英文:
Why can't I get the address of a type conversion in Go?
问题
当我编译这段代码时，编译器告诉我无法获取str(s)的地址。
func main() {
s := "hello, world"
type str string
sp := &str(s)
}
所以我的问题是，类型转换是否会寻找一个新的地址来定位当前的新s，或者还有其他我没有考虑到的情况吗？
英文:
When I compile this code, the compiler tells me that I cannot take the address of str(s).
func main() {
s := "hello, world"
type str string
sp := &str(s)
}
So my question is whether a type conversion may look for a new address to locate the current new s, or something else that I haven't thought of?
答案1
得分: 8
《Go编程语言规范》
表达式
表达式通过将运算符和函数应用于操作数来指定值的计算。
转换
转换是形式为T(x)的表达式，其中T是类型，x是可以转换为类型T的表达式。
地址运算符
对于类型为T的操作数x，地址操作符&x生成一个指向x的类型为*T的指针。操作数必须是可寻址的，即变量、指针间接引用或切片索引操作；或者是可寻址结构操作数的字段选择器；或者是可寻址数组的数组索引操作。作为对可寻址要求的例外，x也可以是一个（可能带括号的）复合字面量。如果对x的求值会导致运行时恐慌，则对&x的求值也会导致恐慌。
表达式是临时的、短暂的值。表达式值没有地址。它可以存储在寄存器中。转换是一种表达式。例如，
package main
import (
"fmt"
)
func main() {
type str string
s := "hello, world"
fmt.Println(&s, s)
// 错误：无法取 str(s) 的地址
sp := &str(s)
fmt.Println(sp, *sp)
}
输出：
main.go:13:8: 无法取 str(s) 的地址
要使值可寻址，它必须是持久的，就像一个变量一样。例如，
package main
import (
"fmt"
)
func main() {
type str string
s := "hello, world"
fmt.Println(&s, s)
ss := str(s)
sp := &ss
fmt.Println(sp, *sp)
}
输出：
0x1040c128 hello, world
0x1040c140 hello, world
英文:
> The Go Programming Language Specification
>
> Expressions
>
> An expression specifies the computation of a value by applying
> operators and functions to operands.
>
> Conversions
>
> Conversions are expressions of the form T(x) where T is a type and x
> is an expression that can be converted to type T.
>
> 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. If the evaluation of x would cause a run-time
> panic, then the evaluation of &x does too.
Expressions are temporary, transient values. The expression value has no address. It may be stored in a register. A comversion is an expression. For example,
package main
import (
"fmt"
)
func main() {
type str string
s := "hello, world"
fmt.Println(&s, s)
// error: cannot take the address of str(s)
sp := &str(s)
fmt.Println(sp, *sp)
}
Output:
main.go:13:8: cannot take the address of str(s)
To be addressable a value must be persistent, like a variable. For example,
package main
import (
"fmt"
)
func main() {
type str string
s := "hello, world"
fmt.Println(&s, s)
ss := str(s)
sp := &ss
fmt.Println(sp, *sp)
}
Output:
0x1040c128 hello, world
0x1040c140 hello, world
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