COMP26020p1 - Introduction to Pointers

class: center, middle

### COMP26020 Programming Languages and Paradigms Part 1: C Programming
***
# Introduction to Pointers

---
# Program Memory Layout

- All of the program's code and data are stored somewhere in memory
- The area of memory accessible to the program is its **address space**
  - It is a large array of contiguous bytes
  - **Addresses** are indexes in that array

- Note that this is <ins>virtual</ins> memory:
  - Size of the address space is independent of the amount of RAM
  - Each program gets its own address space

---

# Program Memory Layout

- **Address of a variable**: the address in memory of the first byte used to
  store that variable

???
- So, as I said, all the code executed and the data manipulated by the program reside in memory
- Each variable has an address, which is the address of the first byte of its memory location

```c
int glob = 12;             // glob's address is 0x3300
char string[] = "abcd";    // string's address is 0x2f0

typedef struct {
    int member1;
    float member2;
} mystruct;

int main(int argc, char **argv) {
    mystruct ms;            // ms's address is 0x120
    ms.member1 = 42;
    ms.member2 = 4.2;
    printf("ms member1: %d, member2: %f\n", ms.member1, ms.member2);
    return 0;
}
```

---
# Addresses

- Use the `&` operator to get the address of a variable

```c
int glob = 12;
char string[] = "abcd";

typedef struct {
    int member1;
    float member2;
} mystruct;

int main(int argc, char **argv) {
    mystruct ms = {1, 2.0};

// Addresses are printed with the "%p" format specifier. On most modern systems an
    // address is a 64-bit value, and "%p" prints it in hexadecimal prefixed by "0x",
    // for example "0x12345" (the exact output format is implementation-defined)
*   printf("ms address: %p, glob address: %p, string address: %p\n", &ms, &glob, &string);
    return 0;
}
```
.codelink[<a href="src/ampersand.c" download>`09-pointers-introduction/ampersand.c`</a> <a href="https://github.com/olivierpierre/comp26020-devcontainer" target="_blank" style="text-decoration: none"><img src="include/gh-logo.svg" style="height: 1em" ></a>]

---
# Pointers

.center[***Pointer*: variable whose value is an address**]
.center[(possibly the address of another variable)]

--
name:ptrintro

- Declaration: `<pointed-to type> *<pointer name>;`

```c
int v = 42;
int *ptr = &v;
```

---
template: ptrintro

---
template: ptrintro

- We say that `ptr` ***points to*** `v`

---
# Pointers

- **Through a pointer it is possible to access the variable it points to**
- This is called **dereferencing**
- It is done with the indirection operator `*`

```c
int v = 42;
int *ptr = &v;

printf("value pointed to by ptr: %d\n", *ptr);   // 42, dereference ptr to read v's value
printf("v's value:               %d\n", v);      // 42

*ptr = 0; // dereference ptr to write to v

printf("value pointed to by ptr: %d\n", *ptr);   // 0
printf("v's value:               %d\n", v);      // 0
```
.codelink[<a href="src/pointer-update.c" download>`09-pointers-introduction/pointer-update.c`</a> <a href="https://github.com/olivierpierre/comp26020-devcontainer" target="_blank" style="text-decoration: none"><img src="include/gh-logo.svg" style="height: 1em" ></a>]

---
name:ptr

# Pointers

```c
int glob = 12;
double glob2 = 4.4;

typedef struct { int member1; double member2; } mystruct;

int main(int argc, char **argv) {
    mystruct ms = {55, 2.23};

int *ptr1 = &glob;
    double *ptr2 = &glob2;
    mystruct *ptr3 = &ms;

/* Print each pointer's value (the address it holds) and the pointed-to value */
    printf("ptr1 = %p, *ptr1 = %d\n", ptr1, *ptr1);
    printf("ptr2 = %p, *ptr2 = %f\n", ptr2, *ptr2);
    printf("ptr3 = %p, (*ptr3).member1 = %d, (*ptr3).member2 = %f\n",
            ptr3, (*ptr3).member1, (*ptr3).member2);
}
```
.codelink[<a href="src/pointer.c" download>`09-pointers-introduction/pointer.c`</a> <a href="https://github.com/olivierpierre/comp26020-devcontainer" target="_blank" style="text-decoration: none"><img src="include/gh-logo.svg" style="height: 1em" ></a>]

---
template: ptr
<div style="text-align:center"><img src="include/pointer2-1.svg" width=700 /></div>

---
template: ptr
<div style="text-align:center"><img src="include/pointer2-2.svg" width=700 /></div>

---
template: ptr
<div style="text-align:center"><img src="include/pointer2-3.svg" width=700 /></div>

---
template: ptr
<div style="text-align:center"><img src="include/pointer2-4.svg" width=700 /></div>

---

# Summary

- A pointer: variable that stores an address corresponding to a memory location
- The data at that location can be accessed through the pointer (dereferencing)

----
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<div style="text-align:center"><img src="include/qr-code.png" height=150 /></div>

???

- Let's recap
- A pointer is a variable that holds an address corresponding to a particular memory location
- And we can access that location through the pointer
- In the next video we will see how useful pointers are