Variables, Types, Pointers & References

Variables, Types, Pointers & References

Fundamental Types

#include <iostream>

int main() {
    int    i  = 42;
    double d  = 3.14;
    char   c  = 'A';
    bool   b  = true;
    float  f  = 2.71f;   // f suffix = float literal
    long   l  = 1000000L;

    // C++11 auto type deduction
    auto x = 5;          // int
    auto y = 3.14;       // double
    auto z = "hello";    // const char*

    std::cout << i << " " << d << " " << c << std::endl;
    return 0;
}

const

const int MAX = 100;          // compile-time constant
const double PI = 3.14159;

// const parameter — function cannot modify it
void print(const std::string& s) {
    std::cout << s << std::endl;
}

Pointers

int x = 10;
int* p = &x;  // p holds the address of x

std::cout << p  << std::endl; // address: 0x7fff...
std::cout << *p << std::endl; // dereference: 10

*p = 20;           // modify x through pointer
std::cout << x;    // 20

int* null_ptr = nullptr; // null pointer (C++11)

References

A reference is an alias — it must be initialized and always refers to the same variable:

int a = 5;
int& ref = a;  // ref IS a (not a copy)

ref = 10;
std::cout << a; // 10 — a was changed through ref

// References are the preferred way to pass large objects
void increment(int& n) {
    n++;
}

int main() {
    int val = 7;
    increment(val);
    std::cout << val; // 8
}

Pointer vs Reference

const with Pointers

int x = 5, y = 10;

const int* p1 = &x;    // pointer to const: can't change *p1
int* const p2 = &x;    // const pointer: can't change p2 itself
const int* const p3 = &x; // both const

p1 = &y;   // ok — pointer can change
// *p1 = 9; // ERROR — can't modify through const pointer

sizeof and size_t

#include <cstddef>

std::cout << sizeof(int)    << " bytes\n"; // 4
std::cout << sizeof(double) << " bytes\n"; // 8
std::cout << sizeof(char)   << " bytes\n"; // 1

size_t n = 42; // use for sizes and indices