Structs and Unions

Structs

• In Computer Systems, structs are collections of heterogeneous data
  • laid out in consecutive memory locations (used paddings to ensure alignment)
  • reordering can reduce size!

struct node {
  short w;
  char *p;
};

... turns into ...

0   2 paddings! 8
|w|w|-|-|-|-|-|-|p|p|p|p|p|p|p|p|

Paddings

• Elements will be padded to match the size.
• If the struct has int, 4 bytes in size, it needs to start at a multiple of 4.

Unions

• Union allows elements to read/write in the same memory.
• All elements start at offset 0.
• The size of the union is the biggest member.
• Elements must be plain old data or at least default constructible.
• Bytes can be stored in reversed order depending on the endianness.