The University of Iowa
The College of Liberal Arts and Sciences
Department of Computer Science

Programming Languages and Tools:

CS:3210:0001

Lecture/Lab #19

Programming with C++

Arrays, array vs vector, smart pointers

Warm up

What will be the output of the following program?

int main() {
    int* n = new int( 17 );
    int* m = n;
    *m = 99;
    std::cout << "n: " << *n << std::endl;
    std::cout << "m: " << *m << std::endl;
    delete n;
    n = new int;
    *n = *m;
    std::cout << "n: " << *n << std::endl;
    std::cout << "m: " << *m << std::endl;
    delete m;
    delete n;
}

Member access through pointer:

car* c = new car;
(*c).start();
c->start();

Arrays, pointer arithmetic

https://repl.it/@agurtovoy/lab19-arrays

int main() {
    int a[10] = { 5, -1, 0, 12, 
                  -7, 13, 99, 64, 
                  11, -22 };

    std::cout << a[0] << std::endl;
    std::cout << a[1] << std::endl;
    
    int* ptr = &a[0];
    ptr += 2;
    *ptr = 33;
    
    std::cout << a[2] << std::endl;
 }

a[6]

Stack

a[5]

a[4]

a[3]

a[2]

a[1]

a[0]

a[7]

a[8]

a[9]

0xffffffd0

0xfffffff8

0xfffffff4

0xfffffff0

0xffffffec

0xffffffe8

0xffffffe4

0xffffffd4

0xffffffdc

0xffffffd8

0xffffffe0

Array vs vector

https://repl.it/@agurtovoy/lab19-array-vs-vector

int main() {
    std::vector<int> a = { 
                  5, -1, 0, 12, 
                  -7, 13, 99, 64, 
                  11, -22 };

    std::cout << a[0] << std::endl;
    std::cout << a[1] << std::endl;
 }

a[6]

Heap

a[5]

a[4]

a[3]

a[2]

a[1]

a[0]

a[7]

a[8]

a[9]

a (ptr=0xff000050, size=10)

Stack

0xff000050

Exercise 1

Finish implementation of a user-defined class `int_vector`

https://repl.it/@agurtovoy/lab19-exercise1

Open the exercise template

Write your code, press Run to test

When you're done, grab your Repl's link and send it as a direct message to me (agurtovoy)

Click on the corresponding option in the "Lab19 exercises" poll in #general

Problems with raw pointers

The pointer declaration (Thingy* ptr) doesn’t indicate whether it points to a single object or to an array.

It also doesn't say anything about whether we should destroy what it points to when we're done using it — i.e., if the pointer owns the thing it points to.

If we determine that we should destroy what the pointer points to, there’s no way to tell how.

Assuming we sorted out the above, it’s difficult to ensure that we perform the destruction exactly once along every path in our code.

There’s typically no way to tell if the pointer dangles, i.e., points to memory that no longer holds the object the pointer is supposed to point to.

std::unique_ptr

Idiomatic replacement for

Exclusive ownership of heap allocated objects

https://repl.it/@agurtovoy/lab19-unique_ptr

widget* w = new widget();

// ...

delete w;

auto w = std::make_unique<widget>();

// ...

Zero overhead (unless you're using a stateful custom deleter)

Non-copyable but movable

Primary characteristics:

Can be stored in standard containers (e.g. std::vector)

Familiar dereference/member access syntax ( *w, w->memfun() ), no pointer arithmetics