Structures

A struct is just a container, packaging different related variables.

struct Rectangle { // (1)!
    int width = 10; // (2)!
    int length;
    char* label;
} rect1; // (3)!

int main () {
    struct Rectangle rect2; // (4)!
    return 0;
}

1. The name of the struct (Rectangle in this case) is optional.
2. struct is just a blueprint for defining a type, therefore the variables inside (called the members) do not allocate storage unless a variable of type Rectangle is declared. The struct can be defined within a scope as well.
3. Defines a global variable rect1 of type struct Rectangle.
4. Defines a local variable rect2 of type struct Rectangle.

The struct cannot contain itself but can contain pointer to it.

struct Node {
    int value;
    struct Node* leftNode;
    struct Node* rightNode;
};

Padding and Size

You can also use sizeof operator to find out the size taken up by a struct.

struct Foo {
    char x;
    char y;
};

int main () {
    printf("%d\n", sizeof(Foo)); // (1)!
}

1. Outputs 2.

struct Foo {
    char x;
    int y;
};

int main () {
    printf("%d\n", sizeof(Foo)); // (1)!
}

1. Outputs 8. Because the int takes 4 bytes, the compiler adds some padding bytes to the char to make it aligned.

Each green block represents a byte for corresponding type within the struct.