nil detection in Go

Question

I see a lot of code in Go to detect nil  like this   if err    nil           handle the error         however  I have a struct like this   type Config struct       host string       port float64     and config is an instance of Config  when I do   if config    nil       there is compile error  saying      cannot convert nil to type Config

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In Go 1 13 and later  you can use Value IsZero method offered in reflect package   if reflect ValueOf v  IsZero            v is zero  do something     Apart from basic types  it also works for Array  Chan  Func  Interface  Map  Ptr  Slice  UnsafePointer  and Struct  See this for reference

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In addition to Oleiade  see the spec on zero values      When memory is allocated to store a value  either through a declaration or a call of make or new  and no explicit initialization is provided  the memory is given a default initialization  Each element of such a value is set to the zero value for its type  false for booleans  0 for integers  0 0 for floats     for strings  and nil for pointers  functions  interfaces  slices  channels  and maps  This initialization is done recursively  so for instance each element of an array of structs will have its fields zeroed if no value is specified    As you can see  nil is not the zero value for every type but only for pointers  functions  interfaces  slices  channels and maps  This is the reason why config    nil is an error and  amp config    nil is not   To check whether your struct is uninitialized you d have to check every member for its respective zero value  e g  host        port    0  etc   or have a private field which is set by an internal initialization method  Example   type Config struct       Host string       Port float64     setup bool    func NewConfig host string  port float64   Config       return  amp Config host  port  true     func  c  Config  Initialized   bool   return c    nil  amp  amp  c setup

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I have created some sample code which creates new variables using a variety of ways that I can think of  It looks like the first 3 ways create values  and the last two create references   package main  import  fmt   type Config struct       host string     port float64    func main           value     var c1 Config     c2    Config       c3     new Config         reference     c4     amp Config       c5    new Config       fmt Println  amp c1    nil      fmt Println  amp c2    nil      fmt Println  amp c3    nil      fmt Println c4    nil      fmt Println c5    nil       fmt Println c1  c2  c3  c4  c5      which outputs   false false false false false   0    0    0   amp   0   amp   0

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The language spec mentions comparison operators  behaviors   comparison operators     In any comparison  the first operand must be assignable to the type   of the second operand  or vice versa      Assignability     A value x is assignable to a variable of type T   x is assignable to   T   in any of these cases          x s type is identical to T    x s type V and T have identical underlying types and at least one of V or T is not a named type    T is an interface type and x implements T    x is a bidirectional channel value  T is a channel type  x s type V and T have identical element types  and at least one of V or T is not   a named type    x is the predeclared identifier nil and T is a pointer  function  slice  map  channel  or interface type    x is an untyped constant representable by a value of type T

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The compiler is pointing the error to you  you re comparing a structure instance and nil  They re not of the same type so it considers it as an invalid comparison and yells at you   What you want to do here is to compare a pointer to your config instance to nil  which is a valid comparison  To do that you can either use the golang new builtin  or initialize a pointer to it   config    new Config     not nil   or   config     amp Config                    host   myhost com                      port  22                        not nil   or  var config  Config    nil   Then you ll be able to check if   if config    nil          then

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You can also check like struct var     struct     This does not allow you to compare to nil but it does check if it is initialized or not  Be careful while using this method  If your struct can have zero values for all of its fields you won t have great time   package main  import  fmt   type A struct       Name string    func main         a    A  Hello       var b A      if a     A              fmt Println  A is empty      Does not print             if b     A              fmt Println  B is empty      Prints            http   play golang org p RXcE06chxE

[go] nil detection in Go

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