In Go language, you need to pay attention to its underlying structure to determine whether the interface is nil. 1. The interface contains type information and value information. The interface is nil only if neither is assigned to the interface; 2. If the specific type of nil is assigned to the interface, the interface is not nil, because the type information is still saved; 3. The correct method is to directly use == nil to judge, which is suitable for most scenarios; 4. The reflection method can be reflected.Value.IsNil(), but attention should be paid to type limitations and panic risks, which are suitable for advanced dynamic processing; 5. It is recommended to avoid assigning specific type nil to the interface, and clarify the document to explain the meaning of nil to ensure that the actual type behind the interface is correctly understood.
In Go, it is not as simple as it seems to determine whether an interface value is nil
. Many people think that just using == nil
to judge is done, but in fact, you may encounter some situations where "it seems to be nil, but it does not equal nil".

The underlying structure of the interface determines the judgment logic
Go's interface variables actually have two pointers: one pointing to type information and the other pointing to the actual value. That is to say, even if you set a specific value to nil
, as long as the type information still exists, the interface variable will not be nil
.

For example:
var val *int // val is type *int, the current value is nil var i interface{} = val fmt.Println(i == nil) // Output false!
The output here is not true
, because although val
is nil
, after assigning the value to the interface, the interface saves *int
type information and nil
value. In this case, the interface itself is not nil
.

Method to correctly determine whether the interface is nil
To determine whether an interface variable is nil
, the easiest and safest way is to use == nil
directly to compare:
var i interface{} fmt.Println(i == nil) // true
But if you pass in a nil
with a specific type (such as *int(nil)
), the result will be false
. So the key is: it is the real nil only if the interface variable itself is not assigned any type and value .
Common misunderstandings include:
- Assign a specific type of
nil
to an interface, mistakenly thinking that the interface isnil
- Don't pay attention to type erasing when using reflection judgment
Reflection method checks whether the interface value is nil (advanced)
If you need to use reflection to determine whether the interface value is nil
, you can use reflect.Value.IsNil()
method. This method is suitable for processing pointers, interfaces, etc. that can be nil
:
var i interface{} = (*int)(nil) v := reflect.ValueOf(i) if v.Kind() == reflect.Interface || v.Kind() == reflect.Ptr { fmt.Println(v.IsNil()) // Output true }
But pay attention to the following points:
- If the incoming primitive type (such as int, string, etc.), calling
IsNil()
will panic - You need to first determine whether
Kind()
supportsIsNil()
- Not recommended for daily basic judgment unless you do need to deal with various types dynamically
Suggestions in actual development
When writing code in daily life, in most cases we only need to make simple == nil
judgments, without introducing reflection or other complex logic. More complex means may be required only when dealing with generics, middleware, or framework-layer logic.
A few tips:
- Try to avoid making judgments after assigning specific types of
nil
to interface variables. - If the function returns the interface type, make sure the document states clearly what nil means
- When using the interface value returned by a third-party library, pay attention to the real type behind it
Basically that's it. Go's interface nil judgment seems simple, but only after understanding its mechanism can you avoid trampling on pitfalls.
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