How Can I Programmatically Determine the Size of a Go Struct?
Dec 18, 2024 pm 01:22 PMObtaining 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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