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
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
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`
-
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
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
Programming with C++, Fall 2019, Lecture #19
By Aleksey Gurtovoy
Programming with C++, Fall 2019, Lecture #19
Arrays, array vs vector, smart pointers
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