Go类型方法集如何在内存中分配？

Question

C++ avoids allocating memory for class methods every time an instance is created. My gut feeling is to assume that Go also mitigates this kind of duplication. Just to confirm, does Go store the method set of a custom struct only once?

type Custom struct {
    value string
}

func (c Custom) TurnItUp() {
    c.value = "up"
}

func (c Custom) TurnItDown() {
    c.value = "down"
}

... // Many more methods defined for Custom. 
    // (Positive and negative directions in 100 dimensions)

func main() {
    var many []Custom
    fmt.Println("Memory: ", foo.memory()) // Measure memory used.
    for i := 0; i < 10000; i++ {
        append(many, Custom{value: "nowhere"})
    } 
    fmt.Println("Memory: ", foo.memory()) // Measure memory used. 
}

Answer 1

The runtime allocates an itable when a concrete type is assigned to an interface type. The itable for the concrete type and interface type is cached and used on later assignments.

As an example, this code will allocate one itable:

type Volume interface {
    TurnItUp()
    TurnItDown()
}
var many []Volume
for i := 0; i < 10000; i++ {
    many = append(many, Custom{value: "nowhere"})
}

and this code will allocate two itables, one for (Custom, Upper) and one for (Custom, Downer):

type Upper interface {
    TurnItUp()
}
type Downer interface {
    TurnItDown()
}
var uppers []Upper
var downers []Downer
for i := 0; i < 10000; i++ {
    uppers = append(uppers, Custom{value: "nowhere"})
    downers = append(downers, Custom{value: "nowhere"})
}

Because the example in the question does not assign a Custom value to an interface, no itables are created.

The runtime uses static metadata data to construct itables. The static data is allocated and initialized once.

See Go Data Structures: Interfaces for more details.