The University of Iowa
The College of Liberal Arts and Sciences
Department of Computer Science
Lecture/Lab #17
Stack vs heap, pointers, storage duration, dynamic memory allocation
Application code
(Typical, simplified) memory model
Static storage
Stack
Heap
Local variables
Dynamically allocated memory
Global aka namespace variables
0x00000000
0xFFFFFFFF
int n; // static storage
void foo( int m ) // automatic storage
{
point p; // automatic storage
}
int main()
{
double d; // automatic storage
int e; // automatic storage
if ( true )
{
bool b; // automatic storage
foo( 42 );
}
}
double d
0xffffffe8
bool b
0xffffffe4
int m
0xffffffe3
point p
0xffffffac
int e
0xffffffa0
Stack
void foo( int* m )
{
point* p = new point;
}
int main()
{
double* d = new double;
int* e = new int;
if ( true )
{
bool* b = new bool;
foo( e );
}
}
double
bool
int
point
0xff000040
0xff000000
Heap
0xff000020
0xff000060
Automatically managed by the compiler
Efficient allocation/deallocation
Variables can't outlive functions they've been defined in
Can't allocate an arbitrary amount of memory that is determined at run time
Stack size is limited
Memory is never fragmented
Explicitly managed by the program author/automatically managed through standard library facilities
Generally expensive allocation/deallocation
Storage can outlive functions it's been allocated in
Prone to fragmentation
Can allocate arbitrary amounts of memory that are determined at run time
Allocation size is limited only by the amount of free memory available on the particular machine running the program
An alias to an existing variable
Has to be initialized, cannot be re-aliased to another variable
Assigning a value modifies the aliased variable:
Stores an address of an existing variable or nullptr
Can be left uninitialized (but dereferencing uninitialized pointer produces undefined behavior)
Can be reassigned a different address:
int i = 17;
int& ref = i;ref = -10; // i == -10int i = 17;
int* ptr = &i;int j = 41;
ptr = &j;Can be dereferenced; dereferencing a pointer gives us a reference:
*ptr = 33; // j == 33, same as:
int& jref = *ptr;
jref = 33;Replace question marks with "stack", "heap" or "global", according to your understanding of the language rules
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 "Lab17 exercises" poll in #general
new type
new type()
new type( value )
delete ptr