# Pointer to object (object)= ## Object :::{lrm} object :fragment: section.3.18 region of data storage that can represent values ::: For example: ```c int number = 42; char name[] = "Partha"; ``` - `number` is the identifier of the object `42`. `42` is stored in an data storage area of 4 bytes. - `name` is the identifier of the object `"Partha"`. This object consists of 7 characters. ## We used pointers before Remember the following diagram from section {ref}`arguments-and-return-value-in-functions`: :::{mermaid} ../industrial-programming/img/function-input-output.mmd ::: :::{card} 🤔 Question to ponder Do you remember 1. what the arrow tips (directions) mean? 1. an example where a function writes to its arguments? ::: ## Meaning :::{figure} https://upload.wikimedia.org/wikipedia/commons/b/b4/Pointers.svg :figwidth: 40% :align: right A pointer variable `a` which has the value `1008`. `1008` is an address. If we *dereference* `a`, we get the value of the variable `b`. [Source](https://commons.wikimedia.org/wiki/File:Pointers.svg) CC-BY-SA 3.0 Unported by User:Sven ::: :::{wpd} pointer :id: Pointer (computer programming) an object that stores a memory address ::: Example: :::{literalinclude} ../code/a_points_to_b.c :name: a_points_to_b :language: c ::: :::{literalinclude} ../code/a_points_to_b.txt ::: ## Declaration ```c int *a; // Pointer to an `int` char *c; // Pointer to a `char` int **d; // Pointer to a pointer to an `int` float *e = nullptr; // Initialization with zero // Better than `0`, because `nullptr` identifier shows intent ``` ## Usage :::{list-table} Pointer operators :header-rows: 1 * - Operator - Meaning - Example * - `*` - *dereference* - `*a`{l=c} gives the value which is stored at the address stored in the pointer variable `a`{l=c}. * - `&` - *address of* - `&b`{l=c} gives the address of the value variable `b`{l=c}. ::: :::{exercise} Meaning of pointer operators :label: meaning-of-pointer-operators Which are correct for the [code above](project:#a_points_to_b)? 1. ` a == 42`{l=c} 1. ` *a == 42`{l=c} 1. ` &a == 42`{l=c} 1. `&*b == 42`{l=c} 1. `*&b == 42`{l=c} ::: :::{activity} Declaring and using a pointer Code the following: 1. Declare and initialize an `unsigned` number called `n`. 1. Declare and initialize a pointer to `n` called `p1`. 1. Declare and initialize a pointer to `n` called `p2`. 1. Print the value that `p1` points to. 1. Increment `n`. 1. Print the value that `p2` points to. What are printed values? ::: ## Arrays can be used as pointers :::{literalinclude} ../code/arrays_can_be_used_as_pointers.c :language: c ::: :::{literalinclude} ../code/arrays_can_be_used_as_pointers.txt :language: text ::: We observe: - `msg[1]` gives the same value as `*(msg + 1)`. Note that we can do pointer arithmetic like we do with other types. We repeat our experiment with an `int` array: :::{literalinclude} ../code/arrays_can_be_used_as_pointers-with_integer_array.c :language: c ::: :::{literalinclude} ../code/arrays_can_be_used_as_pointers-with_integer_array.txt :language: text ::: ::::{exercise} Pointer arithmetic with `char` vs `int` :label: pointer-arithmetic-with-char-vs-int Something is odd. We added 1 to the pointer but the address increased by four 😮. What could be the reason? :::{dropdown} Hint What is `sizeof(int)`? ::: :::: :::{exercise} Calculating `pointer + n` :label: calculating-pointer-plus-n 1. What would be the difference if you use a `double`? Guess and check your answer by modifying the code above. 2. Can you write a general formula for `pointer + n` on the paper using (1) the size of the data type and (2) address stored in the `pointer`? ::: ## Pointers can be used as arrays :::{literalinclude} ../code/pointers_can_be_used_as_arrays.c :language: c ::: :::{literalinclude} ../code/pointers_can_be_used_as_arrays.txt :language: text ::: Pointer does not know its boundaries if used as an array, but an array variable does. ## Pass by address vs pass by value ```c void set_to_42(int *n); // pass by address void set_to_58(int n); // pass by value. Value is copied and given to the function ``` :::{wpd} pass by address :id: Evaluation_strategy#Call_by_address a parameter passing method where the address of the argument is passed. ::: :::{wpd} pass by value :id: Evaluation_strategy#Call_by_value the value of the argument is bound to the corresponding variable in the function typically by copying the value into a new memory region. ::: :::{activity} Pass by address vs value using scalar values and array :label: pass-by-address-vs-value-using-scalars-and-array 1. Implement the functions above in using the following template and use them. 1. Guess the output before you run your program. ```{literalinclude} ../code/masked/call_by_address_and_value.c :language: c ``` ::: :::{warning} The following argument which tries to get a pointer to the array does not make sense. An array is passed as an address anyway. ```c void set_first_element_to_42(int *arr[]) ``` ::: :::{note} In literature, you will also see the term *pass by reference* instead of *pass by address*. A reference in C is always an address and an address is always a pointer. When you talk about it, use *pass by reference* and *pass by value* – every C programmer will understand it. One stands for *having remote access* and the other for *copying the data*. However if you dig deeper, then you see that in C [everything is pass by value](https://stackoverflow.com/a/78891184/13870816), because the pointers are copied when a function is executed. But it is more important how objects are modified/copied and not pointers themselves. ::: (copying-an-array-for-passing-by-value)= ## Copying an array for passing-by-value Arrays are always passed by reference to functions. To create a passing-by-value effect, we have to copy it: ::::{card} ⚡ Live programming Programming the analogy from {numref}`advantage-of-pointers`. :::{literalinclude} ../code/birthday_collage.c :language: c ::: Why did we not copy the array inside of `careless_friend`? Because then the storage area will not be available. ::::