Why reflect.MakeSlice Returns Un-Addressable Value

In Go, the reflect package provides an interface to inspect and manipulate types and values. reflect.MakeSlice creates a slice of a given type. However, the returned value is un-addressable. This means that you cannot take its address or assign it to a pointer.

Solution: Using reflect.New()

To obtain an addressable slice using reflection, you can use the reflect.New() function to create a pointer to the slice. This can be done as follows:

// Create the slice type
sliceType := reflect.SliceOf(SomeType)

// Create a slice using reflect.MakeSlice
slice := reflect.MakeSlice(sliceType, 10, 10)

// Create a pointer to the slice
slicePtr := reflect.New(slice.Type())

// Set the slice pointer to the slice
slicePtr.Elem().Set(slice)

Now, you have an addressable slice that can be passed as an argument to functions that require a slice pointer.

Why reflect.MakeSlice Returns Un-Addressable Value

Local stack variables in Go are not addressable because the runtime may move them to the heap at any time. reflect.MakeSlice creates a local slice variable, which is why the returned value is un-addressable.

Why a Pointer to a Slice is Required

Some APIs, such as the one you mentioned from the mgo package, require a pointer to a slice as an argument. This is because when appending to the slice, a new slice with increased capacity may be allocated. If you pass a non-pointer slice, the changes made to the slice will not be visible outside the function.