How to Calculate the Size of a Go Struct?

How to Determine the Size of a Struct in Go

Go's unsafe package provides a Sizeof method that returns the size of a value in bytes. For example, to determine the size of the Coord3d struct defined below, you can use the following code:

import "unsafe"

type Coord3d struct {
    X, Y, Z int64
}

func main() {
    fmt.Println(unsafe.Sizeof(Coord3d{}))
}

However, it's important to note that the size of a struct does not include the memory referenced by its fields. For instance, if a struct contains a slice, the Sizeof method will return the size of the slice descriptor, not the size of the memory referenced by the slice.

Calculating Struct Size Manually

In addition to using the Sizeof method, you can also calculate the size of a struct manually by following these rules:

• The size of a struct depends on the types of its fields and their order. Two structs with the same fields can have different sizes due to different padding requirements.
• The alignment of a field is determined by its type. For example, int64 has an alignment of 8 bytes, while bool has an alignment of 1 byte.
• The padding is the space added to the end of a field to align it to its required alignment. It ensures that the struct is aligned to its largest field.

Based on these rules, you can calculate the size of any struct by summing the sizes of its fields and adding any necessary padding. For example, the Coord3d struct has three int64 fields, each of which has an alignment of 8 bytes. Therefore, the size of the Coord3d struct is 24 bytes. You can verify your calculations using a service like gostructsize.appspot.com.

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