Alignment of struct and Class Members

Natural alignment

To ensure the stability and efficiency of CPU operations, the storage addresses of the basic data types in the memory must be naturally aligned. The so-called natural alignment means that variables of the basic data type cannot be simply stored in any address in the memory, and their starting addresses must be divisible by their size.

For example, on a 32-bit platform, the addresses of int and pointer variables can be divisible by four.

 

Natural alignment of struct Variables

For objects of the composite type (generally struct and class), if its starting address can meetStrictest requirements (highest)The data member's natural alignment requirements, then it is natural alignment.

What is "the strictest natural alignment requirement? For distance, the spaces occupied by double, Int, and bool are 8, 4, and 1 in sequence, and their alignment rigor is reduced in turn.

Alignment of the member variables in the struct according to the alignment requirements of the basic type variables with the largest sizeof size. When necessary, the end of the object must be filled with bytes.

Summary:The length of a struct must be an integer multiple of its longest member variable.

Example

 struct {// sizeof 8, aligned in 4 bytes, 2-byte long A1; short A2;} B; struct stest // sizeof 16 at the end, which is aligned by 8 bytes and filled with 7-byte {Double B; char ;}; struct threeeshort {// sizeof 10, aligned by 2 bytes, with 1-byte short a; short B; short C; short D; char ch ;};