How to check variable type at runtime in Go language

Question

I have few C functions declared like this  CURLcode curl wrapper easy setopt long CURL  curl  CURLoption option  long param   CURLcode curl wrapper easy setopt str CURL  curl  CURLoption option  char  param     I would like to expose those as one Go function like this  func  e  Easy SetOption option Option  param interface      so I need to be able to check param type at runtime  How do I do that and is this good idea  if not what is good practice in this case

User · Answer

It seems that Go have special form of switch dedicate to this  it is called type switch    func  e  Easy SetOption option Option  param interface           switch v    param  type         default          fmt Printf  unexpected type  T   v      case uint64          e code   Code C curl wrapper easy setopt long e curl  C CURLoption option   C long v        case string          e code   Code C curl wrapper easy setopt str e curl  C CURLoption option   C CString v

User · Answer

The answer by  Darius is the most idiomatic  and probably more performant  method  One limitation is that the type you are checking has to be of type interface     If you use a concrete type it will fail   An alternative way to determine the type of something at run-time  including concrete types  is to use the Go reflect package   Chaining TypeOf x  Kind   together you can get a reflect Kind value which is a uint type  http   golang org pkg reflect  Kind  You can then do checks for types outside of a switch block  like so   import        fmt       reflect              x    42 y    float32 43 3  z     hello   xt    reflect TypeOf x  Kind   yt    reflect TypeOf y  Kind   zt    reflect TypeOf z  Kind    fmt Printf   T   s n   xt  xt  fmt Printf   T   s n   yt  yt  fmt Printf   T   s n   zt  zt   if xt    reflect Int       println   gt  gt  x is int     if yt    reflect Float32       println   gt  gt  y is float32     if zt    reflect String       println   gt  gt  z is string       Which prints outs   reflect Kind  int reflect Kind  float32 reflect Kind  string  gt  gt  x is int  gt  gt  y is float32  gt  gt  z is string   Again  this is probably not the preferred way to do it  but it s good to know alternative options

User · Answer

quux00 s answer only tells about comparing basic types   If you need to compare types you defined  you shouldn t use reflect TypeOf xxx   Instead  use reflect TypeOf xxx  Kind     There are two categories of types    direct types  the types you defined directly  basic types  int  float64  struct         Here is a full example   type MyFloat float64 type Vertex struct       X  Y float64    type EmptyInterface interface     type Abser interface       Abs   float64    func  v Vertex  Abs   float64       return math Sqrt v X v X   v Y v Y     func  f MyFloat  Abs   float64       return math Abs float64 f      var ia  ib Abser ia   Vertex 1  2  ib   MyFloat 1  fmt Println reflect TypeOf ia   fmt Println reflect TypeOf ia  Kind    fmt Println reflect TypeOf ib   fmt Println reflect TypeOf ib  Kind     if reflect TypeOf ia     reflect TypeOf ib        fmt Println  Not equal typeOf     if reflect TypeOf ia  Kind      reflect TypeOf ib  Kind         fmt Println  Not equal kind      ib   Vertex 3  4  if reflect TypeOf ia     reflect TypeOf ib        fmt Println  Equal typeOf     if reflect TypeOf ia  Kind      reflect TypeOf ib  Kind         fmt Println  Equal kind       The output would be   main Vertex struct main MyFloat float64 Not equal typeOf Not equal kind Equal typeOf Equal kind   As you can see  reflect TypeOf xxx  returns the direct types which you might want to use  while reflect TypeOf xxx  Kind   returns the basic types     Here s the conclusion  If you need to compare with basic types  use reflect TypeOf xxx  Kind    and if you need to compare with self-defined types  use reflect TypeOf xxx    if reflect TypeOf ia     reflect TypeOf Vertex          fmt Println  self-defined     else if reflect TypeOf ia  Kind      reflect Float64       fmt Println  basic types

User · Answer

What s wrong with  func  e  Easy SetStringOption option Option  param string  func  e  Easy SetLongOption option Option  param long    and so on

User · Answer

See type assertions here   http   golang org ref spec Type assertions  I d assert a sensible type  string  uint64  etc only and keep it as loose as possible  performing a conversion to the native type last

[go] How to check variable type at runtime in Go language

Examples related to go