What is the difference between a nil interface and an interface holding a nil pointer?

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Accepted Answer

type MyError struct{ msg string }
func (e *MyError) Error() string { return e.msg }

// BUGGY — returns non-nil interface holding nil pointer
func riskyOperation(fail bool) error {
    var err *MyError // nil pointer
    if fail {
        err = &MyError{"something failed"}
    }
    return err // WRONG: even if err is nil, interface is not nil!
}

result := riskyOperation(false)
if result != nil { // this is TRUE even though err pointer is nil!
    fmt.Println("unexpected error:", result)
}

// CORRECT — return untyped nil
func safeOperation(fail bool) error {
    if fail {
        return &MyError{"something failed"}
    }
    return nil // returns nil interface, not typed nil pointer
}

// Inspect the internals
fmt.Printf("%T %v
", result, result) // *MyError <nil> An interface value is nil only when both its type and value fields are nil Assigning a typed nil pointer to an interface creates a non-nil interface Always return untyped nil (return nil) for error return values Use errors.As() to inspect error types rather than direct type assertions on possibly-nil errors This is one of the most common subtle bugs in Go code — affects all interface types, not just error

What is the difference between a nil interface and an interface holding a nil pointer?

A nil interface has no type and no value. An interface holding a nil pointer has a concrete type but a nil pointer value — this interface is NOT nil, which is a classic Go gotcha in error handling.

What is the difference between a nil interface and an interface holding a nil pointer?

A nil interface has no type and no value. An interface holding a nil pointer has a concrete type but a nil pointer value — this interface is NOT nil, which is a classic Go gotcha in error handling.

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