C++ — Variables

Declaration and initialization

C++ offers multiple ways to initialize variables:

#include <iostream>
using namespace std;

int main() {
    int a = 10;       // Copy initialization
    int b(20);        // Direct initialization
    int c{30};        // Brace initialization (C++11, preferred)
    int d = {40};     // Copy-list initialization

    cout << a << " " << b << " " << c << " " << d << endl;
    return 0;
}

auto keyword

Let the compiler deduce the type:

#include <iostream>
using namespace std;

int main() {
    auto x = 42;        // int
    auto y = 3.14;      // double
    auto z = "Hello";   // const char*
    auto flag = true;   // bool

    cout << typeid(x).name() << endl; // i (int)
    cout << typeid(y).name() << endl; // d (double)
    return 0;
}

Reference variables

Create aliases for existing variables:

#include <iostream>
using namespace std;

int main() {
    int x = 10;
    int &ref = x; // ref is an alias for x

    ref = 20;
    cout << x << endl; // 20 — x changed!

    const int &cref = x; // Const reference — can't modify
    // cref = 30; // Error
    cout << cref << endl;

    return 0;
}

Scope and lifetime

#include <iostream>
using namespace std;

int globalVar = 100; // Global scope

int main() {
    int localVar = 50; // Local to main

    {
        int blockVar = 25; // Local to this block
        cout << "Block: " << blockVar << endl;
    }
    // cout << blockVar; // Error: out of scope

    static int count = 0; // Static: persists between calls
    count++;
    cout << "Count: " << count << endl;

    return 0;
}

Constants

#include <iostream>
using namespace std;

int main() {
    const int MAX = 100;    // Compile-time constant
    constexpr int SIZE = 10; // Guaranteed compile-time

    // MAX = 200; // Error: cannot modify const

    // #define (old style, avoid)
    #define PI 3.14159

    cout << "MAX: " << MAX << endl;
    cout << "PI: " << PI << endl;

    return 0;
}

Type deduction with decltype

#include <iostream>
using namespace std;

int main() {
    int x = 42;
    decltype(x) y = 100; // y is int

    auto add = [](int a, int b) { return a + b; };
    decltype(add) lambda2 = add; // Same type as add

    cout << y << endl;
    return 0;
}

Type sizes

#include <iostream>
using namespace std;

int main() {
    cout << "char:     " << sizeof(char) << " bytes" << endl;
    cout << "short:    " << sizeof(short) << " bytes" << endl;
    cout << "int:      " << sizeof(int) << " bytes" << endl;
    cout << "long:     " << sizeof(long) << " bytes" << endl;
    cout << "long long:" << sizeof(long long) << " bytes" << endl;
    cout << "float:    " << sizeof(float) << " bytes" << endl;
    cout << "double:   " << sizeof(double) << " bytes" << endl;

    return 0;
}

Scoped enums (enum class)

#include <iostream>
using namespace std;

enum class Color { RED, GREEN, BLUE };
enum class Direction { UP, DOWN, LEFT, RIGHT };

int main() {
    Color c = Color::RED;
    Direction d = Direction::UP;

    // Must cast to int for output
    cout << "Color: " << static_cast<int>(c) << endl;
    cout << "Direction: " << static_cast<int>(d) << endl;

    return 0;
}

Best practices

• Use auto when the type is obvious from context
• Prefer constexpr over #define for constants
• Use const for values that shouldn't change
• Prefer brace initialization {} to prevent narrowing
• Use enum class over plain enum for type safety

Mini Practice

Write C++ code that:

1. Declares variables using all three initialization styles
2. Uses auto to deduce types for different values
3. Creates a reference variable and shows it modifies the original
4. Demonstrates const and constexpr

Up Next

In the next lesson, you'll learn about Data Types — the full set of types in C++.