How Do I Determine the Size of a Go Structure in Bytes?

Determining the Size of a Go Structure

In Go, accessing the size of a structure can be achieved programmatically using the unsafe.Sizeof method. This method takes a value of a specific type, in this case a structure, and returns its memory size in bytes.

For example, to determine the size of the following structure:

type Coord3d struct {
    X, Y, Z int64
}

You can use the following code:

import (
    "fmt"
    "unsafe"
)

func main() {
    // Create a Coord3d structure
    var coord Coord3d

    // Get the size of the Coord3d structure
    size := unsafe.Sizeof(coord)

    fmt.Println("Size of Coord3d:", size, "bytes")
}

Additional Insights

While unsafe.Sizeof provides the memory size, it's crucial to consider that the reported size does not include any memory referenced by the structure. For instance, if the structure contains a reference to a slice, the size returned will represent only the size of the slice descriptor, not the size of the referenced data.

Calculating Structure Size Intuitively

To calculate the size of a structure manually, consider the following rules:

• The size of the structure depends on the types of the fields it contains and their order.
• Different padding is used based on the order of fields.
• A bool, int8/uint8 occupies 1 byte.
• int16, uint16 occupy 2 bytes.
• int32, uint32, float32 occupy 4 bytes.
• int64, uint64, float64, pointers occupy 8 bytes.
• A string is 16 bytes long, using 2 alignments of 8 bytes.
• Any slice occupies 24 bytes, with 3 alignments of 8 bytes.
• An array of length n occupies n times the size of its elements.

Using these rules, you can manually determine the size of a structure. However, it's advisable to cross-check your calculations using the unsafe.Sizeof method for verification.

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