 bool                                typedef
 void                                using

Fundamental (built-in) data types

Data Type | Description                           | Typical Size | Example
--------- | ------------------------------------- | ------------ | ---------------------
int       | Stores whole numbers                  | 4 bytes      | 25
float     | Stores decimal numbers                | 4 bytes      | 3.14
double    | Stores high-precision decimal numbers | 8 bytes      | 99.999
char      | Stores a single character             | 1 byte       | 'A'
bool      | Stores true or false                  | 1 byte       | true
void      | Represents no value                   | —            | Function return type

Size varies by compiler and OS.

Integer (int)

Int stores whole numbers, no decimals.

#include <iostream>
using namespace std;

int main()
{
    int age = 20;
    cout << age << endl;
    return 0;
}

Floating-point (float)

Float stores decimal numbers.

#include <iostream>
using namespace std;

int main()
{
    float price = 99.99f;
    cout << price << endl;
    return 0;
}

Double (double)

Double gives more precision than float.

#include <iostream>
using namespace std;

int main()
{
    double pi = 3.14159265358979;
    cout << pi << endl;
    return 0;
}

Character (char)

Char stores one character in single quotes.

#include <iostream>
using namespace std;

int main()
{
    char grade = 'A';
    cout << grade << endl;
    return 0;
}

Boolean (bool)

Bool stores only true or false.

#include <iostream>
using namespace std;

int main()
{
    bool isPassed = true;
    cout << isPassed << endl;
    return 0;
}

Output shows 1 for true.

Void (void)

Void means no returned value.

#include <iostream>
using namespace std;

void greet()
{
    cout << "Welcome!" << endl;
}

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

Derived data types

Built from fundamental types.

• Arrays
• Pointers
• References
• Functions

User-defined data types

Programmers build custom structures.

• Class
• Structure (struct)
• Union (union)
• Enumeration (enum)
• Type aliases (typedef, using)

Data type modifiers

Modifiers change size or range.

Modifier | Example
-------- | --------------
short    | short int
long     | long int
signed   | signed int
unsigned | unsigned int

#include <iostream>
using namespace std;

int main()
{
    unsigned int population = 100000;
    cout << population << endl;
    return 0;
}

Common mistakes and pitfalls

• Int for decimals. Fix: use float or double
• Double quotes for char. Fix: use single quotes
• Large values in float. Fix: use double
• Assuming fixed sizes. Fix: sizes vary by platform

// Incorrect
char grade = "A";

// Correct
char grade = 'A';

Best practices

• Use int for whole numbers
• Prefer double over float generally
• Use bool for logical values
• Choose descriptive variable names
• Pick smallest fitting data type

When not to use certain data types

• Float for money. Use double or fixed-point
• Char for words. Use std::string
• Int for huge values. Use long long
• Double for true/false. Use bool

Summary

• Data type defines what a variable stores
• Types split into fundamental, derived, user-defined
• Fundamental: int, float, double, char, bool, void
• Derived: arrays, pointers, references, functions
• User-defined: classes, structs, unions, enums
• Right type improves memory and readability

Frequently asked questions

What is a data type in C++?

Defines value kind and memory needed.

What is the difference between float and double?

Double holds more precision than float.

Why is char enclosed in single quotes?

Single quotes mark one char. Double quotes mean string.

What is the purpose of the bool data type?

Stores true or false for conditions.

What should I learn after data types?

• Variables
• Constants
• Type Modifiers
• Type Casting
• Operators
• Input and Output
• Control Statements