GoLang-内存分配-[]字节与字符串

Question

In the below code:

c := "fool"
d := []byte("fool")
fmt.Printf("c: %T, %d
", c, unsafe.Sizeof(c)) // 16 bytes
fmt.Printf("d: %T, %d
", d, unsafe.Sizeof(d)) // 24 bytes

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To decide the datatype needed to receive JSON data from CloudFoundry, am testing above sample code to understand the memory allocation for []byte vs string type.

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Expected size of string type variable c is 1 byte x 4 ascii encoded letter = 4 bytes, but the size shows 16 bytes.

For byte type variable d, GO embeds the string in the executable program as a string literal. It converts the string literal to a byte slice at runtime using the runtime.stringtoslicebyte function. Something like... []byte{102, 111, 111, 108}

Expected size of byte type variable d is again 1 byte x 4 ascii values = 4 bytes but the size of variable d shows 24 bytes as it's underlying array capacity.

---

Why the size of both variables is not 4 bytes?

Answer 1

Both slices and strings in Go are struct-like headers:

reflect.SliceHeader:

type SliceHeader struct {
        Data uintptr
        Len  int
        Cap  int
}

reflect.StringHeader:

type StringHeader struct {
        Data uintptr
        Len  int
}

The sizes reported by unsafe.Sizeof() are the sizes of these headers, exluding the size of the pointed arrays:

Sizeof takes an expression x of any type and returns the size in bytes of a hypothetical variable v as if v was declared via var v = x. The size does not include any memory possibly referenced by x. For instance, if x is a slice, Sizeof returns the size of the slice descriptor, not the size of the memory referenced by the slice.

To get the actual ("recursive") size of some arbitrary value, use Go's builtin testing and benchmarking framework. For details, see How to get memory size of variable in Golang?

For strings specifically, see String memory usage in Golang. The complete memory required by a string value can be computed like this:

var str string = "some string"

stringSize := len(str) + unsafe.Sizeof(str)