cannot convert data type interface to type string need type assertion

Question

I am pretty new to go and I was playing with this notify package   At first I had code that looked like this   func doit w http ResponseWriter  r  http Request        notify Post  my event    Hello World        fmt Fprint w    OK       I wanted to append newline to Hello World  but not in the function doit above  because that would be pretty trivial  but in the handler afterwards like this below   func handler w http ResponseWriter  r  http Request        myEventChan    make chan interface        notify Start  my event   myEventChan      data     lt -myEventChan     fmt Fprint w  data     n       After go run     go run lp go    command-line-arguments   lp go 15  invalid operation  data     n   mismatched types interface    and string    After a little bit of Googling I found this question on SO   Then I updated my code to   func handler w http ResponseWriter  r  http Request        myEventChan    make chan interface        notify Start  my event   myEventChan      data     lt -myEventChan     s   data  string      n      fmt Fprint w  s      Is this what I was supposed to do  My compiler errors are gone so I guess that s pretty good  Is this efficient  Should you do it differently

User · Answer

As asked for by    s     an explanation can be found at https   golang org pkg fmt  Sprint  Related explanations can be found at https   stackoverflow com a 44027953 12817546 and at https   stackoverflow com a 42302709 12817546  Here is  Yuanbo s answer in full   package main  import  fmt   func main         var data interface     2     str    fmt Sprint data      fmt Println str

User · Answer

an easy way  str    fmt Sprint data

User · Answer

Type Assertion  This is known as type assertion in golang  and it is a common practice   Here is the explanation from a tour of go      A type assertion provides access to an interface value s underlying concrete value    t    i  T       This statement asserts that the interface value i holds the concrete type T and assigns the underlying T value to the variable t       If i does not hold a T  the statement will trigger a panic       To test whether an interface value holds a specific type  a type assertion can return two values  the underlying value and a boolean value that reports whether the assertion succeeded    t  ok    i  T       If i holds a T  then t will be the underlying value and ok will be true       If not  ok will be false and t will be the zero value of type T  and no panic occurs    NOTE  value i should be interface type   Pitfalls  Even if i is an interface type    i is not interface type  As a result  in order to convert   i to its value type  we have to do it individually      var items   i for    item    range items       value  ok    item  T      dosomethingWith value      Performance  As for performance  it can be slower than direct access to the actual value as show in this stackoverflow answer

User · Answer

According to the Go specification      For an expression x of interface type and a type T  the primary expression x  T  asserts that x is not nil and that the value stored in x is of type T    A  type assertion  allows you to declare an interface value contains a certain concrete type or that its concrete type satisfies another interface   In your example  you were asserting data  type interface    has the concrete type string  If you are wrong  the program will panic at runtime  You do not need to worry about efficiency  checking just requires comparing two pointer values   If you were unsure if it was a string or not  you could test using the two return syntax   str  ok    data  string    If data is not a string  ok will be false  It is then common to wrap such a statement into an if statement like so   if str  ok    data  string   ok          act on str      else          not string

[go] cannot convert data (type interface {}) to type string: need type assertion

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