How Can I Programmatically Determine the Size of a Go Struct?

Obtaining the Size of Go Structs Programmatically

Calculating the size of a Go struct involves understanding the underlying types and the alignment requirements.

Unsafe Package Method

The Sizeof method from the unsafe package returns the size of a variable or struct. However, it's important to note that this size does not include memory referenced by the variable.

import "unsafe"

func GetStructSize(s interface{}) int {
    return int(unsafe.Sizeof(s))
}

Calculating Struct Size Manually

The size of a struct depends on the types of its fields and the order in which they appear. Here are some guidelines:

• Primitive types: 1 byte (bool, int8, uint8), 2 bytes (int16, uint16), 4 bytes (int32, uint32, float32), 8 bytes (int64, uint64, float64, pointer).
• string: 16 bytes (two 8-byte alignments)
• slice: 24 bytes (three 8-byte alignments)
• array: n * size of its element type
• Fields are aligned to their size. If a field's size is not divisible by its alignment, padding is added.

For example:

type Coord3d struct {
    X, Y, Z int64
}

// Size of Coord3d is 24 bytes (3 * 8 bytes)

Verifying Size with an Online Service

You can also verify your calculated size using an online service like [Go Playground](https://play.golang.org/):

func main() {
    type Coord3d struct {
        X, Y, Z int64
    }
    
    fmt.Println(unsafe.Sizeof(Coord3d{})) // Outputs: 24
}

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