How to initialize a struct using a pointer

Question

When I want to initialize all the components of a struct I do it in the main function like this:

This is the struct:

typedef struct {
    int data[1000];
    int oc;
} Table;

And this is how I initialize all the components to be 0 (the array and the int now are 0 with this).

int main() {
    Table x = {0};

Now I want to do exactly the same but using a function. I want to do something like this:

void initialize(Table *y) {
    y = {0};
}

I think it does not work because to initialize it I should do it when I declare it, so how can I initialize a struct using a function?

There is no point passing a struct to a function if you are going to treat it as a local variable and overwrite whatever was in it. — Weather Vane
– Weather Vane, Commented Dec 1, 2021 at 18:06

Some programmer dude · Accepted Answer · 2021-12-01 18:26:20Z

6

Remember that y us a pointer so you must dereference it to assign the object itself.

Also you need to tell the compiler that the assignment is from a Table object, which is done with a compound literal.

All in all:

void initialize(Table *y){
    *y = (Table){0};
}

The compound literal creates (Table){0} creates a temporary Table structure object, with the initializer for the structure. Then this temporary structure object is assigned (copied to) the Table structure object that y points to.

It's somewhat similar to the following:

void initialize(Table *y){
    Table temp_struct_object = {0};  // Normal initialization
    *y = temp_struct_object;  // Normal assignment (copy of object)
}

edited Dec 1, 2021 at 18:26

answered Dec 1, 2021 at 18:11

Some programmer dude

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6 Comments

0___________ Over a year ago

And it is usually the most efficient way as well (assuming compiler optimizations enabled)\

Tenko Over a year ago

This method is only to initialize structs or can it be used in arrays like this(It doesn´t work so how can I use that method to initialize whatever I want but not doing it in the declaration as the first example I have put? int main(){ int table[5]; table = table{0};

Some programmer dude Over a year ago

@Tenko Arrays can't be assigned to from the beginning, only copied to.

Some programmer dude Over a year ago

@Tenko It's possible to create array like that as well, for example (int[5]){1, 2, 3, 4, 5}. But as mentioned it's not possible to assign to array.

Tenko Over a year ago

``` int main(){ int table[5]; int table2[5]={0}; table=table2; ``` To assign to array you mean doing this? Why this can not be done as the structs?

|

pmg · Accepted Answer · 2021-12-01 18:07:35Z

6

Just memset it.

void initialize(Table *y) {
    memset(y, 0, sizeof *y);
}

answered Dec 1, 2021 at 18:07

pmg

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