How to determine an interface value s real type

Question

I have not found a good resource for using interface   types  For example  package main  import  fmt   func weirdFunc i int  interface         if i    0           return  zero            return i   func main         var i   5     var w   weirdFunc 5          this example works      if tmp  ok    w  int   ok           i    tmp            fmt Println  i     i      Do you know of a good introduction to using Go s interface     specific questions    how do I get the  real  Type of w  is there any way to get the string representation of a type  is there any way to use the string representation of a type to convert a value

User · Answer

You can use reflection  reflect TypeOf    to get the type of something  and the value it gives  Type  has a string representation  String method  that you can print

User · Answer

You also can do type switches   switch v    myInterface  type    case int         v is an int here  so e g  v   1 is possible      fmt Printf  Integer   v   v  case float64         v is a float64 here  so e g  v   1 0 is possible      fmt Printf  Float64   v   v  case string         v is a string here  so e g  v     Yeah   is possible      fmt Printf  String   v   v  default         And here I m feeling dumb         fmt Printf  I don t know  ask stackoverflow

User · Answer

Type switches can also be used with reflection stuff   var str    hello   var obj   reflect ValueOf  amp str   switch obj Elem   Interface    type    case string      log Println  obj contains a pointer to a string   default      log Println  obj contains something else

User · Answer

I m going to offer up a way to return a boolean based on passing an argument of a reflection Kinds to a local type receiver  because I couldn t find anything like this    First  we declare our anonymous type of type reflect Value   type AnonymousType reflect Value   Then we add a builder for our local type AnonymousType which can take in any potential type  as an interface    func ToAnonymousType obj interface    AnonymousType       return AnonymousType reflect ValueOf obj       Then we add a function for our AnonymousType struct which asserts against a reflect Kind   func  a AnonymousType  IsA typeToAssert reflect Kind  bool       return typeToAssert    reflect Value a  Kind       This allows us to call the following   var f float64   3 4  anon    ToAnonymousType f   if anon IsA reflect String        fmt Println  Its A String      else if anon IsA reflect Float32        fmt Println  Its A Float32      else if anon IsA reflect Float64        fmt Println  Its A Float64      else       fmt Println  Failed       Can see a longer  working version here https   play golang org p EIAp0z62B7

User · Answer

Here is an example of decoding a generic map using both switch and reflection  so if you don t match the type  use reflection to figure it out and then add the type in next time   var data map string interface          for k  v    range data       fmt Printf  pair  s t s n   k  v          switch t    v  type        case int          fmt Printf  Integer   v n   t      case float64          fmt Printf  Float64   v n   t      case string          fmt Printf  String   v n   t      case bool          fmt Printf  Bool   v n   t      case   interface             for i n    range t               fmt Printf  Item   v   v n   i  n                default          var r   reflect TypeOf t          fmt Printf  Other  v n   r

User · Answer

There are multiple ways to get a string representation of a type  Switches can also be used with user types   var user interface   user   User name   Eugene         type  can only be used inside a switch switch v    user  type    case int         Built-in types are possible  int  float64  string  etc       fmt Printf  Integer   v   v  case User         User defined types work as well       fmt Printf  It s a user   s n   user  User  name        You can use reflection to get  reflect rtype userType    reflect TypeOf user  fmt Printf    v n   userType      You can also use  T to get a string value fmt Printf   T   user      You can even get it into a string userTypeAsString    fmt Sprintf   T   user   if userTypeAsString     main User        fmt Printf   nIt s definitely a user       Link to a playground  https   play golang org p VDeNDUd9uK6

User · Answer

Your example does work  Here s a simplified version   package main  import  fmt   func weird i int  interface         if i  lt  0           return  negative            return i    func main         var i   42     if w  ok    weird 7   int   ok           i    w           if w  ok    weird -100   int   ok           i    w           fmt Println  i     i     Output  i   49   It uses Type assertions

[types] How to determine an interface{} value's "real" type?

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