go - 当接收者方法 T 不能接受 *T 时？

Question

Go 官方网站 writes as follows :

As the Go specification says, the method set of a type T consists of all methods with receiver type T, while that of the corresponding pointer type *T consists of all methods with receiver *T or T. That means the method set of *T includes that of T, but not the reverse.

This distinction arises because if an interface value contains a pointer *T, a method call can obtain a value by dereferencing the pointer, but if an interface value contains a value T, there is no safe way for a method call to obtain a pointer. (Doing so would allow a method to modify the contents of the value inside the interface, which is not permitted by the language specification.)

Even in cases where the compiler could take the address of a value to pass to the method, if the method modifies the value the changes will be lost in the caller.

我的问题是，编译器何时不能将值传递给指针接收器值？

Answer 1

Addressable 定义于 https://golang.org/ref/spec#Address_operators :

For an operand x of type T, the address operation &x generates a pointer of type *T to x. The operand must be addressable, that is, either a variable, pointer indirection, or slice indexing operation; or a field selector of an addressable struct operand; or an array indexing operation of an addressable array. As an exception to the addressability requirement, x may also be a (possibly parenthesized) composite literal.

计数器示例包括映射值和函数:

func f() {}

func main() {
    var m map[string]string
    p1 := &m["foo"] // cannot take the address of m["foo"]
    p2 := &f        // cannot take the address of f
}