3 ms·

My second thought was that go will pass the struct by value, so even if it didn’t crash that member would not have been set in the parent function.

TheDong 6y ago
Since the argument is "context.Context", an interface type, it's not necessarily passing a value. It is actually a pointer since the implementation, if you look at the definition of `context.TODO()`, is of type `* context.emptyContext`. Looking at just a function definition, you can't know if a variable is a value or a pointer unless you first determine if it's an interface type, and if it is an interface type, you have to know the concrete type of the caller to know. However, even a struct passed by value can be mutated since interior mutability can be managed with internal pointers. For example: type value struct { interior *int } func(v value) mutate(to int) { *v.interior = to } func(v value) String() string { return fmt.Sprintf("%d", *v.interior) } func newValue(i int) value { return value{&i} } func mutateValue(v value, to int) { v.mutate(to) } func main() { val := newValue(10) mutateValue(val, 20) fmt.Println(val) }
- tsimionescu 6y ago
  Aren't all interface variables pointers in Go, regardless of whether the type implementing the interface is a pointer or a value?
  - morelisp 6y ago
    Interfaces are internally pointers, insofar as the value passed when you pass around an interface is a pointer to something so that the value passed is a fixed size regardless of the underlying type (otherwise you couldn't have any arrays of interfaces). But if the interface is implemented by a pointer-to-T and you pass a T, you will get an error; and if you have wrapped a T in one interface and try to convert it to another interface that is only satisfied by a pointer-to-T, the conversion will fail. So the answer to "are interfaces are always pointers?" is yes if you're asking about implementing the Go compiler or worried about the size of some allocation; but no if you are asking about Go's type system.

TheDong 6y ago

Since the argument is "context.Context", an interface type, it's not necessarily passing a value. It is actually a pointer since the implementation, if you look at the definition of `context.TODO()`, is of type `* context.emptyContext`. Looking at just a function definition, you can't know if a variable is a value or a pointer unless you first determine if it's an interface type, and if it is an interface type, you have to know the concrete type of the caller to know. However, even a struct passed by value can be mutated since interior mutability can be managed with internal pointers. For example: type value struct { interior *int } func(v value) mutate(to int) { *v.interior = to } func(v value) String() string { return fmt.Sprintf("%d", *v.interior) } func newValue(i int) value { return value{&i} } func mutateValue(v value, to int) { v.mutate(to) } func main() { val := newValue(10) mutateValue(val, 20) fmt.Println(val) }

tsimionescu 6y ago
Aren't all interface variables pointers in Go, regardless of whether the type implementing the interface is a pointer or a value?
- morelisp 6y ago
  Interfaces are internally pointers, insofar as the value passed when you pass around an interface is a pointer to something so that the value passed is a fixed size regardless of the underlying type (otherwise you couldn't have any arrays of interfaces). But if the interface is implemented by a pointer-to-T and you pass a T, you will get an error; and if you have wrapped a T in one interface and try to convert it to another interface that is only satisfied by a pointer-to-T, the conversion will fail. So the answer to "are interfaces are always pointers?" is yes if you're asking about implementing the Go compiler or worried about the size of some allocation; but no if you are asking about Go's type system.