TIL: Initializing C objects

When working in C, there are a few ways to intialize objects. The simplest is the direct assignment:

int obj = 15;
size_t hash = 14695981039346656037ULL; // FNV offset basis constant
char* ptr = 0;
char a = 'a';
char* str = "a string";

For simple object types, you can just put a correctly formatted value on the righthand side of the = assignment operator.

For fixed-length array types, there is another initializer syntax that uses curly braces:

int nums[3] = { 1, 2, 3 };
int nums[5] = { 1, 2, 3 }; // the remainder of the values are set to 0
int nums[5] = { // set to [0, 2, 0, 0, 3]
	[1] = 2,
	[4] = 3,
};

For structs, initializer syntax looks like:

struct A {
	int B;
	int C;
};

struct A a = {
	.B = 1,
	.C = 2,
};

struct A a2 = { 0 }; // sets all fields to 0; can be used for any object to set to its 0 value.

For variable length arrays or malloc’d arrays, intialization requires looping:

int* arr = malloc(sizeof(int)*10);
for (size_t i = 0; i < 10; i++) {
	arr[i] = i;
}

Tags: til c