C++ Pointers

A pointer is a variable which keeps the memory address of another variable. This is the most important and most difficult idea of C++.

Address and dereference

#include <iostream>

int main() {
    int n = 42;
    int* p = &n;                 // & gives the address of n
    std::cout << *p << '\n';     // * follows the pointer to the value
    *p = 99;                     // change n through the pointer
    std::cout << n << '\n';
    std::cout << std::boolalpha << (p == &n) << '\n';
}

nullptr

A pointer which points to nothing should be nullptr (not 0 or NULL). Always check before you use it.

#include <iostream>

int main() {
    int* p = nullptr;
    if (p == nullptr) {
        std::cout << "p points to nothing\n";
    }
    if (!p) {
        std::cout << "same check, shorter\n";
    }
}

Pointers and arrays

#include <iostream>

int main() {
    int a[4] = {10, 20, 30, 40};
    int* p = a;                              // the name of the array is the address of its first item

    std::cout << *p << ' ' << *(p + 1) << ' ' << p[2] << '\n';
    p++;                                     // move to the next int
    std::cout << *p << '\n';
    std::cout << (a + 3) - a << '\n';        // distance in items
}

const and pointers

Declaration	Meaning
const int* p	Pointer to a const int: you can not change the value through p
int* const p	A const pointer: p can not point to another place
const int* const p	Both are const

int main() {
    int a = 1, b = 2;
    const int* p = &a;
    *p = 5;                  // error: the value is const through this pointer
}

Pointer to a struct or object: the arrow

#include <iostream>
#include <string>

struct Student {
    std::string name;
    int marks;
};

int main() {
    Student s{"Riya", 88};
    Student* p = &s;
    std::cout << p->name << ' ' << (*p).marks << '\n';
}

Common mistakes

• Using a pointer which was never set (a wild pointer).
• Using a pointer after the memory was freed (a dangling pointer).
• Dereferencing nullptr: the program crashes.
• Going outside an array with pointer arithmetic.

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