Using a struct vs a pointer to a struct

Question

I was working on the following as an example to see the differences between passing an object directly and then passing a pointer to it:

#include "stdio.h"

// Car object
typedef struct Car {
    char*           name;
    unsigned int    price;
} Car;

void print_car(Car car) {
    printf("<Car: %s, Price: $%d>", car.name, car.price);
};

void print_car2(Car *car) {
    printf("<Car: %s, Price: $%d>", car->name, car->price);
};

int main(int argc, char* argv[]) {

    Car chevy = {chevy.name = "Chevy", chevy.price = 45000};
    print_car(chevy);

    Car mazda = {chevy.name = "Mazda", chevy.price = 30000};
    print_car2(&mazda);

    return 1;

}

Other than the first approach being much more readable and easier to understand for me, what are the differences between the two? When would passing a pointer be the only option, and why do both work in the above case?

Answer 1

There are two reasons to use the second approach:

1. If you want to avoid copying the whole struct. If the struct is big, this can affect performance.
2. If you want to modify struct that you're passing.

Answer 2

In general (not only for structs) passing a variable to a function will make a copy of this variable so if you want to alter this variable you ll have to return the value of the altered copy but you may want to alter the variable and return something else, in this case you have no other choice of passing a pointer as argument exemple :

first exemple with passing a variable

int useless_func(int nb) /*nb is actually a copy of the variable passed as argument */
{
    nb++; /* the copy was incremented, not the real variable */
    return nb; /* the new value is returned */
}
int main()
{
    int nb = 1;

    useless_func(nb); /* here nb is still = 1 cause it wasn't altered by the function */
    nb = useless_func(nb); /* here nb is = 2 cause it took the return value of the func */
}

now a second stupid exemple with pointer :

char *useless_func(int *nb) /* nb is now a pointer to the variable */
{
    *nb++; /* the derefencement of the pointer (so the variable value) was incremented */
    return strdup("This is a useless return"); /* we return some other stupid stuff */
}
int main()
{
    int nb = 1;
    char *str = useless_func(&nb); /* now nb is = 2 and str is an allocated useless string woohoo */
}