dongtan9465
dongtan9465
2015-11-19 04:12
浏览 52

Golang MongoDB(mgo)查找反射错误

With the following code

func (s Store) Lookup(department string, number string) (*types.Course, error) {
    var result *types.Course
    err := s.collection.Find(bson.M{
        "department":    department,
        "course_number": number,
    }).One(result)

    if err != nil {
        switch err {
        case mgo.ErrNotFound:
            return nil, ErrNotFound
        default:
            log.Error(err)
            return nil, ErrInternal
        }
    }

    return result, nil
}

I came across the error:

reflect: reflect.Value.Set using unaddressable value

If I change the first line from var result *types.Course to result := &types.Course{} there is no error. What exactly is the difference between these two?

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1条回答 默认 最新

  • douzhang1299
    douzhang1299 2015-11-19 04:40
    已采纳

    The two otions both declare a variable of type *types.Course. The first pointer value is nil. The second is initialized to point at a value of type types.Course.

     var result *types.Course    // result == nil
     result := &types.Course{}   // result != nil, points to a value.
     result := new(types.Course) // basically the same as the second
    

    The mgo function requires a pointer to a value. A nil pointer does not point to a value.

    The typical way to write this code is:

    var result types.Course   // declare variable of result type, not a pointer
    err := s.collection.Find(bson.M{
        "department":    department,
        "course_number": number,
    }).One(&result)           // pass address of value to function
    
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