英文:
Unsafe pointers, emptyInterface word
问题
我正在尝试理解Go语言中的unsafe.Pointers内部机制。在结构体中只有一个字段并且返回指向Foo的指针时，是否有办法始终返回指向Foo而不是Bar的指针？只有当*Foo或Foo具有多个字段时才起作用。这是编译器优化，还是与堆/栈分配有关？
type Bar struct {
ID int
Name string
Price float64
}
type Foo struct {
Bar *Bar
}
aFoo := Foo{
Bar: &Bar{},
}
// xunsafe.AsPointer 实现：
//
// func AsPointer(v interface{}) unsafe.Pointer {
// empty := (*emptyInterface)(unsafe.Pointer(&v))
// return empty.word
//}
fmt.Printf("Foo ptr: %v, Bar ptr: %v\n", AsPointer(aFoo), unsafe.Pointer(aFoo.Bar))
// 输出: Foo ptr: 0xc00010a060, Bar ptr: 0xc00010a060
编辑：我正在使用go1.17版本。
英文:
I am trying to understand Go unsafe.Pointers internal. In case when there is only one field in the struct and returns me a pointer to the Foo, returns a pointer to the Bar.
Is there a way to actually return allways pointer to the Foo not Bar? It works only when *Foo or Foo has more than one field. Is it compiler optimization, or is it related with Heap/Stack allocation?
type Bar struct {
ID int
Name string
Price float64
}
type Foo struct {
Bar *Bar
}
aFoo := Foo{
Bar: &Bar{},
}
//xunsafe.AsPointer implementation:
//
// func AsPointer(v interface{}) unsafe.Pointer {
// empty := (*emptyInterface)(unsafe.Pointer(&v))
// return empty.word
//}
fmt.Printf("Foo ptr: %v, Bar ptr: %v\n", AsPointer(aFoo), unsafe.Pointer(aFoo.Bar))
//Output: Foo ptr: 0xc00010a060, Bar ptr: 0xc00010a060
Edit: I am using the go1.17.
答案1
得分: 2
使用&aFoo来获取指向Foo的指针。
类型unsafe.Pointer是编译器理解的一种特殊类型。编译器允许在unsafe.Pointer和其他类型之间进行转换，而这些转换在语言规范中通常是不允许的。
关于xunsafe.Pointer：你会看到不同的结果，因为这两种类型在存储在接口值中时具有不同的表示形式。
type Foo struct {
Bar *Bar
}
type Foo struct {
Bar *Bar
ASecondField int
}
这是因为这些类型在存储在接口值中时具有不同的表示形式。大于一个字的值被复制到接口值私有的内存中，并且接口值存储指向该内存的指针。xunsafe.AsPointer函数确实有处理这两种表示形式的代码。
英文:
Use &aFoo to get a pointer to the Foo.
The type unsafe.Pointer is a special type understood by the compiler. The compiler allows conversions between unsafe.Pointer and other types that are not normally allowed by the language specification.
Regarding xunsafe.Pointer: You see different results for
type Foo struct {
Bar *Bar
}
and
type Foo struct {
Bar *Bar
ASecondField int
}
because those types have different representations when stored in an interface value. Values larger than a word are copied to memory private to the interface value and the interface value stores a pointer to that memory. The xunsafe.AsPointer function does have code to handle both of representations.
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