Memory and pointers · Lesson 11 of 21

Pointers

Addresses, * and &, nullptr, and pointers with arrays.

11 minutes3 games

Every variable lives somewhere in memory, and that place has an address (a number). A pointer is a variable whose value is another variable's address: it "points" at it.

&x means "the address of x".
int* p means "p points to an int".
*p means "the value p points to" (a dereference).

C++

#include <iostream>

int main() {
    int xp = 100;
    int* p = &xp;          // p holds xp's address

    std::cout << "xp  = " << xp << "\n";
    std::cout << "*p  = " << *p << "\n";
    std::cout << "same address? " << (p == &xp) << "\n";

    *p = 250;              // change xp through the pointer
    std::cout << "xp is now " << xp << "\n";

    int* nothing = nullptr;
    if (nothing == nullptr) std::cout << "nothing points nowhere\n";
    return 0;
}

xp250int · at 0x7ffd10

p0x7ffd10int* → xp

nothingnullptrint* → (nothing)

p holds xp's address, so *p = 250 changed xp itself. (The real address changes every run.)

Step by step1 / 6

▶int a = 5;
2int b = 9;
3int* p = &a;
4*p = *p + 1;
5p = &b;
6*p = 0;

Variables

Output

…

A pointer can change what it points at.

Pointers and arrays

C++

#include <iostream>

int main() {
    int scores[4] = {90, 75, 88, 60};
    int* p = scores;            // an array name turns into a pointer to its first element
    std::cout << *p << " " << *(p + 2) << " " << p[3] << "\n";

    for (int* it = scores; it != scores + 4; ++it) {
        std::cout << *it << " ";
    }
    std::cout << "\n";
    return 0;
}

What's the output?

#include <iostream>
int main() {
    int x = 3, y = 7;
    int* p = &x;
    int* q = &y;
    *p = *q;
    p = q;
    *p = 1;
    std::cout << x << " " << y << "\n";
}

What will this code print?

Spot the bug

This function returns a pointer to something that's gone. Which line is the problem?

Exercise

Write void swapPtr(int* a, int* b) that swaps the two values through the pointers. The program must print 9 4.

Expected output: 9 4

C++

#include <iostream>

void swapPtr(int* a, int* b) {
    
}

int main() {
    int x = 4, y = 9;
    swapPtr(&x, &y);
    std::cout << x << " " << y << "\n";
    return 0;
}

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#include <iostream> int main() { int xp = 100; int* p = &xp; // p holds xp's address std::cout << "xp = " << xp << "\n"; std::cout << "*p = " << *p << "\n"; std::cout << "same address? " << (p == &xp) << "\n"; *p = 250; // change xp through the pointer std::cout << "xp is now " << xp << "\n"; int* nothing = nullptr; if (nothing == nullptr) std::cout << "nothing points nowhere\n"; return 0; }