cpp core · high

Struct Alignment & Padding

Struct layout follows declaration order. Each member must start at an offset that is a multiple of its alignment, so the compiler inserts padding between fields and at the end (rounding sizeof up to a multiple of alignof). Reordering members from largest to smallest alignment eliminates most internal padding — here it shrinks the struct from 12 to 8 bytes.

The code

struct Bad { // declaration order IS the memory order
    char a;  //  offset 0   (1 byte)
    int b;   //  needs a 4-byte-aligned offset -> goes to 4
    char c;  //  offset 8
}; //  sizeof == 12, alignof == 4  (6 bytes wasted)

struct Good { // reordered: largest member first
    int b;    //  offset 0
    char a;   //  offset 4
    char c;   //  offset 5
}; //  sizeof == 8,  alignof == 4  (2 trailing pad)

static_assert(sizeof(Bad) == 12);
static_assert(sizeof(Good) == 8);

What this lesson walks through

1. 01Members keep their declared order
2. 02char a — alignment 1
3. 03int needs a 4-aligned offset
4. 04int b lands at offset 4
5. 05Trailing padding rounds the size up
6. 06Reorder largest -> smallest
7. 07Pack hot structs tight

A struct's fields are laid out in the order you declare them. The compiler may leave gaps between them so that each member starts at an address its type can tolerate.