go - 为什么结构内存大小与我的期望不一致？

Question

我初始化了一个结构并计算了内存，这与我的期望不一致。

代码是:

type Man struct {
Name     string
Money    int
Age      int32
}

func main() {

m := Man{Name:"james", Money:1000000, Age:30}

fmt.Println("man size:", unsafe.Sizeof(m))
fmt.Println("name size:", unsafe.Sizeof(m.Name))
fmt.Println("money size:", unsafe.Sizeof(m.Money))
fmt.Println("age size:", unsafe.Sizeof(m.Age))
}

结果是:

man size: 32
name size: 16
money size: 8
age size: 4

我预期的'man'大小是16 + 8 + 4 = 28，但事实是32

期待您的帮助!

Answer 1

Size and alignment guarantees:

For the numeric types, the following sizes are guaranteed:

type                                 size in bytes

byte, uint8, int8                     1
uint16, int16                         2
uint32, int32, float32                4
uint64, int64, float64, complex64     8
complex128                           16

The following minimal alignment properties are guaranteed:
1. For a variable x of any type: unsafe.Alignof(x) is at least 1.
2. For a variable x of struct type: unsafe.Alignof(x) is the largest of all the values unsafe.Alignof(x.f) for each field f of x, but at least 1.
3. For a variable x of array type: unsafe.Alignof(x) is the same as the alignment of a variable of the array's element type.
A struct or array type has size zero if it contains no fields (or elements, respectively) that have a size greater than zero. Two distinct zero-size variables may have the same address in memory.

然后，阅读以下内容:
Memory Layouts
Padding is hard - Dave Cheney

在64位计算机上的说明:

您的类型是struct，其大小取决于基础类型的定义方式。具有相同字段但顺序不同的两个结构可能具有不同的大小。结构的大小通过填充和对齐规则进行相应计数。



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